Showing papers in "Journal of Polymer Science Part A in 2013"

Standing on the shoulders of Hermann Staudinger: Post-polymerization modification from past to present

Abstract: With a span as long as the history of polymer science itself, post-polymerization modification represents a versatile platform for the preparation of diversely functionalized polymers from a single precursor. Starting with the initial efforts by Staudinger in the 1920s, many of the early developments in modern polymer science can be attributed to the utilization of post-polymerization modification reactions. The scope of post-polymerization modification has greatly expanded since the 1990s due to the development of functional group tolerant controlled/living polymerization techniques combined with the (re)discovery of highly efficient coupling chemistries that allow quantitative, chemoselective, and orthogonal functionalization of reactive polymer precursors. After some basic mechanistic considerations, this Highlight will provide an overview of the development and evolution of eight main classes of post-polymerization modification reactions. (C) 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2013

High molecular weight poly(ethylene-2,5-furanoate); critical aspects in synthesis and mechanical property determination

Abstract: Furan-2,5-dicarboxylic acid (FDCA) is a widely advocated renewable substitute for terephthalic acid (TA). Preparation of high molecular weight FDCA based polyesters by an industrially common combination of melt polymerization and subsequent solid state post condensation is described. Ultimately, poly(ethylene 2,5-furanoate) (PEF) with absolute Mn = 83,000 g mol−1 is obtained, determined by triple detection Size Exclusion Chromatography. The bulk polymer properties of FDCA based polyesters, necessary to evaluate their industrial potential were determined the Young's modulus of PEF is determined to be 2450 ± 220 MPa and the maximum stress 35 ± 8 MPa. The influence of crystallinity on the mechanical properties as function of temperature was determined by dynamic mechanical thermal analysis. A detailed differential scanning calorimetry study on the crystallization behavior of high molecular weight PEF allowed to calculate the equilibrium melting temperature (Tm0) of 239.3 and 239.7 °C for the first and second melting peak, respectively. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013, 51, 4191–4199

Donor–acceptor semiconducting polymers for organic solar cells

Abstract: This review describes the synthesis and photovoltaic performance of donor–acceptor (D–A) semiconducting polymers that have been reported during the last decade. 9,9-Dialkyl-2,7- fluorene, 2,7-carbazole, cyclopenta[2,1-b:3,4-b′]dithiophene, dithieno[3,2-b:2′,3′-d]silole, dithieno[3,2-b:2′,3′-d]pyrrole, benzo[1,2-b:4,5-b′]dithiophene, benzo[1,2 b:4,5 b′]difuran building blocks, and their D–A copolymers are described in this review. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2013

Designing responsive microgels for drug delivery applications

Abstract: Microgels based on thermally responsive polymers have been widely investigated in the context of controlled release applications, with increasing recent interest on developing a clearer understanding of what physical, chemical, and biological parameters must be considered to rationally design a microgel to deliver a specific drug at a specific rate in a specific physiological context. In this contribution, we outline these key design parameters associated with engineering responsive microgels for drug delivery and discuss several recent examples of how these principles have been applied to the synthesis of microgels or microgel-based composites. Overall, we suggest that in vivo assessment of these materials is essential to bridge the existing gap between the fascinating properties observed in the lab and the practical use of microgels in the clinic. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013, 51, 3027–3043

Synthesis and characterization of novel renewable polyesters based on 2,5‐furandicarboxylic acid and 2,3‐butanediol

Abstract: Novel polyesters from 2,5-furandicarboxylic acid or 2,5-dimethyl-furandicarboxylate and 2,3-butanediol have been synthesized via bulk polycondensation catalyzed by titanium (IV) n-butoxide, tin (IV) ethylhexanoate, or zirconium (IV) butoxide. The polymers were analyzed by size exclusion chromatography, nuclear magnetic resonance spectroscopy, Fourier transform infrared spectroscopy (FTIR), matrix-assisted laser ionization-desorption time-of-flight mass spectrometry, electrospray ionization time-of-flight mass spectrometry, electrospray ionization quadruple time-of-flight mass spectroscopy, thermogravimetric analysis, and differential scanning calorimetry. Fully bio-based polyesters with number average molecular weights ranging from 2 to 7 kg/mol were obtained which can be suitable for coating applications. The analysis of their thermal properties proved that these polyesters are thermally stable up to 270-300 °C, whereas their glass transition temperature (Tg) values were found between 70 and 110 °C. Furthermore, a material was prepared with a molecular weight of 13 kg/mol, with a Tg of 113 °C. This high Tg would make this material possibly suitable for hot-fill applications

A renaissance of color: New structures and building blocks for organic electronics

Abstract: Natural dyes and pigments like indigo and its derivatives valued for their bright colors and photochemical stability has been used since antiquity. Recently, the need for better performing materials in the organic electronics field has inspired a resurgence of these historical molecules and their subsequent transformation into new families of π-conjugated building blocks used to construct new (macro)molecular semiconductors. This Highlight will explore the renaissance of notable building blocks including diketopyrrolopyrrole, (iso)indigo, benzodipyrrolidone, and benzodifuranone, as well as nonfullerene acceptor structures 9,9′-bifluorenylidene and quinacridone. In addition, as the organic electronics field continues to evolve, the design of molecules with precise structure and function embodies a new paradigm for the next generation of materials. Representative examples will be described that embrace this new model and point the direction for advanced technologies. © 2013 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2013

Photo‐control of biological systems with azobenzene polymers

Abstract: Azobenzene-containing polymers offer tremendous advantages and opportunities over other stimuli-responsive materials to interface with biology. Azobenzene's fast, reversible, and innocuous cis–trans geometrical isomerization can be leveraged into dramatic intra- and inter-molecular changes when incorporated in polymeric materials. Azobenzene use has grown from a colorant, through to optical storage materials, and most recently in a variety of biologically themed applications. This review highlights the broad impact this photo-switch has had in recent years and offers a snapshot of the research landscape at the interface between photochemistry and biology. From photo-reversible micelles and peptides to controlled drug release and sensing, the versatility of azobenzene makes it a favored photo-switch found in many emerging applications. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013, 51, 3058–3070

Aluminum salen and salan complexes in the ring-opening polymerization of cyclic esters: Controlled immortal and copolymerization of rac-β-butyrolactone and rac-lactide

Abstract: Aluminum-based salen and salan complexes mediate the ring-opening polymerization (ROP) of rac-β-butyrolactone (β-BL), rac-lactide, and e-caprolactone. Al-salen and Al-salan complexes exhibit excellent control over the ROP of rac-β-butyrolactone, yielding atactic poly(3-hydroxybutyrate) (PHB) with narrow PDIs of <1.15 for Al-salen and <1.05 for Al-salan. Kinetic studies reveal pseudo-first-order polymerization kinetics and a linear relationship between molecular weight and percent conversion. These complexes also mediate the immortal ROP of rac-β-BL and rac-lactide, through the addition of excess benzyl alcohol of up to 50 mol eq., with excellent control observed. A novel methyl/adamantyl-substituted Al-salen system further improves control over the ROP of rac-lactide and rac-β-BL, yielding atactic PHB and highly isotactic poly(lactic acid) (Pm = 0.88). Control over the copolymerization of rac-lactide and rac-β-BL was also achieved, yielding poly(lactic acid)-co-poly(3-hydroxybutyrate) with narrow PDIs of <1.10. 1H NMR spectra of the copolymers indicate a strong bias for the insertion of rac-lactide over rac-β-BL. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2013

Solution-processable colorless polyimides derived from hydrogenated pyromellitic dianhydride with controlled steric structure

Abstract: This work presents novel colorless polyimides (PIs) derived from 1R,2S,4S,5R-cyclohexanetetracarboxylic dianhydride (H″-PMDA). Isomer effects were also discussed by comparing with PI systems derived from conventional hydrogenated pyromellitic dianhydride, that is, 1S,2R,4S,5R-cyclohexanetetracarboxylic dianhydride (H-PMDA). H″-PMDA was much more reactive with various diamines than H-PMDA, and the former led to PI precursors with much higher molecular weights. The results can be explained from the quite different steric structures of these isomers. The thermally imidized H″-PMDA-based films were colorless regardless of diamines because of inhibited charge-transfer interaction. In particular, the H″-PMDA/4,4′-oxydianiline system simultaneously achieved a very high Tg exceeding 300 °C, high toughness (elongation at break > 70%), and good solution processability. In contrast, the H-PMDA-based counterparts were essentially insoluble. The outstanding solubility of the former probably results from disturbed chain stacking by its nonplanar steric structure. An advantage of chemical imidization process is also proposed. In some cases, a copolymerization approach with an aromatic tetracarboxylic dianhydride was effective to improve the thermal expansion property. The results suggest that the H″-PMDA-based PI systems can be promising candidates for novel high-temperature plastic substrate materials in electronic paper displays. A potential application as optical compensation film materials in liquid crystal displays (LCD) is also proposed in this work. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2013

A non‐phosgene process to homopolycarbonate and copolycarbonates of isosorbide using dimethyl carbonate: Synthesis, characterization, and properties

Abstract: Poly(isosorbide carbonate) (PIC) was synthesized by melt polycondensation of dimethyl carbonate (DMC) and isosorbide using lithium acetylacetonate (LiAcac) as the catalyst. The reaction conditions were optimized to achieve PIC with relatively high number-average molecular weight (Mn) of 28,800 g/mol and isosorbide conversion of 95.2%. A series of poly(aliphatic diol-co-isosorbide carbonate)s (PAICs) were also synthesized by melt polycondensation of DMC with isosorbide and equimolar amounts of aliphatic diols (1,4-butanediol, 1,5-pentanediol, 1,6-hexanediol, and 1,4-cyclohexane dimethanol) in the presence of LiAcac and the TiO2/SiO2-based catalyst (TSP-44). PAICs with Mn values ranging from 18,700 to 34,400 g/mol and polydispersities between 1.64 and 1.69 were obtained. The 13C NMR analysis revealed the random microstructure of PAICs. The differential scanning calorimetry results demonstrated that all the PAICs were amorphous with a unique Tg ranging from 46 to 88 °C. The dynamic analysis results showed that the incorporation of linear or cyclohexane structure changed the dynamic mechanical properties of PIC drastically. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2013

Poly(amidoamine)s: Past, present, and perspectives

Abstract: Poly(amidoamine)s (PAAs) are a family of synthetic polymers obtained by stepwise polyaddition of prim- or sec-amines to bisacrylamides. Nearly all conceivable bisacrylamides and prim- or sec-amines can be employed as monomers endowing PAAs of a structural versatility nearly unique among stepwise polyaddition polymers. PAAs are degradable in aqueous media, including physiological fluids. Many of them are remarkably biocompatible notwithstanding their cationic character. PAAs are per se highly functional polymers and, in addition, can be further functionalized giving rise to an endless variety of polymeric structures meeting the requisites for applications in such apparently disparate fields as inorganic water pollutants scavengers, sensors, drug and protein intracellular carriers, transfection promoters, peptidomimetic antiviral and antimalarial agents. In this review, the unique chemistry of PAAs is discussed and a vast library of PAA structures and PAA applications from the beginning to the present days reported. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013, 51, 2319–2353

Deep eutectic solvents as both active fillers and monomers for frontal polymerization

Abstract: The deep eutectic solvents (DESs) based on the mixtures of a variety of ammonium salts and hydrogen bond donors containing acrylic acids and acrylamides are capable of sustaining frontal polymerization (FP). The selection of ammonium salt affects the reactivity and allows FP at relatively low temperature but with full conversion. Also, full conversion allows us to use these polymers for biomedical applications (e.g., drug delivery systems) as the unreactive ammonium salts can be released from the resulting polymer without by-products. We call these components “active fillers,” which can be ammonium salts with biological or pharmaceutical importance. For instance, we prepared poly(acrylic acid) loaded with lidocaine hydrochloride (a common anesthetic), the release of which was found to occur in a controlled fashion. The ammonium salts also create a sufficiently high viscosity to suppress buoyancy-driven convection without additional materials. The DES here described played an all-in-one role, providing the monomer, the active filler, and the polymerization medium for FPs. © 2013 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2013

Renewable benzoxazine monomers from “lignin‐like” naturally occurring phenolic derivatives

Abstract: The benzoxazines of three naturally occurring phenylpropanoid phenols: ferulic, coumaric, and phloretic acids, and their esters are described. Benzoxazines with conjugated unsaturated chains exhibit unusual poor thermal stability and degrade partially at the polymerization temperature making necessary the use of a catalyst (BF3.Et2O) to low the polymerization temperature and prevent degradation. Polybenzoxazines are prepared thermally and characterized by DSC and TGA techniques. The resulting materials have superior Tgs when compared with those prepared from an unsubstituted monofuctional benzoxazine due to the additional crosslinking through the ester and carboxylic moieties. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013, 51, 4894–4903

Diketopyrrolopyrrole-based conjugated polymers and small molecules for organic ambipolar transistors and solar cells

Abstract: The present highlight discusses major work in the synthesis of low bandgap diketopyrrolopyrrole (DPP)-based polymers with donor-acceptor-donor (D-A-D) approach and their application in organic electronics. It examines the past and recent significant advances which have led to development of low bandgap DPP-based materials with phenyl and thiophene as donors. (c) 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013, 51, 4241-4260

Optimization of direct arylation polymerization conditions for the synthesis of poly(3‐hexylthiophene)

Abstract: Direct arylation polymerization (DArP) is an emerging alternative to Stille and Suzuki polymerizations. This method is attractive as it allows preparation of high-molecular-weight conjugated polymers in good yield without the need to metallate monomers. Despite this promise, for poly(3-hexylthiophene) (P3HT) and related polymers that have β-protons on the thiophene ring, DArP is known to produce β-defects, which make the polymer properties different from polymers produced by traditional methods. Here, we demonstrate that DArP conditions based on simple, inexpensive, and bench-stable reagents can be tuned to limit the amount of defects and produce P3HT with properties remarkably similar to Stille P3HT. Specifically, lowering the reaction temperature, lowering the amount of catalyst, and using a bulkier carboxylate ligand is critical. Optimized conditions include reacting 2-bromo-3-hexylthiophene with 0.25 mol % of Pd(OAc)2, 1.5 equivalents of K2CO3, and 0.3 equivalents of neodecanoic acid in N,N-dimethylacetamide at 70 °C and give DArP P3HT with ∼60% yield, regioregularity of 93.5%, molecular weight of 20 kDa, polydispersity of 2.8, and melting point of 217 °C, providing a very close match to Stille P3HT, which is obtained with 70–80% yield, 91–94% regioregularity, molecular weight of 15–25 kDa, polydispersity of 2.5–2.8, and melting point of 214–221 °C. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013, 51, 2660–2668

Precision synthesis of acrylate multiblock copolymers from consecutive microreactor RAFT polymerizations

Abstract: Univ Hasselt, Polymer React Design Grp, Inst Mat Res IMO, B-3590 Diepenbeek, Belgium. Univ Sao Paulo, Escola Polytecn, Assoc Engn Quim, BR-05508010 Butanta Sao Paulo, Brazil.

A comparative study of the stimuli-responsive properties of DMAEA and DMAEMA containing polymers

Abstract: Reversible addition-fragmentation chain transfer copolymerization of dimethylaminoethyl acrylate (DMAEA) and methyl acrylate (MA) and their methacrylate counterparts (MMA) has been performed with good control over molecular weight and polydispersity. A screening in composition of copolymers has been performed from 0 to 75% of MA (or MMA). The behavior of these pH and temperature-sensitive copolymers has been studied in aqueous solution by measuring the cloud point (CP) and the acid dissociation constants (pKa). The higher incorporation of the hydrophobic monomer in the copolymer resulted in an increase in the pKa values due to the larger distance between charges thus facilitating the protonation of adjacent nitrogens for both, the acrylate and methacrylate derivatives. The CP behavior of the copolymers has been studied in pure water and the CP values have been found to be irreproducible for the acrylate polymers, as a consequence of the self-hydrolysis of DMAEA. Hence, kinetic studies have been performed to quantify the degree of self-hydrolysis at different temperatures and polymer concentrations to explore the full potential and application of these versatile polymers. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013, 51, 3333–3338

Synthesis of polystyrene nanoparticles “armoured” with nanodimensional graphene oxide sheets by miniemulsion polymerization

Abstract: Polystyrene particles “armoured” with nanosized graphene oxide (GO) sheets have been prepared by aqueous miniemulsion polymerization of styrene, exploiting the amphiphilic properties of GO in the absence of conventional surfactants. The nanoscale GO sheets were prepared from graphite nanofibers of diameter approximately 100 nm based on a novel procedure, thus effectively ensuring the absence of larger sheets. Polymerization proceeded to high conversion with minor coagulation, with final number-average particle diameters of approximately 500 nm, but relatively broad particle size distributions. Scanning electron microscopy analysis revealed particles with a textured surface, consistent with the expected morphology. Interestingly, analysis of GO sheets recovered from the polymerization revealed that the GO sheets are partially reduced during the polymerization—approximately 50% of the initial carboxyl groups of the GO were lost, consistent with some loss in colloidal stability at high conversion. The overall approach offers a convenient and attractive synthetic route to novel graphene-based polymeric nanostructures. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2013

Main-chain 1,2,3-triazolium-based poly(ionic liquid)s issued from AB + AB click chemistry polyaddition

Abstract: INTRODUCTION Poly(ionic liquid)s (PILs) are peculiar polyelectrolytes where the cationic or anionic centers are included in the repeating units of the polymer chain. PILs have been actively applied in the fields of polymer chemistry and materials science because they combine the unique properties of ionic liquids (ionic conductivity, thermal stability, versatile anion exchange, and chemical stability) with those of polymer materials (mechanical stability, improved processability, durability, and tunable architecture from macromolecular design); therefore, enabling the creation of an infinite pool of new materials with enhanced properties and functions. PILs are promising components in applications such as dye sensitized solar cells, lithium batteries, actuators, field effect transistors, light-emitting electrochemical cells, electrochromic devices, switchable surfaces, sensors, responsive nanostructured assemblies, membranes, composites, electromagnetic devices, and catalysis. PILs can be synthesized by either direct polymerization of IL monomers or by chemical modification of existing polymers. Most studied structures are based on organic cations such as dialkylimidazolium, alkylpyrrodilonium, tetraalkylammonium, or alkylpyridinium groups in combination with for instance halogenated (Br and I ) or hydrophobic counter-anions such as tetrafluoroborate (BF4 ), hexafluorophosphate (PF6 ), or fluorinated amides [(CF3CF2SO2)2N and (CF3SO2)2N ]. An infinitely high number of PILs can thus be synthesized due to all of the potential combinations of cations, anions, polymerization methods, and polymer architectures. However, imidazolium-based PILs are by far the most widespread and investigated materials in this highly competitive research field. Thus, the design of new PILs using simple synthetic procedures is of outmost importance to develop innovative materials with enhanced properties as required by emerging technologies. With the striking development of the copper-catalyzed azidealkyne cycloaddition (CuAAC, the most developed example of click chemistry reactions), 1,2,3-triazole groups have been increasingly popular linking units in a broad variety of molecular and macromolecular structures. Recently, several 1,4-disubstituted-1,2,3-triazolium ionic liquids have been developed, and their potential as efficient reaction media, anion-templated formation of rotaxanes, receptors for anion recognition, stereoselective organocatalysts, ligands for ruthenium-catalyzed olefin metathesis, as well as precursors of N-heterocyclic carbene ligands for palladium-catalyzed Suzuki coupling have been established. The scope of this communication is to demonstrate the unprecedented synthesis of 1,2,3-triazolium-based PILs and their potential as new ionic conducting materials. As an initial proof of concept a 1,2,3-triazolium-based PIL is obtained by the chemical modification of a poly(1,2,3-triazole) precursor obtained by AB þ AB CuAAC step growth polymerization.

Sugar Overcomes Oxygen Inhibition in Photoinitiated Free Radical Polymerization

Abstract: Faruk Oytun, Muhammet U. Kahveci, Yusuf Yagci Department of Chemistry, Faculty of Science and Letters, Istanbul Technical University, Maslak, Istanbul 34469, Turkey Department of Bioengineering, Faculty of Chemical and Metallurgical Engineering, Yildiz Technical University, Davutpasa, Esenler, Istanbul 34210, Turkey Department of Chemistry, Faculty of Science, King Abdulaziz University, Jeddah 21589, Kingdom of Saudi Arabia Correspondence to: Y. Yagci (E-mail: yusuf@itu.edu.tr)

Reversible addition–fragmentation chain transfer synthesis of amidine‐based, CO2‐responsive homo and AB diblock (Co)polymers comprised of histamine and their gas‐triggered self‐assembly in water

Abstract: 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.

Cationic polymerization of isobutylene at room temperature

Abstract: This review highlights recent approaches toward polyisobutylene (PIB) by an energy efficient room temperature cationic polymerization. Special focus is laid on our own work using modified Lewis acids and nitrile-ligated metal complexes associated with weakly coordinating anions. In both cases, suitable conditions have been found for efficient production of PIB characterized by medium to low molar masses and a high content of exo double bonds as end groups—the typical features of highly reactive PIB, an important commercial intermediate toward oil and gasoline additives. These and other approaches demonstrate that the cationic polymerization of isobutylene is still not fully explored, and new innovative catalyst systems can lead to surprising results of high commercial interest. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2013

Efficient stabilization of thiol-ene formulations in radical photopolymerization

Abstract: INTRODUCTION The thiol-ene reaction was first suggested by Posner in 1905, but academic interest in this potential polymerization reaction remained fairly limited until the last two decades. Interest in the thiol-ene reaction mechanism increased as distinct advantages over acrylate homopolymerization were discovered. The thiol-ene polymerization is unique in that it proceeds in general by a step growth radical polymerization. The simplified reaction mechanism of a photo-initiated thiol-ene step growth polymerization for an ene that cannot homopolymerize is well known and is presented in the Supporting Information. Various researchers have shown that unlike (meth)acrylates, thiol-ene reactions have reduced oxygen inhibition with significant lower shrinkage providing materials with often better mechanical properties. In particular, addition of thiols to (meth)acrlyate polymerizations contributes to significant improvement in impact resistance. The thiol-ene step growth radical polymerization leads to a homogeneous crosslinked network with low volume shrinkage and delayed gelation, explaining the reduced brittleness. Because of the wide availability of monomers with different terminal ene groups, it is possible to tailor the physical and mechanical properties of the network structures to meet a diversity of applications.

Three-Dimensional Microfabrication of Protein Hydrogels via Two-Photon-Excited Thiol-Vinyl Ester Photopolymerization

Abstract: Engineering three-dimensional (3D) hydrogels with well-defined architectures has become increasingly important for tissue engineering and basic research in biomaterials sci- ence. To fabricate 3D hydrogels with (sub)cellular-scale fea- tures, two-photon polymerization (2PP) shows great promise although the technique is limited by the selection of appropri- ate hydrogel precursors. In this study, we report the synthesis of gelatin hydrolysate vinyl esters (GH-VE) and its copolymer- ization with reduced derivatives of bovine serum albumin (acting as macrothiols). Photorheology of the thiol-ene copoly- merization shows a much more rapid onset of polymerization and a higher end modulus in reference to neat GH-VE. This allowed 2PP to provide well-defined and stable hydrogel microstructures. Efficiency of the radical-mediated thiol-vinyl ester photopolymerization allows high 2PP writing speed (as high as 50 mm s 21 ) with low laser power (as low as 20 mW). MTT assays indicate negligible cytotoxicities of the GH-VE macromers and of the thiol-ene hydrogel pellets. Osteosar- coma cells seeded onto GH-VE/BSA hydrogels with different macromer relative ratios showed a preference for hydrogels with higher percentage of GH-VE. This can be attributed both to a favorable modulus and preferable protein environment since gelatin favors cell adhesion and albumin incurs nonspe- cific binding. V C 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013, 51, 4799-4810

Stimuli‐responsive star polymers

Abstract: Stimuli-responsive star polymers gain more and more interest over the last decades due to their unique properties compared to their linear counterparts. The branched structure for instance has influence on the responsive behavior of these polymers. This review offers an overview of stimuli-responsive star polymers generated by different polymerization techniques, e.g. anionic and controlled radical polymerization (CRP). Beside conventional branched homopolymers different other types like block copolymers, miktoarm star copolymers, core crosslinked star polymers (CCS) and comb polymers are also presented. Furthermore their responsive behavior in solution or immobilized on a substrate, and their applications are outlined. VC 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013, 51, 2980–2994