Peter Roth studied chemistry at the University of Mainz (Germany), the University of Massachusetts (USA) and Seoul National University (South Korea) and obtained his PhD from Mainz in 2009. Jointly supervised by Prof. Rudolf Zentel and Prof. Patrick Theato, his PhD thesis dealt with the orthogonal end group modification of RAFT polymers.After completing a pre-diploma in psychology at the University of Mainz, Peter moved to Sydney for a postdoc at the Centre for Advanced Macromolecular Design (CAMD) at the University of New South Wales (UNSW) working with Prof. Tom Davis and Prof. Andrew Lowe.Peter was awarded a Discovery Early Career Researcher Award (DECRA) in 2012 and was promoted to senior research associate in 2013. Peter's group in Sydney worked on the development of novel multi-responsive polymers and nanoparticles and exploited multicomponent reactions for the design of reactive polymers.In 2015 Peter moved to Curtin University in Perth (Western Australia) as a lecturer in chemistry to conduct research and teach within the Department of Chemistry and the Nanochemistry Research Institute.Peter returned to Europe in 2016 to join the University of Surrey as a lecturer in organic/polymer chemistry.
Our group are experts in synthesizing polymers and polymer (nano-)materials. We exploit the whole spectrum of organic chemistry to design smart and multi-functional polymers. We have expertise in:
- Polymer Chemistry: controlled radical polymerization (specifically RAFT) and various metal-catalyzed polymerization methods; multicomponent reactions, click-type chemistries
- Smart Polymers: temperature-responsive materials (especially such with upper critical solution temperature (UCST) in water), multi-responsive polymers, CO2-responsive polymers, metal ion-sensitive polymers
- Biological Interface: protein bio-conjugation, non-toxic polymers, antifouling materials
- Nanoparticle Synthesis: stimulus-triggered self-assembly, polymerization-induced self-assembly, stimulus-triggered 'shape-shifting'
- Radical ring-opening polymerization and biodegradable polymers
Our group works in the newly refurbished Joseph Kenyon laboratory which features state-of-the-art facilities for organic chemistry and polymer synthesis, a cold room, triple detection gel permeation chromatography (GPC), dynamic light scattering, and an adjacent spectroscopy suite including several UV-vis, FT-IR, and Raman spectrometers. The department has two (400 and 500 MHz) NMR spectrometers, a brand-new triple-quad LC-MS, a state-of-the-art ICP-MS, a brand new Raman microscope, and several GC-MS instruments.
- CHE1038/ENG1086 Industrial Chemistry (coordinator)
- CHE1041 Organic Structure, Reactivity and Functional Groups
- CHE2044 Organic Carbon–Carbon Bond Formation and Heterocyclic Chemistry (coordinator)
- CHE3047/CHEM029 Research Project
- CHE3049/CHEM032 Medicinal Chemistry II / Advanced Medicinal Chemistry
- CHE3052 Topics in Polymer Chemistry / Advanced Polymer Materials & Nanotechnology
Courses I teach on
The radical ring-opening polymerization (RROP) of thionolactones provides access to thioester backbone-functional copolymers but has, to date, only been demonstrated on acrylic copolymers. Herein, the thionolactone dibenzo[c,e]oxepane-5-thione (DOT) was subjected to azobisisobutyronitrile (AIBN)-initiated free-radical homopolymerization, which produced a thioester-functional homopolymer with a glass-transition temperature of 95 °C and the ability to degrade exclusively into predetermined small molecules. However, the homopolymerization was impractically slow and precluded the introduction of functionality. Conversely, the reversible addition–fragmentation chain-transfer (RAFT)-mediated copolymerization of DOT with N-methylmaleimide (MeMI), N-phenylmaleimide (PhMI), and N-2,3,4,5,6-pentafluorophenylmaleimide (PFPMI) rapidly produced well-defined copolymers with the tendency to form alternating sequences increasing in the order MeMI ≪ PhMI < PFPMI, with estimated reactivity ratios of rDOT = 0.198 and rPFPMI = 0.0078 for the latter system. Interestingly, defects in the alternating structure were more likely caused by (degradable) DOT–DOT sequences rather than (nondegradable) MI–MI sequences, which was confirmed through the paper spray mass spectrometric analysis of the products from aminolytic degradation. Upon the aminolysis of backbone thioesters, maleimide repeating units were ring-opened, forming bisamide structures. Conversely, copolymer degradation through a thiolate did not result in imide substitution but nucleophilic para-fluoro substitution on PFPMI comonomer units, indicating the ability of DOT–MI copolymers to degrade under different conditions and to form differently functional products. The RROP of thionolactones has distinct advantages over the RROP of cyclic ketene acetals and is anticipated to find use in the development of well-defined degradable polymer materials.
The synthesis of functional polymers has been enriched dramatically by post-polymerization modifications. Even though it represents a synthetically very appealing approach, different synthetic concepts of organic reactions are utilized in polymer science for the synthesis of architecturally well-defined multifunctional polymers. The different classes of reactions that provide the synthetic polymer chemist with tools of unprecedented precision, thereby opening the doors for materials synthesis in an interdisciplinary world, will be summarized.
The use of 2,3,4,5,6-pentafluorobenzyl methacrylate (PFBMA) as a core-forming monomer in ethanolic RAFT dispersion polymerization formulations is presented. Poly[poly(ethylene glycol) methyl ether methacrylate] (pPEGMA) macromolecular chain transfer agents were chain extended with PFBMA leading to nanoparticle formation via polymerization-induced self-assembly (PISA). pPEGMA-pPFBMA particles exhibited the full range of morphologies (spheres, worms, and vesicles) including pure and mixed phases. Worm phases formed gels that underwent a thermo-reversible degelation and morphological transition to spheres (or spheres and vesicles) upon heating. Post-synthesis, the pPFBMA cores were modified through thiol–para-fluoro substitution reactions in ethanol using 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) as the base. For monothiols, conversions were 64% (1-octanethiol) and 94% (benzyl mercaptan). Spherical and worm-shaped nano-objects were core cross-linked using 1,8-octanedithiol, which prevented their dissociation in non-selective solvents. For a temperature-responsive worm sample, cross-linking additionally resulted in the loss of the temperature-triggered morphological transition. The use of the reactive monomer PFBMA in PISA formulations presents a simple method to prepare well-defined nano-objects similar to those produced with non-reactive monomers (e.g. benzyl methacrylate) and to retain morphologies independent of solvent and temperature.
Well-defined poly[pentafluorophenyl (meth)acrylate] (PPFP(M)A) homopolymers are prepared by RAFT radical polymerization mediated by a novel chain transfer agent containing two cholesteryl groups in the R-group fragment. Subsequent reaction with a series of small-molecule amines in the presence of an appropriate Michael acceptor for ω-group end-capping yields a library of novel bischolesteryl functional hydrophilic homopolymers. Two examples of statistical copolymers are also prepared including a biologically relevant sugar derivative. Specific examples of these homopolymers are examined with respect to their ability to self assemble in aqueous media-a process driven entirely by the cholesteryl end groups. In all instances evaluated, and under the preparation conditions examined, the homopolymers aggregate clearly forming polymersomes spanning an impressive size range.
The influence of the chemical structure of both end groups onto the lower critical solution temperature (LCST) of poly[oligo(ethylene glycol) monomethyl ether methacrylate] (POEGMA) in water was systematically investigated. POEGMA of Mn = 3550 g/mol and Mw/Mn = 1.14 prepared by reversible addition-fragmentation chain transfer (RAFT) polymerization was equipped with two different functional end groups in a one-step postpolymerization reaction combining activated esters, functional amines, and functional methane thiosulfonates. As end groups, n-propyl, n-hexadecyl, di(n-octadecyl), poly(ethylene glycol)-550 (PEG), 1H,1H-perfluorononyl, azobenzene, and trimethylethylammonium groups were systematically combined with methyl, n-hexadecyl, and 1H,1H,2H,2H-perfluorooctyl groups. Polymers were characterized by gel permeation chromatography, dynamic light scattering, and turbidimetry. Hydrophobic end groups at either end of the polymer chain decreased the LCST. For hydrophobic groups at both ends of the chain their influence was additive. Two large hydrophobic end groups allowed micelle formation below the LCST and an LCST higher than to be expected from nonaggregated polymers. The strongest hydrophobic effect was found for rigid aromatic end groups, which was attributed to their incompatibility with the flexible polymer chain. Charged end groups increased the LCST and could compensate for the effect of hydrophobic end groups at the opposite end group. PEG end groups could mask a hydrophobic influence of the opposite end group and stabilized the LCST.
In this work, we synthesize a polydisperse aqueous colloidal system composed of small and large zwitterionic particles, as well as medium sized standard acrylic particles. By assembling these dispersions into films by drying, we show using atomic force microscopy (AFM) how their top surfaces can be mostly covered by zwitterionic groups for a wide range of evaporation rates. We probe underneath the top film surface using Fourier-transform infrared (FTIR) spectroscopy - attenuated total reflection (ATR), observing that the content in zwitterionic particles of the film upper layer increases for faster evaporation rates. We show how polydisperse systems hold great potential to overcome the evaporation rate dependence of size segregation processes in drying colloidal blends, and we provide further insights into the assembly mechanisms involved. Polydisperse blends enhance the robustness of such processes for application in coatings and other soft products where evaporation rate can not be tuned.
A methacrylic polymer undergoing highly efficient para-fluoro substitution reactions is presented. A series of well-defined poly(2,3,4,5,6-pentafluorobenzyl methacrylate) (pPFBMA) homopolymers with degrees of polymerization from 28 to 132 and Ð ≤ 1.29 was prepared by the RAFT process. pPFBMA samples were atactic (with triad tacticity apparent in 1H and 19F NMR spectra) and soluble in most organic solvents. pPFBMA reacted quantitatively through parafluoro substitution with a range of thiols (typically 1.1 equiv thiol, base, RT, < 1h) in the absence of any observed side reactions. Para-fluoro substitution with different (thio)carbonylthio reagents was possible and allowed for subsequent one-pot cleavage of dithioester pendent groups with concurrent thia-Michael side group modification. Reactions with aliphatic amines (typically 2.5 equiv amine, 50–60 °C, overnight) resulted in complete substitution of the para-fluorides without any observed ester cleavage reactions. However, for primary amines, H2NR, double substitution reactions yielding tertiary (–C6F4)2NR amine bridges were observed, which were absent with secondary amine reagents. No reactions were found for attempted modifications of pPFBMA with bromide, iodide, methanethiosulfonate, or thiourea, indicating a highly selective reactivity toward nucleophiles. The versatility of this reactive platform is demonstrated through the synthesis of a pH-responsive polymer and novel thermoresponsive polymers: an oligo(ethylene glycol)-functional species with an LCST in water and two zwitterionic polymers with UCSTs in water and aqueous salt solution (NaCl concentration up to 178 mM).
Polymerization-induced self-assembly (PISA) is an extremely versatile method for the in situ preparation of soft-matter nanoparticles of defined size and morphologies at high concentrations suitable for large-scale production. Recently, certain PISA-prepared nanoparticles have been shown to exhibit reversible polymorphism (“shape-shifting”), typically between micellar, worm-like, and vesicular phases (order–order transitions), in response to external stimuli including temperature, pH, electrolytes, and chemical modification. This review summarises the literature to date and describes molecular requirements for the design of stimulus-responsive nano-objects. Reversible pH-responsive behaviour is rationalised in terms of increased solvation of reversibly ionised groups. Temperature-triggered order–order transitions, conversely, do not rely on inherently thermo-responsive polymers, but are explained based on interfacial LCST or UCST behaviour that affects the volume fractions of the core and stabilizer blocks. Irreversible morphology transitions, on the other hand, can result from chemical post-modification of reactive PISA-made particles. Emerging applications and future research directions of this ‘smart’ nanoparticle behaviour are reviewed.
The synthesis of poly(methyl methacrylate) (PMMA) exhibiting one fluorescent dye (Texas Red) and one methyl disulfide end group is described. It is shown that the latter end group enabled the exchange of both oleic amine on gold nanoparticles (AuNP) and of oleic acid on CdSe/ZnS quantum dots (QD), allowing for a phase transfer of both types of nanoparticles (NP) from hexane into dimethylformamide due to the solubility provided by the PMMA chains. For AuNP, a fluorescence quenching of the dye was found due to fluorescence resonance energy transfer (FRET) from the dye to the AuNP, while QDs caused a fluorescence enhancement by FRET from the QD to the attached dyes. Due to the hetero-telechelic geometry of the polymer, the separation between NP and dye is governed by the end-to-end distance of the polymer.
Five different polymers, poly[methyl methacrylate] (PMMA), poly[lauryl methacrylate] (PLMA), poly[diethylene glycol methacrylate] (PDEGMA), poly[N-iso-propylacrylamide] (PNIPA), and poly[styrene] (PS) prepared by the RAFT process and thus terminated with dithioesters were aminolyzed in the presence of S-3-butynyl methane thiosulfonate (MTS), which was synthesized in two steps. Analysis of the polymers by 2D NMR, UV–vis absorbance, and gel permeation chromatogra-phy revealed them to quantitatively carry acetylene end groups connected with disul-ﬁde bridges, indicating that functional MTS reagents can be employed for end group functionalization of RAFT polymers. This versatile method is of advantage compared with conjugations with functional maleimides, where isolation of terminal thiols is of-ten required but inexpedient for poly[(meth)acrylates] because their terminal thiols may undergo backbiting and thus avoid conjugation. The acetylene-terminated poly-mers were bound to an azide functionalized glass surface in a Cu(I) catalyzed cyclo-addition. The modiﬁed surfaces exhibited water contact angles corresponding to the polarity of the attached polymers. In the case of the stimulus responsive polymers PNIPA and PDEGMA, the surfaces showed temperature-dependent contact angles. The disulﬁde bond connecting the polymers to the surface could be selectively cleaved and resulted in all surfaces having the same contact angle, independent of the nature of the polymer prior attached to the surface.
The direct synthesis of methacrylic-based soft polymeric nanoparticles via reversible addition– fragmentation chain transfer dispersion polymerization (RAFTDP) is described. The use of poly[2-(dimethylamino)ethyl methacrylate]s, of varying average degree of polymerization ( Xn), as the stabilizing blocks for the RAFTDP of 3-phenylpropyl methacrylate (PPMA) in ethanol at 70 C, at various total solids contents, yielded the full spectrum of self-assembled nanoparticles (spherical and worm aggregates and polymersomes). We also demonstrate that nanoparticle morphology can be tuned simply by controlling temperature. This is especially evident in the case of worm aggregates undergoing a thermoreversible transition to spherical species – a process that is accompanied by a macroscopic degelation–gelation process.
Reversible addition-fragmentation chain transfer (RAFT) polymerization was used to synthesize poly[diethylene glycol monomethylether methacrylate] (PDEGMA) (Mn ) 6250 g/mol, PDI ) 1.14) with a pentafluorophenyl (PFP) activated ester and a dithioester end group. The hormone thyroxin (T4) was quantitatively attached to the PFP activated ester R end group via its amino group. The ω-terminal dithioester was not harmed by this reaction and was subsequently aminolyzed in the presence of N-biotinylaminoethyl methanethiosulfonate, yielding a polymer with a thyroxin and a biotin end group with very high heterotelechelic functionality. The polymer was characterized by 1 H, 13C, and 19F NMR, UV-vis, and IR spectroscopy and gel permeation chromatography. The thyroxin transport protein prealbumin with two thyroxin binding sites and streptavidin, which has four biotin binding sites, was conjugated using the biotarget labeled polymer, resulting in the formation of a protein-polymer network, confirming the heterotelechelic nature of the polymer. Polymer-protein microgel formation was observed with dynamic light scattering. To realize a directed protein assembly, prealbumin was immobilized onto a surface, exposing one of its two thyroxin binding groups and thus allowing the conjugation with the thyroxin R end group of the heterotelechelic polymer. The biotin ω end group of the attached polymer layer enabled the subsequent immobilization of streptavidin, yielding a defined multilayer system of two proteins connected with the synthetic polymer (efficiency of streptavidin immobilization 81% based on prealbumin). Without the polymer, no streptavidin immobilization occurred. The layer depositions were monitored by surface plasmon resonance. The synthetic approach of combining PFP activated esters with functional MTS reagents presents a powerful method for obtaining well-defined heterotelechelic (bio-) functionalized polymers.
The key designing in reliable biosensors is the preparation of thin films in which biomolecular functions may be immobilized and addressed in a controlled and reproducible manner. This requires the controlled preparation of specific binding sites on planar surfaces. Poly(methylsilsesquioxane)-poly(pentafluorophenyl acrylates) (PMSSQ-PFPA) are promising materials to produce stable and adherent thin reactive coatings on various substrates. Those reactive surface coatings could be applied onto various materials, for example, gold, polycarbonate (PC), poly(tetrafluoroethylene) (PTFE), and glass. By dipping those substrates in a solution of a desired amine, specific binding sites for protein adsorption could be immobilized on the surface. The versatile strategy allowed the attachment of various linkers, for example, biotin, L-thyroxine, and folic acid. The adsorption processes of streptavidin, pre-albumin, and folate-binding protein were monitored using surface plasmon resonance (SPR), Fourier transform infrared (FTIR) spectroscopy, fluorescence spectroscopy, and atomic force microscopy (AFM). The presented protein immobilization strategy, consisting of four steps (a) spin-coating of PMSSQ-PFPA hybrid polymer from tetrahydrofuran (THF) solution, (b) annealing at 130 C for 2 h to induce thermal cross-linking of the PMSSQ part, (c) surface analogues reaction with different amino-functionalized specific binding sites for proteins, and (d) controlled assembly of proteins on the surface, may find various applications in future biosensor design.
The synthesis of a vinyl polymer with two different fluorescent dye end groups using reversible addition-fragmentation chain transfer (RAFT) polymerization is described. Use of a pentafluorophenyl (PFP) activated ester chain transfer agent (CTA) provided a polymer with an R end group that was reactive toward amines and a dithioester ω end group. The R PFP ester was amidated with Oregon Green Cadaverin. This did not harm the ω dithioester, which was subsequently aminolyzed with an excess of n-propylamine in the presence of Texas Red-2-sulfonamidoethyl methanethiosulfonate, resulting in a disulfide bond connecting the second dye to the polymer chain. Excess dyes and side products were removed by thin layer chromatography (TLC). Gel permeation chromatography (GPC) using a UV-vis detector could verify the presence of each dye on the polymer chain and the absence of free dyes. The synthesis of the polymer by a living radical technique and the mild complementary conjugation methods conducted after polymerization at each end group allowed to introduce complex dye residues possessing high brightness and photostability. In particular, fluorescent dyes capable of acting as donor and acceptor for electronic excitation energy transfer were chosen. Time-resolved fluorescence measurements were used to determine the time constant of energy transfer between the end groups of isolated polymer chains. Assuming a Forster-type process, an average end- € to-end distance of 4.5 nm was calculated, which was in reasonable agreement with data obtained from light scattering.
The synthesis and aqueous solution properties of a family of zwitterionic homo-, co-, and terpolymers derived from poly(2-vinyl-4,4-dimethylazlactone) (pVDMA) with tunable lower and upper critical solution temperatures (LCST and UCST) are presented. A RAFT-made pVDMA precursor was reacted with mixtures of zwitterionic sulfopro-pylbetaine (SPB) amine or sulfobutylbetaine (SBB) amine, tetrahydrofurfurylamine (THF amine), and benzylamine (Bz amine) in varying molar ratios. Products were characterized by variable temperature (VT) NMR spectroscopy, FT-IR spec-troscopy, size exclusion chromatography, turbidity, and VT dynamic light scattering in order to conﬁrm quantitative postpolymerization modiﬁcation, determine molar compositions, and elucidate structure−property relationships. Polymers comprising large molar fractions of THF groups showed LCST behavior due to a polarity change of the THF-functional segments, while SPB/SBB-rich samples, including the zwitterionic homopolymers, showed UCST behavior in ultrapure water based on electrostatic polymer−polymer attractions. Binary SPB−THF copolymers were water-soluble between 0 and 90 °C for a large compositional range. Terpolymers comprising molar SPB:THF:Bz ratios of approximately 50:25:25 showed a low LCST and a high UCST (LCST < UCST) with a miscibility gap in which the SPB groups and THF groups were not fully hydrated. In the one-phase regions below the LCST and above the UCST, polymer chains were presumed to be unimerically dissolved with partially solvated domains undergoing intrachain associations. Addition of NaCl caused LCST and UCST behavior to disappear, resulting in temperature-independent solubility. Molecular insights presented herein are anticipated to aid in the development of smart materials with double LCST < UCST or UCST < LCST thermoresponsiveness.
Telechelic thermo- and light-responsive polymers based on poly(oligo(ethylene glycol) methyl ether methacrylate) P(OEGMA) with azobenzene functionalities at the end groups were synthesized. In a reversible addition-fragmentation chain transfer (RAFT) polymerization using a functionalized chain transfer agent (CTA) containing a pentafluorophenyl (PFP) activated ester, oligo(ethylene glycol) methyl ether methacrylate (OEGMA, Mn ∼ 300 g mol-1 ) could successfully be polymerized with good control over molecular weight, very high conversions, and narrow molecular weight distributions. Polymers derived from this CTA possessed an activated ester at the R-end of the polymer chain as well as a dithioester ω-terminus. The ω-dithioester group of each polymer chain could quantitatively be either removed with AIBN treatment or substituted with a PFP ester by using a modified diazo compound. As a consequence, a postmodifiaction of the telechelic reactive end groups was possible through a polymer analogous reaction with aminofunctionalized azobenzene. P(OEGMA) polymers containing azobenzene end groups showed a reversible light- and temperature-controlled phase transition in water. Higher values for the lower critical solution temperature (LCST) were measured after irradiation of the aqueous polymer solutions due to the higher polarity of cis-azobenzene. The LCST differences between irradiated and nonirradiated solutions increased linearly upon the ratio of azobenzene units up to 4.3 C
Postpolymerization modification is a powerful strategy to change the chemical functionality of pre-made polymers, but only limited approaches exist to modify functionality as well as the shape and behaviour of nano-particles. Herein, poly[poly(ethylene glycol) methyl ether methacrylate]-poly(2,3,4,5,6-pentafluorobenzyl methacrylate) nano-objects (pPEGMA-pPFBMA) prepared via RAFT dispersion polymerization with concurrent polymerization-induced self-assembly (PISA) in ethanol with either spherical or worm-shaped morphology were modified, post-synthesis, with a selection of 15 different thiols through thiol–para-fluoro substitution reactions in the nano-object cores. Depending on the choice of thiol, spherical nano-objects underwent an order–disorder transition to form unimers, increased in size, or underwent an order–order transition to form worm-shaped nano-objects. The core solvophobicity was found to be more important in driving a morphological transition than the modification efficiency, mass increase of the core block, or the glass transition temperature of the (partially) modified cores. These findings are relevant to the development of a “universal nanoparticle precursor” approach that allows the tuning of functionality, behaviour, size, and shape of a pre-made nano-object sample on demand.
Functionalized gold nanoparticles have been prepared in an organic solvent by a two-phase reduction method in ethyl acetate and water using bis(6-hydroxyhexyl) disulﬁde bis(2-bromoisobutyl) ester, bis(6-acetyloxyhexyl) disulﬁde, and bis(5-carboxypentyl) disulﬁde bis(pentaﬂuorophenyl) ester as stabilizing ligands. This procedure features the advantages that no phase transfer agent was necessary during the preparation of the gold nanoparticles and that the reducing conditions were mild enough to utilize functional disulﬁde ligands. The obtained gold nanoparticles with typical sizes between 2 and 5 nm could be precipitated and redispersed without any irreversible aggregation. Using these nanoparticles the stimuli-responsive polymers poly(N-isopropylacrylamide) and poly(N-cyclopropylacrylamide) could be grafted from the surface. Also, the grafting of polymers onto gold nanoparticles could be demonstrated with nanoparticles featuring pentaﬂuorophenyl ester groups. The reactive character of gold nanoparticles featuring a pentaﬂuorophenyl ester groups on the surface could also be applied in the preparation of multilayers on the basis of covalent bonds between the gold nanoparticles and polyallylamine.
Mechanical stimulation of supersaturated aqueous CO2 solutions is accompanied by a pH increase within seconds. In solutions of tailored homo- and AB diblock copolymers this is exploited to induce micelle formation, or, taking advantage of an aqueous upper critical solution temperature transition, nanoparticle disassembly
A novel monomer, 4-azido-2,3,5,6-tetrafluorobenzyl methacrylate (ABMA), enabled the selective and efficient postpolymerization modification of RAFT-made homopolymers and diblock copolymer nanoparticles prepared through polymerization-induced self-assembly (PISA). Poly(ABMA) homopolymers were modified postpolymerization in (near-)quantitative conversions with phosphines to give stable iminophosphoranes and in a multicomponent reaction with phenylacetaldehyde and morpholine, piperidine, or the crosslinker N,N’-dimethylethylene diamine to give the corresponding amidine derivatives in one step. Product polymers were characterized by NMR and FT-IR spectroscopy, size-exclusion chromatography, and differential scanning calorimetry. Unlike its monomer, poly(ABMA) was insoluble in ethanol and enabled the preparation of well-defined spherical, worm-shaped, and vesicular nanoparticles with azide-functional cores through RAFT dispersion polymerization with concurrent PISA. Worm-shaped particles formed physical gels that underwent thermally reversible degelation. Multicomponent modification of spherical nanoparticles with phenyl acetaldehyde and morpholine or piperidine led to (near-)quantitative core modification and, for morpholine, a significant increase in sphere diameter. UV-irradiation of nanoparticles led to crosslinking through the formation of reactive nitrene intermediates which prevented the disassembly of nanoparticles in non-selective solvents, representing a simple and reagent-free crosslinking strategy, and expanding the scope of azide-based polymer chemistry.
A series of uniform, macroporous poly(styrene-co-divinylbenzene) microspheres with diameters ranging from 6.6 0.6 to 8.6 0.2 mm was prepared in a multistep procedure involving precipitation polymerization synthesis of polystyrene seed particles, swelling of seed particles with plasticiser and porogen, and polymerization of styrene–divinylbenzene (S–DVB) inside the seed particles. Particles prepared with varying DVB feed ratios had comparable diameters (as evidenced by scanning electron microscopy) with speciﬁc surface areas increasing with DVB content from 11 to 467 m2 g1 (measured by nitrogen adsorption). Residual double bonds were converted into azide functionality (through HBr addition and bromo-azide substitution) or alkyne functionality (Br2 addition followed by double elimination) which allowed for CuAAC-click chemistry conjugation with reagents carrying the respective complimentary alkyne or azide functional groups including the ﬂuorescent dye derivatives 7-nitro-4-(prop-2-ynylamino)benzofuran (NBD-alkyne) and Rhodamine B hexylazide synthesised for this purpose. Eﬃciency of chemical transformations was determined using a combination of CHN and IC elemental analyses, solid state NMR spectroscopy, FT-IR spectroscopy, Raman spectroscopy, and confocal scanning ﬂuorescence microscopy. Although the respective second steps in each modiﬁcation route (substitution and elimination) suﬀered from lower yields (35%), porous particles with azide loadings of up to 0.71 mmol g1 and alkyne loadings of up to 0.78 mmol g1 were prepared. Confocal laser scanning microscopy on Rhodamine B-labelled microspheres indicated functionalization throughout the particles featuring a core–shell structure with higher functionalization in the outer layer of particles. Results are expected to contribute to the development of advanced, well-deﬁned, macroporous particles with high, chemically accessible surface areas.
A library of novel well-deﬁned (bis)amide-based (co)polymers was prepared through postpolymeriza-tion modiﬁcation of poly(pentaﬂuorophenyl acrylate) with amines including ammonia and amide de-rivatives of amino acids. Products were characterized using a combination of NMR and FT-IR spectroscopies and size exclusion chromatography; results conformed to the expected structures ob-tained through quantitative conversion. The series of (bis)amide (co)polymers displayed rich phase behavior in aqueous solution such as thermoreversible gelation at low temperature and high concen-tration while other samples displayed inverse temperature dependent solubility (lower critical solution temperature (LCST)-type) behavior. A hydrophobically modiﬁed polyacrylamide copolymer displayed upper critical solution temperature (UCST) behavior in aqueous solution. Signiﬁcantly, driven by polymer epolymer hydrogen bonding, copolymers self-associated into highly ordered, regular structures of several tens to hundreds of micrometers in size. Morphologies included sheet-like, rod-like and honeycomb-like structures and depended strongly on the chemical composition of copolymers.
A novel class of thiol-reactive (meth)acrylate monomers and the quantitative postpolymerization modifi- cation of their RAFT-made (co)polymers with aromatic, glycosidic, and aliphatic thiols are presented. A set of 6 different N-functional 2-(meth)acryloyloxy-2-(pentafluorophenyl)acetamide monomers bearing pentafluorophenyl groups was prepared by a Passerini three-component reaction of (meth)acrylic acid, 2,3,4,5,6-pentafluorobenzaldehyde, and various isocyanides in water in up to near-quantitative isolated yields. RAFT polymerization was used to produce well-defined homopolymers and copolymers with methyl methacrylate, tert-butyl methacrylate, poly(ethylene glycol) methyl ether (meth)acrylate, and pentafluorophenyl acrylate, with low polydispersity indices of generally ĐM ≤ 1.23. In the presence of base, (co)polymers underwent selective para-fluoro substitution reactions with thiols in the absence of any side reactions observable by 1 H and 19F NMR spectroscopy and size exclusion chromatography. The selection of employed thiols included various alkanethiols, a thiolated glucose derivative, mercaptopropionic acid, L-cysteine and the drug captopril. 19F NMR kinetic measurements indicated quantitative thiol–para- fluoro substitutions after primary aliphatic > secondary aliphatic > tertiary aliphatic) and the choice of a suitable base (triethylamine or 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU)). The versatility of thiol-reactive (meth)acrylate species is demonstrated by the examples of a thermoresponsive copolymer showing a thiol-sensitive lower critical solution temperature (LCST) and the selective sequential modification with thiols and amines of a doubly reactive copolymer containing activated pentafluorophenyl esters
The multicomponent Passerini reaction of aldehydes, carboxylic acids, and isocyanides is used to produce a series of novel reactive (meth)acrylic and styrenic monomers carrying pendant double bond, (trimethylsilyl protected) triple bond, diene, acetate, or pentafluorophenyl functionality. Dichloromethane and water were compared as solvents in the synthesis of 15 different monomers, with water resulting in significantly higher, up to quantitative, isolated yields with minimal purification. Characterization by 1H, 13C, and 19F NMR spectroscopy, FT-IR spectroscopy and mass spectrometry confirmed the synthesis and high purity of the functional α-acyloxycarboxamide products. The monomers are shown to be well suited for the RAFT-synthesis of well-defined homopolymers, statistical copolymers with methyl methacrylate, poly(ethylene glycol) methyl ether methacrylate, and styrene, statistical copolymers produced from two different Passerini-derived monomers, and AB diblock copolymers. SEC-measured polydispersities were generally low, ĐM ≤ 1.29, and 1H NMR spectroscopy confirmed copolymer molar compositions in good agreement with comonomer feed ratios. We expect this synthetic strategy to provide access to a wide range of novel multifunctional materials and demonstrate preliminary postpolymerization modification of a polystyrene derivative by cleavage of its pendent acetate groups and coupling of the dye Methyl Red to the resulting alcohol groups
RAFT dispersion polymerization (RAFTDP) is used to prepare reactive nanoparticles via the incorporation of Passerini-derived methacrylic comonomers containing pentafluorophenyl (PFP) groups. Copolymerization of 2-(dimethylamino)ethyl methacrylate with a Passerini comonomer gives copolymers suitable as macro-CTAs for ethanolic RAFTDP of 3-phenylpropyl methacrylate. Reaction of the PFP residues with functional thiols offers an approach for preparing surface modified nanoparticles.
We report the ring-opening metathesis polymerization (ROMP) synthesis of novel (co)polymers containing the multiresponsive morpholino functional group [(3aR,7aS)−2-(2-morpholinoethyl)−3a,4,7,7a-tetrahydro-1H−4,7-epoxyisoindole-1,3(2H)-dione (M1)]. All (co)polymers were prepared with the Grubbs' first generation initiator, RuCl2(PCy3)2CHPh, in CH2Cl2 or CH2Cl2/2,2,2-trifluoroethanol solvent mixtures. M1 homopolymers exhibit a pH dependent aqueous solubility being fully soluble below pH 5.0 and above pH 6.0. At these intermediate values, the polymers exhibit molecular weight (MW) independent inverse temperature dependent solubility with measured cloud points (TCP) of 86 °C at pH 5.0 and 79 °C at pH 6.0. In the case of the lowest MW homopolymer (absolute MW of 9950 g/mol), there was a clear dependence of the TCP on the homopolymer solution concentration and varied over the range 78–88 °C. The TCP could be further tuned via the preparation of novel AB statistical copolymers. Incorporation of a permanently cationic comonomer as a more hydrophilic species resulted in an increase of the TCP at low incorporations (up to 10 mol %) and the complete disappearance of any temperature dependent solubility at 20 mol %. In a complementary approach, the TCP could also be lowered by the preparation of statistical copolymers of M1 with a more hydrophobic comonomer. Finally, we note that M1 homopolymers are also responsive to Na2SO4 and could be readily salted-out of an aqueous solution salt at a [Na2SO4] of 2.0 M giving a third trigger for controlling aqueous solubility. These copolymers represent examples of new multiresponsive materials and demonstrate the effectiveness of ROMP as a synthetic tool for the preparation of new and interesting materials.
Uniform benzophenone-covered AuNP (Bph-AuNP) were prepared in a two-phase ethyl acetate/water reduction of auric acid employing a benzophenone-functionalized disulfi de. Upon UV irradiation, the benzophenone carbonyl bond was excited allowing for insertion into a nearby C–H bond and thus a covalent attachment of the AuNP onto a substrate exhibiting C–H bonds. Using this technique, polycarbonate–gold core–shell particles were easily obtained, providing a simple synthetic pathway toward AuNP hybrids with polycondensation polymers. Bph-AuNP were also covalently attached to alkylated silica microspheres or cellulose paper. UV irradiation through a photo mask enabled a photolithographic surface patterning.
Well-deﬁned stimulus-responsive polymer gels were prepared from poly(2-vinyl-4,4-dimethylazlatone) (PVDMA) via one-pot post-polymerization modiﬁcation. VDMA homo-polymers were reacted with diamine crosslinking agents and functional 1 or 2 amines to form polymer gels that swelled in organic solvents and, in many cases, aqueous solu-tions. A series of functional amine reagents, including N,N-dimethylethylenediamine (DMEDA), N,N-diethylethylenediamine (DEEDA), morpholine, 3-morpholinopropylamine (MPPA) and tetrahydrofurfurylamine (THFA), were chosen as functional amines to produce polymer gels containing environmentally sensitive species. 13C solid-state NMR and FTIR spectroscopic measurements conﬁrmed complete conversion of the reactive scaffolds. pH-dependent swelling behavior at ambient temperature was observed in DMEDA-, DEE-DA- and MPPA-modiﬁed hydrogels. Kinetic studies showed the swelling behaviors of DME-DA-modiﬁed hydrogels were regulated by cross-linker type and concentration in acidic water (pH = 4) at ambient temperature. The swelling ratio of hydrogels modiﬁed by DEE-DA, MPPA and THFA also depended strongly on temperature, indicating successful synthe-sis of thermoresponsive gels. Furthermore, the concentration of added sodium sulfate played a signiﬁcant role with respect to the swelling properties of MPPA-modiﬁed hydro-gels. These smart materials may be of interest in the biomedical ﬁeld as well as in other applications.
The upper critical solution temperature (UCST) behaviour of poly[oligo(ethylene glycol) monomethylether methacrylate] (POEGMA) in 1-octanol was exploited to self-assemble and crosslink micellar aggregates. Four diblock copolymers, POEGMA-block-poly(N-isopropylacrylamide) (POEGMA-b-PNIPAM), POEGMA-b-poly(N,N-diethylacrylamide) (POEGMA-b-PDEAM), and two examples of POEGMA-b-[PNIPAM-co-poly(pentafluorophenyl acrylate)] (POEGMA-b- (PNIPAM-co-PPFPA)), containing different amounts of activated PFP esters, were found to reversibly self-assemble into well-defined spherical micelles upon cooling in 1-octanol, as evidenced by dynamic light scattering (DLS) and electron microscopy. Transition temperatures, measured by turbidity and DLS, were around room temperature and the PNIPAM and PDEAM blocks did not significantly influence the critical temperature of the POEGMA block compared to homo POEGMA. The aggregates exhibited an inverted morphology compared to PNIPAM core–POEGMA shell micelles accessible through a thermally triggered self-assembly in water exploiting the lower critical solution temperature (LCST) behaviour of PNIPAM. Inverted micelles with PNIPAM-co-PPFPA coronae in 1-octanol were shell crosslinked using a diamine and subsequently transferred into water. This procedure yielded cage-like structures with swollen POEGMA cores and crosslinked PNIPAM shells. The shells reversibly collapsed onto the cores when heated above the LCST of PNIPAM, providing particles with novel architecture and the potential to host and release guest molecules by a thermal trigger.
Polysulfobutylbetaine (SBB) (co)polymers, zwitterionic species bearing ammonium and sulfonate groups separated by a butyl spacer in every repeat unit, were prepared through three different synthetic routes and their aqueous solution behaviour was studied. Postpolymerization quaternization of poly[2-(dimethylamino)ethyl methacrylate] with 1,4-butanesultone resulted in incomplete modification due to the low reactivity of this alkylating agent. RAFT radical polymerization of SBB-functional (meth)acrylate monomers and their copolymerization with a sulfopropylbetaine (SPB) methacrylate yielded well-defined (co)polymers with low dispersities 1.13 ≤ ĐM ≤ 1.23 at monomer conversions of 75–92%. For a series of SBB methacrylate homopolymers with increasing degrees of polymerization from 66–186 measured upper critical solution temperature (UCST) cloud points increased from 27–77 °C. Cloud points of statistical SPB-SBB copolymers with similar degrees of polymerization, but varying molar compositions, increased linearly with SBB content offering a simple means of UCST tuning. Additionally, novel SBB acrylamide homo- and copolymers were prepared by postpolymerization modification of poly(pentafluorophenyl acrylate) with an SBB-functional amine and in mixtures with benzylamine as a hydrophobic modifier. In all cases, the SBB (co)polymers had significantly higher UCSTs than their more common SPB counterparts, greatly extending the temperature range of tuneable UCST transitions and making the investigated SBB (co)polymers advantageous for exploiting their ‘smart’ behaviour. In this respect, combining SBB functionality with hydrophobic benzylacrylamide comonomers is presented as a simple means of increasing the maximum salt concentration at which UCST behaviour (which shows an antipolyelectrolyte effect) can be observed, enabling UCST transitions in aqueous solutions containing a physiological concentration (9 g L−1 ) of NaCl.
The introduction of functional groups into microparticles is commonly accomplished through, at times, low-yielding post-synthesis modiﬁcation. In this detailed study, the introduction of azide functionality into uniform, crosslinked, macroporous microparticles through direct copolymerization of styrene, divinyl-benzene (DVB), and 4-vinylbenzyl azide (VBA) in varying ratios inside swollen polystyrene seed (template) particles is investigated. Formulations contained up to 40 wt% of VBA in the monomer mixture. Resulting microspheres were characterised by SEM, porosimetry, FT-IR spectroscopy, and CHN elemental analysis. Uniform spherical particles with diameters ranging from 7.3 to 10.8 μm with diameter dispersities typically below 1.01 and with tuneable azide loadings from 0.11 to 1.17 mmol g−1 were obtained. Interestingly, severe eﬀects of VBA addition on porosity, surface smoothness, and particle shape were observed. Speciﬁc surface areas and cumulative pore volumes increased with the amount of DVB in feed, decreased with increasing VBA feed ratio, and increased drastically for the use of azide-functional template particles with measured cumulative pore volumes reaching up to 0.60 cm3 g−1. With increasing VBA feed, formation of smaller, secondary particles was observed and attributed to an incomplete swelling of VBA into seed particles, which is discussed as a main reason for lower-than-expected azide contents in product particles. For high VBA feed ratios (>25 wt%), dented, hollow, or hollow collapsed azide-functional particles were found, presumably due to immiscibility of the growing azide-functional copolymer with the polystyrene seeds. Finally, successful click-modiﬁcation is demonstrated with phenyl-acetylene and an alkyne-functional Rhodamine B dye allowing for mapping of functionalization density through confocal ﬂuorescence microscopy.
Doubly thermoresponsive polymers consisting of a poly[oligo(ethylene glycol) methyl ether methacrylate] (POEGMA) block displaying UCST behavior in alcohols and a block of poly(N-isopropylacrylamide) (PNIPAM) or poly- (N,N-diethylacrylamide) (PDEAM), each of which has an LCST in water, were synthesized using RAFT polymerization followed by simultaneous activated ester/amine and nucleophilic thiol−ene postpolymerization conversions. Upon heating aqueous solutions of POEGMA-b-PNIPAM, 1 H NMR spectroscopy confirmed a sudden decrease of the PNIPAM signals at the LCST, indicating dehydration and chain collapse. Dynamic light scattering (DLS) and turbidity measurements observed the macroscopic phase separation of the PNIPAM block at the same temperature. In 2-propanol, 1 H NMR spectroscopy showed a gradual decrease of the POEGMA signals over a range of more than 30 °C during its UCST transition, indicating early stages of chain crumpling up to 20 °C above the macroscopic phase separation. The OEG side chains were found to collapse onto the backbone starting at the ester linkages, indicating the most unfavorable enthalpic polymer− solvent interactions occur adjacent to the ester group. Although the diblock copolymers displayed a strong concentrationdependent cloud point, 1 H NMR spectroscopy revealed a concentration-independent desolvation, indicating the potential for applications that are not based on phase separation but on changes of polymer conformation. The phase separation occurred within a narrow temperature range of ∼6 °C as evidenced by turbidity and DLS. This transition could be exploited to selfassemble POEGMA-b-PDEAM into micellar structures with POEGMA cores in 1-octanol. Cooling to ∼15 °C below the cloud point was necessary to produce compact structures. Upon heating, the aggregates remained compact until redissolving entirely within a range of 1 °C, making the UCST of POEGMA in alcohols a valuable tool for reversible self-assembly applications.
Polymer analogous reactions represent a synthetically very appealing approach for the synthesis of functional polymers. Different synthetic concepts of organic reactions are merging in polymer science leading toward the synthesis of architecturally well-defined multifunctional polymers. The different classes of reactions provide the synthetic polymer chemist with tools of unprecedented precision, thereby opening the doors for materials synthesis in an interdisciplinary world.
(Co)Polymers containing pentafluorophenylacetylene (F5PA) have been prepared for the first time mediated by [Rh(nbd)Cl]2/NEt3 to give materials with properties typical of poly(phenylacetylene)s prepared with this catalyst/co-catalyst combination. We demonstrate that the F5PA repeat units in these new (co)polymers serve as convenient reactive species for postpolymerization modification with thiols via para-fluoro aromatic nucleophilic substitution reactions to give an entirely new family of novel thioether-functional polyene materials accompanied by absorption maxima shifts of up to 130 nm. Finally, we briefly examine the electrochemical properties of these new fluorinated polyene materials and highlight the distinct difference in behaviour of the F5PA homopolymer versus polyphenylacetylene, copolymers and functional derivatives.
A library of (meth)acrylamido (co)polymers was prepared by reacting poly(pentafluorophenyl (meth)- acrylate) with α-amino, ω-methoxy functionalized di(ethylene glycol), tri(ethylene glycol), and poly(ethylene glycol) (PEG)- 350, PEG-750, and PEG-5k, in combination with hexylamine or thyroxine. The resulting copolymers showed an improved solubility in water (higher or absent LCST values) and in alcohols (lower or absent UCST values) than the analogous common series of poly[oligo(ethylene glycol) methyl ether (meth)acrylates]. The polyacrylamido species showed a better solubility than the corresponding polymethacrylamido derivatives of similar molecular weight with all polyacrylamides investigated being water-soluble at temperatures exceeding 90.0 °C. Tunable thermosensitive behavior could be effected by the incorporation of the hydrophobic hexylamide comonomer. Similarly, an acrylamido backbone with grafted oligo(propylene glycol 600) amides exhibited a sharp LCST-type transition around 22.0 °C. The UCST-type transitions of the (meth)acrylamido homopolymers were evaluated in 2-propanol and 1-octanol and were found to increase with an increasing ethylene glycol side chain length, but were essentially independent of the alcohol chain length with polymers exhibiting higher UCST transitions in 2-propanol vs 1-octanol. Cytotoxicity tests on MRC5 fibroblast cells of the di- and tri(ethylene glycol) methyl ether acrylamido homopolymers revealed no toxicity up to concentrations of 10.0 g/L. By employing mixtures of di(ethylene glycol) methyl ether amine and the prohormone thyroxine (T4), water-soluble copolymers containing varying amounts of T4 could be easily synthesized. Because of enhanced solubility, low toxicity, and higher hydrolytic stability of amides versus ester linkages, activated ester polymers in combination with amino-functionalized ethylene glycol based side chains are presented as a versatile platform for highly soluble, biocompatible, bioconjugated materials.
This review highlights the chemistry of thiocarbonylthio groups with an emphasis on chemistry conducted at v or a and v chain-ends in copolymers prepared by reversible addition–fragmentation chain-transfer (RAFT) radical polymerization. We begin by giving a general overview of reactions associated with the thiocarbonylthio groups, followed by examples associated with macromolecular thiols.
The synthesis and characterisation of soft matter nanoparticles based on AB diblock copolymers of oligo(ethylene glycol)methyl ether methacrylate (OEGMA) with 3-phenylpropyl methacrylate (PPMA) is described. Reversible addition–fragmentation chain transfer dispersion polymerization formulations that result in polymerization-induced self-assembly (RAFTDP-PISA) in methanol were utilized to access a range of poly(OEGMA-b-PPMA) (p(OEGMA-b-PPMA)) nanoparticles with the sphere-to-worm-to-vesicle order–order transitions being readily observed with increasing average degree of polymerization ([X with combining macron]n) of the pPPMA block for a fixed [X with combining macron]n of 28 for the pOEGMA block. Similarly the effect of total copolymer concentration on the resulting nanoparticle morphology is also demonstrated whereby we highlight how tuning of worm micelle diameters can be accomplished simply by varying the concentration of a formulation. The block copolymer nanoparticles were characterized by size exclusion chromatography (SEC), 1H NMR spectroscopy, transmission electron microscopy (TEM) and small angle X-ray scattering (SAXS). Additionally, we report the first examples utilizing 3D electron tomography and in situ atomic force microscopy (AFM) in methanol as convenient and powerful complementary techniques for the characterization of the resulting soft matter nano-objects with an emphasis on the direct visualization of worm nanoparticles.
Well-defined homopolymers of pentafluorophenyl acrylate (PFPA) and AB diblock copolymers of N,N-dimethylacrylamide (DMA) and poly(ethylene glycol) methyl ether acrylate (PEGA) with PFPA were prepared by reversible addition–fragmentation chain transfer (RAFT) radical polymerization. Three PFPA homopolymers of different molecular weights were reacted with the commercially available amidine and guanidine species histamine (HIS) dihydrochloride and L-arginine methyl ester (ARG) dihydrochloride in the presence of S-methyl methanethiosulfonate to yield, quantitatively, the corresponding amidine and guanidinebased acrylamido homopolymers. Both the HIS and ARG homopolymers are known to reversibly bind CO2 with, in the case of the former, CO2 fixation being accompanied with a switch from a hydrophobic to hydrophilic state. The RAFT synthesis of PFPADMA and PEGA-PFPA diblock copolymers yielded well-defined materials with a range of molar compositions. These precursor materials were converted to the corresponding HIS and ARG block copolymers whose structure was confirmed using 1 H NMR spectroscopy. Employing a combination of dynamic light scattering and transmission electron microscopy, we demonstrate that the DMA-HIS and PEGA-HIS diblock copolymers are able to undergo reversible and cyclable self-directed assembly in aqueous media using CO2 and N2 as the triggers between fully hydrophilic and amphiphilic (assembled) states. For example, in the case of the 54:46 DMA-HIS diblock, aggregates with hydrodynamic diameters of about 40.0 nm are readily formed from the molecularly dissolved state.
A series of polymers of 2-vinyl-4,4-dimethylazlactone (VDMA) of varying molecular weights modified with small organic amines have been synthesized. Their performance as kinetic hydrate inhibitors (KHIs) has been investigated in high pressure steel rocking cells using a Structure II-forming synthetic hydrocarbon gas mixture. It was found that the PVDMA polymer with the lowest molecular weight (Mn = 1845 g/mole by 1H NMR) performed the best. It was also found that the n-propylamine derivative performed the best of the amine derivatives. The cloud point of this polymer derivative was found to be lower than ambient temperature, which is considered too low for practical oilfield applications. However, high cloud point PVDMA derivatives such as the ethylamine or pyrrolidine derivatives still gave reasonable KHI performance. The KHI performance of the n-propylamine derivative of PVDMA-I was tested at different concentrations in the range 1000–7000 ppm. It was found that the performance improved as the polymer concentration was increased.
A new functional bis-acylurea molecule allows a two-stage self-organization process. It self-organizes—at first—into 2D nanosheets with disulfide groups at the surface, which act—in the second stage—as a template for gold nanoparticle arrays.
In this communication, we report on a new route to the functionalization of ATRP polymers exploiting their halide end-groups, which were converted successfully into reactive disulfide end-groups, using sodium methanethiosulfonate. The resultant disulfide-terminated polymers could then be reacted with different functional thiols to yield functional polymers exploiting either thiol/disulfide exchange chemistry or thiol/ene ‘‘click’’ reactions.
A poly(oligo(ethylene glycol) monomethyl ether methacrylate)-block-poly(N-isopropyl methacrylamide) (POEGMAb-PNIPMAM) block copolymer with a biotin end group on the PNIPMAM block as a biotarget was synthesized as a model system for temperature-controlled polymer immobilization. The synthesis was based on RAFT polymerization followed by postpolymerization modification of an activated ester precursor block and an exchange of the dithioester end group within one step. NMR, differential scanning calorimetry (DSC), dynamic light scattering (DLS), and turbidimetry measurements were performed to investigate the stimulus-responsive properties. The double thermoresponsive POEGMA-b-PNIPMAM with biotin end group showed a temperature-dependent multistage assembly behavior as it was completely soluble in water at temperatures below the LCST of both blocks, formed micellar structures above the LCST of PNIPMAM but below the LCST of POEGMA, or precipitated from solution above the LCST of both blocks. At room temperature, the polymer could be immobilized onto a streptavidin surface via its biotin end group, as shown in surface plasmon resonance (SPR) experiments. At 50 °C, at which the block copolymer formed micelles trapping the biotin target within the PNIPMAM core, no immobilization was observed, showing that the biological binding ability of the model could be controlled via external stimuli.
Thermoresponsive copolymers carrying di(2-pyridyl)methyl ligands are shown to respond sensitively and selectively to the presence of heavy metal cations, while their metal complexes respond in a likewise selective and sensitive manner to the presence of anions or molecules with higher metal affinity. A set of well-defined copolymers of poly(ethylene glycol) methyl/phenyl ether acrylate, and N-di(2-pyridyl)methylacrylamide was prepared through a combination of RAFT radical polymerization and postpolymerization modification of activated esters. Products were characterised by 1 H and 19F NMR spectroscopy, size exclusion chromatography, FT-IR spectroscopy, and turbidity measurements. Ligand–metal complexation, as observed by UV–vis spectroscopy, was found to increase lower critical solution temperature (LCST) transitions in water drastically (e.g. up to 22 C) for addition of small amounts (e.g. 0.4 mM) of Cu(II), Co(II), Fe(II) and Ag(I) salts, attributed to a tethering of charge to the polymer. Conversely, salts of Mn(II) and Gd(III) did not affect copolymer solubility. Observed LCST transitions of polymer–metal complexes decreased with the addition of anions or molecules which formed more stable complexes, poorly soluble compounds, or underwent redox reactions with the metal cation. Selectivity toward specific anion or molecule analytes could be tuned though the choice of metal. An isothermal phase separation of a polymer–Cu(II) solution (5 g/L) in response to the addition of as little as 0.4 mM sodium cyanide is demonstrated while the addition of an equal amount of sodium azide did not cause any response, signifying the potential of the proposed concept for sensing applications.
Poly[oligo(ethylene glycol) methyl ether methacrylate] (POEGMA) is shown to possess insoluble– soluble transitions (UCST-type phase behavior) in a large variety of aliphatic alcohols. Samples of different molecular weights ranging from 5 kg mol1 to 23 kg mol1 prepared by the RAFT process and featuring different end groups at each end were analyzed by cloud point measurements. Transitions occurred sharply and were fully reversible. The UCST was found to increase with an increasing molecular weight. Hydrophobic (alkyl chain) end groups were found to lower the critical temperature in isopropanol, while rigid aromatic end groups raised the transition temperature. In ternary mixtures of isopropanol/chloroform/POEGMA, the UCST decreased with an increasing chloroform concentration, with 10 vol% of chloroform accounting for a 30 C drop. In mixtures of isopropanol/ hexane/POEGMA, the cloud point increased significantly only with hexane concentrations above 30 vol%, at which level a 2 C transition temperature increase was found. Addition of water to isopropanol solutions had a strong effect, with 1 vol% of water causing a decrease of the transition temperature of 12.5 C. With an increasing chain length of the solvent, the cloud point increased, while a branching of the hydrocarbon chains lowered the cloud point. Samples of 23 kg mol1 POEGMA were for instance found to have cloud points of 22.0 C in ethanol, 35.7 C in isopentanol, and 75.4 C in dodecanol.
Reversible addition fragmentation chain transfer (RAFT) polymerization is one of the most extensively studied reversible deactivation radical polymerization methods for the production of well-defined polymers. After polymerization, the RAFT agent end-group can easily be converted into a thiol, opening manifold opportunities for thiol modification reactions. This review is focused both on the introduction of functional end-groups using well-established methods, such as thiol-ene chemistry, as well as on creating bio-cleavable disulfide linkages via disulfide exchange reactions. We demonstrate that thiol modification is a highly attractive and efficient chemistry for modifying RAFT polymers.
The 2,3,4,5,6-pentafluorobenzyl group has become a popular reactive functionality in polymer chemistry because of its high susceptibility to para-fluoro substitution with thiols. Herein, it is demonstrated postpolymerization that the para-fluoride can be substituted using sodium azide and that the resulting 4-azido-2,3,5,6-tetrafluorobenzyl-functional polymers are versatile precursors for a multitude of onward modifications with click-like efficiencies. Quantitative azide–para-fluoro substitution was found for poly(2,3,4,5,6-pentafluorobenzyl methacrylate) and the related Passerini ester–amide (meth)acrylic (co)polymers when heated in DMF with sodium azide to 80 °C for 60–90 min. Conversely, the azidation of poly(2,3,4,5,6-pentafluorostyrene) under similar conditions resulted in ~90% substitution efficiency. Azide-functional (co-)polymers were thermally stable below 100 °C and were subsequently modified with (i) four different alkynes (CuBr, triethylamine, DMF, 55 °C, overnight) to give 1,4-substituted 1,2,3-triazoles in >95% conversions; (ii) potassium thioacetate (DMF, RT, 15 min) with quantitative amidation to the acetanilide derivative; and (iii) DL-dithiothreitol (methanol/DMF, RT, 90 min) resulting in complete reduction of the azides to primary amines, which were subsequently acylated with two different acyl chlorides. Products were characterized by 1H NMR, 19F NMR, and FT-IR spectroscopies, and size exclusion chromatography. Given their adaptability, perfluorophenylazides have large potential as multi-purpose intermediates in polymer and materials chemistry.
Postpolymerization modifi cation—the installation of functional groups into a premade, reactive polymeric precursor—is emerging as an advantageous synthetic strategy toward tailored materials. In this article, the preparation of environmentally sensitive, “smart,” polymers by virtue of postpolymerization modifi cation is presented. The underlying fundamentals of different types of stimulusresponsiveness are highlighted with an emphasis on thermoresponsiveness, encompassing lower and upper critical solution temperature (LCST and UCST) behavior. Using a range of postpolymerization modifi cations as examples, properties imparted through incorporation of specifi c functional groups and their structure–property relations are discussed. Strategies for an appropriate choice of functionality in order to obtain well-defi ned materials with custom-made behavior are presented.
Polysulfobetaines, polymers carrying highly polar zwitterionic side chains, present a promising research field by virtue of their antifouling properties, hemocompatibility, and stimulus-responsive behavior. However, limited synthetic approaches exist to produce sulfobetaine copolymers comprising hydrophobic components. Postpolymerization modification of an activated ester precursor, poly- (pentafluorophenyl acrylate), employing a zwitterionic amine, 3-((3-aminopropyl)dimethylammonio)propane-1-sulfonate, ADPS, is presented as a novel, one-step synthetic concept toward sulfobetaine (co)polymers. Modifications were performed in homogeneous solution using propylene carbonate as solvent with mixtures of ADPS and pentylamine, benzylamine, and dodecylamine producing a series of well-defined statistical acrylamido sulfobetaine copolymers containing hydrophobic pentyl, benzyl, or dodecylacrylamide comonomers with well-controllable molar composition as evidenced by NMR and FT-IR spectroscopy and size exclusion chromatography. This synthetic strategy was exploited to investigate, for the first time, the influence of hydrophobic modification on the upper critical solution temperature (UCST) of sulfobetaine copolymers in aqueous solution. Surprisingly, incorporation of pentyl groups was found to increase solubility over a wide composition range, whereas benzyl groups decreased solubilityan effect attributed to different entropic and enthalpic contributions of both functional groups. While UCST transitions of polysulfobetaines are typically limited to higher molar mass samples, incorporation of 0−65 mol % of benzyl groups into copolymers with molar masses of 25.5−34.5 kg/mol enabled sharp, reversible transitions from 6 to 82 °C in solutions containing up to 76 mM NaCl, as observed by optical transmittance and dynamic light scattering. Both synthesis and systematic UCST increase of sulfobetaine copolymers presented here are expected to expand the scope and applicability of these smart materials
The radical copolymerization of the thionolactone dibenzo[c,e]oxepane-5-thione with acrylates, acrylonitrile, and N,N-dimethylacrylamide afforded copolymers containing a controllable amount of backbone thioesters which could be selectively cleaved. The process is compatible with RAFT polymerization and promising for the development of advanced degradable polymers.
Radical ring-opening polymerization is a clever strategy to incorporate cleavable linkages into otherwise non-degradable vinyl polymers. But conventional systems suffer from slow copolymerization, harsh non-selective degradation conditions, and limited application potential because the degradation products (often oligomers or polymers themselves) have properties like the intact species. This work presents fast selective degradation accompanied by a drastic change in a key property, aqueous solubility. The thionolactone dibenzo[c,e]oxepane-5-thione was found to copolymerise radically with a range of primary, secondary, and tertiary neutral and zwitterionic acrylamides with rapid incorporation of degradable biphenyl thiocarboxylate repeat units. Intact copolymers displayed temperature-responsive (LCST or UCST-type) aqueous solubility behaviour, tuneable through the molar composition and (exploiting the non-azeotropic copolymerization behaviour) comonomer sequence. Various conditions led to selective and complete degradation of the backbone thioesters through hydrolysis, aminolysis, transthioesterification (including under physiological conditions), and oxidative hydrolysis which drastically increased aqueous solubility. Polymers containing as little as 8 mol-% of thioester repeat units underwent a temperature-independent insoluble–soluble transition upon degradation with cysteine or potassium persulfate. Insoluble polymers were used to block syringe filters which allowed flow of degradant solutions only, relevant relevant to lab-on-a-chip, sensing, and embolic biomedical applications.
Spherical, micrometer-sized, azide-functional particles were produced through dispersion copolymerization of styrene and vinylbenzyl azide (VBA, 1–100 wt-% of monomer feed) in ethanol in the presence of stabilizers. The obtained microspheres were characterized by SEM, disc centrifuge, FT-IR and NMR spectroscopy, elemental analysis, DSC, and TGA, had measured azide loadings of up to 5.58 mmol/g, and average diameters that decreased with increasing azide content from 2.8 to 0.8 μm. Microspheres were irradiated at a wavelength of 254 nm resulting in crosslinking based on azide-to-nitrene decomposition and subsequent C–H insertion and C=C addition reactions. The conversion of azide functionality was monitored by FT-IR spectroscopy, elemental analysis, and DSC and was found to roughly follow first-order kinetics with increased rates found for microspheres with lower azide contents. Photocrosslinking preserved shapes and size distributions and, above a crosslinking degree of 10%, prevented microsphere dissolution in good solvents. By controlling the irradiation time, the amount of azide consumed for photo-crosslinking could be precisely adjusted. Residual azide groups spared during the irradiation were shown to be amenable to highly efficient CuAAC click modification with a fluorescent dye, Rhodamine B propargyl ester. Given the demand for functional crosslinked microspheres and the inherent difficulties associated with common synthetic strategies in producing such materials, this methodology based on two orthogonal chemistries of the azide functionality provides simple access to well-defined microspheres with customizable degrees of crosslinking and functional group densities.
The radical ring-opening polymerization (RROP) of thionolactones provides access to thioester backbone-functional copolymers but has, to date, only been demonstrated on acrylic copolymers. Herein, the thionolactone dibenzo[c,e]oxepane-5-thione (DOT) was subjected to azobisisobutyronitrile (AIBN)-initiated free-radical homopolymerization, which produced a thioester-functional homopolymer with a glass-transition temperature of 95 °C and the ability to degrade exclusively into predetermined small molecules. However, the homopolymerization was impractically slow and precluded the introduction of functionality. Conversely, the reversible addition–fragmentation chain-transfer (RAFT)-mediated copolymerization of DOT with N-methylmaleimide (MeMI), N-phenylmaleimide (PhMI), and N-2,3,4,5,6-pentafluorophenylmaleimide (PFPMI) rapidly produced well-defined copolymers with the tendency to form alternating sequences increasing in the order MeMI ≪ PhMI < PFPMI, with estimated reactivity ratios of r DOT = 0.198 and r PFPMI = 0.0078 for the latter system. Interestingly, defects in the alternating structure were more likely caused by (degradable) DOT–DOT sequences rather than (nondegradable) MI–MI sequences, which was confirmed through the paper spray mass spectrometric analysis of the products from aminolytic degradation. Upon the aminolysis of backbone thioesters, maleimide repeating units were ring-opened, forming bisamide structures. Conversely, copolymer degradation through a thiolate did not result in imide substitution but nucleophilic para-fluoro substitution on PFPMI comonomer units, indicating the ability of DOT–MI copolymers to degrade under different conditions and to form differently functional products. The RROP of thionolactones has distinct advantages over the RROP of cyclic ketene acetals and is anticipated to find use in the development of well-defined degradable polymer materials.