Evaluation of thiol‐ene click chemistry in functionalized polysiloxanes

Polysiloxanes are commonly used in a myriad of applications, and the “click” nature of the thiol‐ene reaction is well suited for introducing alternative functionalities or for crosslinking these ubiquitous polymers. As such, understanding of the thiol‐ene reaction in the presence of silicones is valuable and would lead to enhanced methodologies for modification and crosslinking. Here, the thiol‐ene reaction kinetics were investigated in functionalized oligosiloxanes having varying degrees of thiol functionalization (SH), π–π interactions (from diphenyls, DP), and ene types (C?C). In the ene‐functionalized oligomers, π–π interactions were controlled through the use of dioctyl repeats (DO). The polymerization rate and rate‐limiting steps were determined for all systems containing an allyl‐functionalized oligomer, and rates ranging from 0.10 to 0.54 mol L^?1 min^?1 were seen. The rate‐limiting step varied with the oligomer composition; examples of rate‐limited propagation (5:3:2 C?C:DP:DO/1:1 SH:DP) or chain transfer (5:3:2 C?C:DP:DO/3:1 SH:DP) were found in addition to cases with similar reaction rate constants (5:2:3 C?C:DP:DO/1:1 SH:DP). None of the siloxanes were found to exhibit autoacceleration despite their relatively high viscosities. Instead, the allyl‐, vinyl‐, and acrylate‐functionalized siloxanes were all found to undergo unimolecular termination based on their high α scaling values (0.98, 0.95, and 0.82, respectively) in the relation R_p ∝ R_i^α. © 2013 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2013     

Synthesis of water‐soluble zwitterionic polysiloxanes

A series of water‐soluble siloxane polymers with pendent phosphorylcholine (PC) and sulfobetaine (SB) zwitterions was prepared using thiol‐ene “click” chemistry. Specifically, well‐defined vinyl‐substituted siloxane homopolymers and block copolymers were functionalized with small molecule zwitterionic thiols at room temperature. Rapid and quantitative substitution of the pendent vinyl groups was achieved, and zwitterionic polysiloxanes of narrow molecular weight distribution were obtained. The PC‐ and SB‐substituted polymers were found to be readily soluble in pure, salt‐free water. Critical micelle concentrations (CMCs) of these polymers in water were measured using a pyrene fluorescence probe, with CMC values estimated to be <0.01 g/L. Polymer aggregates were studied by dynamic light scattering, and the micelles generated from the PC block copolymers were visualized, after drying, by transmission electron microscopy. Aqueous solutions of these zwitterionic polysiloxanes significantly reduced the oil‐water interfacial surface tension, functioning as polymer amphiphiles that lend stability to oil‐in‐water emulsions. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016 , 54, 127–134     

Application of thiol‐ene click chemistry to preparation of functional polyethylene with high molecular weight and high polar group content: Influence of thiol structure and vinyl type on reactivity

A series of functional polyethylenes have been simply and efficiently synthesized via the combination of regioselective ethylene/5‐vinyl‐2‐norbornene (VNB) copolymerization using [PhNC(CF₃)CHCO(Ph)]₂TiCl₂ catalyst and following ultraviolet light initiated thiol‐ene click reaction. On treatment of ethylene/VNB copolymer with different thiols including mercaptoethanol, 1‐thioglycerol, methyl mercaptoacetate, methyl mercaptopropionate, 2‐mercaptoethylamine, mercaptoacetic acid, and mercaptopropanoic acid, various polar groups have been successfully introduced into the polyethylene. Except 2‐mercaptoethylamine, the functionalizations are quite efficient with the degree of functionalization higher than 94%, which is independent of thiol structure and double bond content. The content of polar group in functional polyethylene can be tuned in a wide range of 0–30 mol %. Gel permeation chromatography profiles indicate all functional polyethylenes that have very high molecular weights (160–336 kg/mol) with homogeneous formation. Besides, systematic investigation of the influence of vinyl type and thiol structure on reactivity has been also carried out. By treatment of mercaptoethanol with different copolymers (ethylene/VNB, ethylene/5‐ethylidene‐2‐norbornene, and ethylene/dicyclopentadiene copolymer), the order of vinyl reactivity can be summarized as terminal > internal > cyclic double bond. For different thiols, the reactivity has the sequence of SHCH₂COOH > SHCH₂COOCH₃ > SHCH₂CH₂COOH > SHCH₂CH₂COOCH₃ > SHCH₂CH(OH)CH₂OH > SHCH₂CH₂OH > SHCH₂CH₂NH₂, which is depended on the solubility and the electron‐withdrawing inductive effect of polar group. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012     

Cysteine‐Functional Polymers via Thiol‐ene Conjugation

A thiofunctional thiazolidine is introduced as a new low‐molar‐mass building block for the introduction of cysteine residues via a thiol‐ene reaction. Allyl‐functional polyglycidol (PG) is used as a model polymer to demonstrate polymer‐analogue functionalization through reaction with the unsaturated side‐chains. A modified trinitrobenzenesulfonic acid (TNBSA) assay is used for the redox‐insensitive quantification and a precise final cysteine content can be predetermined at the polymerization stage. Native chemical ligation at cysteine‐functional PG is performed as a model reaction for a chemoselective peptide modification of this polymer. The three‐step synthesis of the thiofunctional thiazolidine reactant, together with the standard thiol‐ene coupling and the robust quantification assay, broadens the toolbox for thiol‐ene chemistry and offers a generic and straightforward approach to cysteine‐functional materials.

     

Synthesis of well‐defined poly(dimethylsiloxane) telechelics having nitrobenzoxadiazole fluorescent chain‐ends via thiol‐ene coupling

Well‐defined PDMS telechelics having nitrobenzoxadiazole (NBD) fluorescent probes covalently attached at both chain‐ends were prepared in two steps and a series of fractionation procedures starting from commercially available divinyl‐terminated PDMS having a broad molar mass dispersity. First, thiol‐ene coupling between 6‐mercapto‐1‐hexanol and vinyl chain‐ends allowed the formation of dihydroxy‐terminated PDMS telechelics through the formation of a thioether linkage. The resulting material was then sequentially fractionated using dichloromethane/methanol mixtures to afford several well‐defined dihydroxy‐terminated PDMS fractions having sharp distributions of molar masses (M_n = 99.5–158 kDa and ? < 1.2). The NBD fluorescent probes were then attached at both chain‐ends by N,N′‐dicyclohexylcarbodiimide/4‐(dimethylamino)pyridine esterification coupling between the hydroxyl groups and 6‐(7‐nitrobenzofurazan‐4‐ylamino)hexanoic acid. The resulting fluorescent PDMS telechelics were characterized by SEC, ¹H NMR, UV–visible, and fluorescence spectroscopies. These materials are suitable probes to investigate the dynamics of polymer chains in bulk or at interfaces by the fringe pattern fluorescent recovery after photobleaching technique. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012     

Synthesis and characterization of chiral liquid‐crystalline polysiloxanes containing fluorinated units and sulfonic acid groups

Chiral side‐chain liquid‐crystalline polysiloxanes ( PS‐1 , PS‐2 , PS‐3 , PS‐4 , PS‐5 , PS‐6 ) bearing fluorinated units and sulfonic acid groups were synthesized with poly(methylhydrogeno)siloxane, cholest‐5‐en‐3‐ol(3β)‐4‐(2‐propenyloxy)benzoate, and 3‐trifluoromethyl‐phenyl 3‐sulfo‐4‐undec‐10‐ enoyloxy‐benzoate. The effects of fluorinated units and sulfonic acid groups on characteristic of liquid‐crystalline properties were studied. PS‐1 , PS‐2 , and PS‐3 exhibited both smectic and cholesteric mesophases, while PS‐4 , PS‐5 , and PS‐6 exhibited only cholesteric mesophase. As the polymers contained more fluorinated units and sulfonic acid groups, segregation of the fluorinated segment to the surface and aggregation of hydrogen bonding should occur. Therefore, the highly ordered lamellar mesogen–siloxane matrix systems should be disturbed severely, suggesting that PS‐4 , PS‐5 , and PS‐6 show no smectic phase. The maximum reflection bands become broad and shifted slightly to long wavelength from PS‐1 to PS‐6 . Copyright © 2008 John Wiley & Sons, Ltd.     

Versatile and controlled functionalization of polyferrocenylsilane-b-polyvinylsiloxane block copolymers using a N-hydroxysuccinimidyl ester strategy

A general route for the N-hydroxysuccinimidyl (NHS) ester functionalization of poly(ferrocenyldimethylsilane)-b-polyvinylsiloxane copolymers, which should be readily transferrable to other vinyl containing polymers, has been developed using a simple two step approach. Facile reaction of the NHS ester functionalized polymers with primary amines enables the incorporation of a range of functionalities previously inaccessible using standard thiol-ene “click” reactions. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016, 54, 245–252     

Thiol‐epoxy polymerization via an AB monomer: Synthetic access to high molecular weight poly(β‐hydroxythio‐ether)s

A synthetic route is developed for the preparation of an AB‐type of monomer carrying an epoxy and a thiol group. Base‐catalyzed thiol‐epoxy polymerization of this monomer gave rise to poly(β‐hydroxythio‐ether)s. A systematic variation in the reaction conditions suggested that tetrabutyl ammonium fluoride, lithium hydroxide, and 1,8‐diazabicycloundecene (DBU) were good polymerization catalysts. Triethylamine, in contrast, required higher temperatures and excess amounts to yield polymers. THF and water could be used as polymerization mediums. However, the best results were obtained in bulk conditions. This required the use of a mechanical stirrer due to the high viscosity of the polymerization mixture. The polymers obtained from the AB monomer route exhibited significantly higher molecular weights (M_w = 47,700, M_n = 23,200 g/mol) than the materials prepared from an AA/BB type of the monomer system (M_w = 10,000, M_n = 5400 g/mol). The prepared reactive polymers could be transformed into a fluorescent or a cationic structure through postpolymerization modification of the reactive hydroxyl sites present along the polymer backbone. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014 , 52, 2040–2046     

A Versatile and Efficient Hydrosilylation Route to Functionalized Polyferrocenylsilanes

Summary: The synthesis of a series of polyferrocenylsilanes (PFSs) containing CC functionalities in the side‐group structure and their subsequent derivatization by hydrosilylation chemistry are described. Hydrosilylation is shown to be an effective postpolymerization functionalization method, particularly in the case of poly(ferrocenylmethylvinylsilane), which can be prepared by photolytic anionic ring‐opening polymerization of the corresponding ferrocenophane monomer.

     

RAFT Polymerization and Thiol Chemistry: A Complementary Pairing for Implementing Modern Macromolecular Design

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.

     

Photocaged pendent thiol polymer brush surfaces for postpolymerization modifications via thiol‐click chemistry

In this work, a postpolymerization surface modification approach is reported that provides pendent thiol functionality along the polymer brush backbone using the photolabile protection chemistry of both o‐nitrobenzyl and p‐methoxyphenacyl thioethers. Poly(2‐hydroxyethyl methacrylate) (pHEMA) brushes were synthesized via surface‐initiated atom transfer radical polymerization, after which the pHEMA hydroxyl groups were esterified with 3‐(2‐nitrobenzylthio)propanoic acid or 3‐(2‐(4‐methoxyphenyl)‐2‐oxoethylthio)propanoic acid to provide the photolabile protected pendent thiols. Addressing the protecting groups with light not only affords spatial control of reactive thiol functionality but enables a plethora of thiol‐mediated transformations with isocyanates and maleimides providing a modular route to create functional polymer surfaces. This concept was extended to block copolymer brush architectures enabling the modification of the chemical functionality of both the inner and outer blocks of the block copolymer surface. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2013     

Orthogonal multiple click reactions in synthetic polymer chemistry

This review highlights the concept of multiple click reaction strategy which is utilized for design and synthesis of well‐defined complex macromolecular structures as well as multifunctionalization of well‐defined polymers. This review examines the click combinations mainly from double to quadruple and additionally from the most frequently used to the least. The present review may also be regarded as an update for recent reviews dealing with specifically double and triple click reaction combinations in synthetic polymer chemistry. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014 , 52, 3147–3165     

Synthesis and postfunctionalization of well‐defined star polymers via “double” click chemistry

In this article, the synthesis and the functionalization of well‐defined, narrow polydispersity (polydispersity index < 1.2) star polymers via reversible addition‐fragmentation chain transfer polymerization is detailed. In this arm first approach, the initial synthesis of a poly(pentafluorophenyl acrylate) polymer, and subsequent, cross‐linking using bis‐acrylamide to prepare star polymers, has been achieved by reversible addition fragmentation chain transfer polymerization. These star polymers were functionalized using a variety of amino functional groups via nucleophilic substitution of pentafluorophenyl activated ester to yield star polymers with predesigned chemical functionality. This approach has allowed the synthesis of star glycopolymer using a very simple approach. Finally, the core of the stars was modified via thiol‐ene click chemistry reaction using fluorescein‐o‐acrylate and DyLigh 633 Maleimide. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011     

From NMP to RAFT and Thiol‐Ene Chemistry by In Situ Functionalization of Nitroxide Chain Ends

A straightforward, novel strategy based on the in situ functionalization of polymers prepared by nitroxide‐mediated polymerization (NMP), for the use as an extension toward block copolymers and post‐polymerization modifications, has been investigated. The nitroxide end group is exchanged for a thiocarbonylthio end group by a rapid transfer reaction with bis(thiobenzoyl) disulfide to generate in situ reversible addition–fragmentation chain transfer (RAFT) macroinitiators. Moreover, not only have these macroinitiators been used in chain extension and block copolymerization experiments by the RAFT process but also a thiol‐terminated polymer is synthesized by aminolysis of the RAFT end group and subsequently reacted with dodecyl vinyl ether by thiol‐ene chemistry.     

Visible light‐induced thiol‐ene reaction: A new strategy to prepare Α,ω‐dithiol and Α,ω‐divinyl telechelic polythiolether oligomers

A new strategy is developed to prepare both α,ω‐dithiol and α,ω‐divinyl linear telechelic polythiolether oligomers by visible light induced thiol‐ene chemistry in the presence of a fac‐Ir(ppy)₃ photoredox catalyst. Polythiolether oligomers of well‐defined end groups and controlled molecular weights have been successfully synthesized at varying monomer molar ratios of 1,4‐benzenedimethanethiol (BDMT) to diethylene glycol divinyl ether (DEGVE). ¹H NMR and MALDI‐TOF MS analyses demonstrate that as‐prepared polythiolethers possess high end‐group fidelity, which is further supported by the successful polyaddition of polythiolethers bearing α,ω‐dithiol and α,ω‐divinyl groups. For example, with the α,ω‐dithiol‐ (M_n = 1900 g mol^?1, PDI = 1.25) and α,ω‐divinyl‐terminated (M_n = 2000 g mol^?1, PDI = 1.29) polythiolethers as macromonomers, the molecular weight of resulting polythiolether is up to 7700 g mol^?1 with PDI as 1.67. The reactivity of the terminal thiol group is further confirmed by the addition reaction with N‐(1‐pyrenyl)maleimide. UV‐vis spectra and fluorescene measurements suggest that fac‐Ir(ppy)₃ undergo a redox quenching process reacted with BDMT to generate thiyl free radicals. With these results, the mechanism of the thiol‐ene reaction catalyzed by photoredox catalyst is proposed. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016 , 54, 740–749     

A versatile approach to the synthesis of polyphosphazene derivatives via the thiol–ene reaction

The thiol–ene radical addition reaction has been successfully used to synthesize polyphosphazene derivatives. Poly[bis(allylamino)phosphazene] with pendant allyl groups was reacted with different thiol reagents under UV irradiation. These thiol reagents include 1‐pentanethiol, 3‐mercaptopropionic acid, 3‐mercapto‐1,2‐propane‐diol, and 2,3,4,6‐tetra‐O‐acetyl‐1‐thio‐β‐D ‐glucopyranose. ¹H NMR analyses confirm that the allyl polyphosphazene can be quantitatively modified by the mercaptans. In total, 100% conversion of the allyl groups was reached in <60 min toward the first three mercaptans, whereas about 80% conversion of the allyl groups was reached after 120‐min reaction toward the thioglucose. This method is a facile route for the synthesis of functional polyphosphazenes without the needs for protection/deprotection procedures. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012     

Exhaustive and partial alkoxylation of polymethylhydrosiloxane by copper‐catalyzed dehydrocoupling with alcohols: Synthesis,crosslinking behavior,and thermal properties of the products

Polymethylhydrosiloxane (PMHS) reacts with aliphatic and aromatic alcohols at room temperature in the presence of [CuH(PPh₃)]₆ complex catalyst to give poly[(methyl) (alkoxy)siloxane]s in high yields. Reactivity of alcohols decreases in the order of p‐methoxyphenol > p‐cresol > phenol > benzyl alcohol > allyl alcohol > ethanol > isopropanol > tert‐butyl alcohol. Partially p‐cresylated polymers, which still retain unreacted Si? H bonds, react further with ethylene glycol or water to form cross‐linked polymers, which, depending on the extent of cross linking, gelate during the cross‐linking process. Propargyl alcohol reacts with PMHS very rapidly to give exhaustively and partially propargyloxylated PMHS. Resulting polymers, upon heating, undergo crosslinking. Partially propargyloxylated polymers display high thermal stability [T_d5 (temperature of 5% weight loss) > 500 °C] as compared with starting PMHS (243 °C) and exhaustively propargyloxylated one (414 °C). © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011     

Modification of polysaccharides via thiol‐ene chemistry: A versatile route to functional biomaterials

We present herein a mild and rapid method for the modular functionalization of polysaccharides. Several ene‐functional charged and neutral polysaccharides, that is, hyaluronic acid and dextran, were prepared by esterification of the hydroxyl groups with pentenoic anhydride. The modified polysaccharides were then reacted with six model mercaptans under UV light, leading to linear polymers modified with hydrophobic groups, peptides, or oligosaccharides as well as chemical hydrogels. The thiol‐ene coupling reactions were found to proceed with high efficiency in short reaction times and with nearly no degradation of the polysaccharide backbone. Moreover, they were carried out in aqueous media, without the use of any metal catalysts, enhancing the attractive nature of this process. Notably, we investigated whether it is feasible to prepare cell‐responsive hydrogels by sequential bioconjugation and cross‐linking of the polysaccharide backbone with a bioactive peptide and poly(ethylene glycol)‐dithiol, respectively. All together, these results highlight the potential of this coupling strategy for the modular functionalization of polysaccharides under click chemistry‐like conditions. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012