Thiol‐terminated polyisobutylene: Synthesis,characterization, and derivatization

Thiol‐terminated polyisobutylene (α,ω‐PIB‐SH) was synthesized from thiourea and α,ω‐bromine‐terminated PIB in a three‐step, one‐pot procedure, using a cosolvent system of 1:1 (v:v) heptane:dimethylformamide. The initial alkylisothiouronium salt was produced at 90 °C. Aqueous base hydrolysis at 110 °C resulted in thiolate chain ends, which were re‐acidified to form telechelic PIB‐SH. ¹H and ¹³C NMR confirmed thiol functionality and complete terminal halogen conversion. Thiol‐based “click” reactions were used to demonstrate PIB‐SH utility. Alkyne‐terminated PIB was synthesized by a phosphine‐catalyzed thiol‐ene Michael addition with propargyl acrylate. Reaction of this product with 6‐mercaptohexanol produced tetrahydroxy‐functional PIB by a sequential thiol‐ene/thiol‐yne procedure. ¹H NMR confirmed the structures of both products. PIB‐SH was reacted with isocyanates in the presence of base to produce polythiourethanes. A model reaction used phenyl isocyanate in THF with catalytic triethylamine. Similar conditions were used to produce PIB‐based thiourethanes with and without a small‐molecule chain extender. Increased molecular weights and thiol group conversion were observed with GPC and ¹H NMR, respectively. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2010     

Synthesis and functionalization of dendron‐polymer conjugate based hydrogels via sequential thiol‐ene “click” reactions

Fabrication and functionalization of hydrogels from well‐defined dendron‐polymer‐dendron conjugates is accomplished using sequential radical thiol‐ene “click” reactions. The dendron‐polymer conjugates were synthesized using an azide‐alkyne “click” reaction of alkene‐containing polyester dendrons bearing an alkyne group at their focal point with linear poly(ethylene glycol)‐bisazides. Thiol‐ene “click” reaction was used for crosslinking these alkene functionalized dendron‐polymer conjugates using a tetrathiol‐based crosslinker to provide clear and transparent hydrogels. Hydrogels with residual alkene groups at crosslinking sites were obtained by tuning the alkene‐thiol stoichiometry. The residual alkene groups allow efficient postfunctionalization of these hydrogel matrices with thiol‐containing molecules via a subsequent radical thiol‐ene reaction. The photochemical nature of radical thiol‐ene reaction was exploited to fabricate micropatterned hydrogels. Tunability of functionalization of these hydrogels, by varying dendron generation and polymer chain length was demonstrated by conjugation of a thiol‐containing fluorescent dye. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016 , 54, 926–934     

Synthesis of nonsymmetric chain end functionalized polyisobutylenes

We present the synthesis of nonsymmetric α‐ω‐functionalized polyisobutylenes (PIBs) bearing different functional moieties on their chain ends. Thus, on one chain end either, a short tri‐ethylene oxide chain (TEO) or a phosphine oxide ligand is attached, whereas the other chain end is substituted by hydrogen bonding moieties (thymine/2,6‐diaminotriazine). The nonsymmetric PIBs were synthesized via living cationic polymerization using methyl‐styrene epoxide as initiator, followed by quenching reaction with 3‐bromopropyl‐benzene. Subsequent bromide/azide exchange and the use of the azide/alkyne click reaction allowed the synthesis of (a) (α)‐TEO‐(ω)‐thymine‐telechelic PIB ( 7a ), (b) (α)‐triethyleneoxide‐(ω)‐triazine telechelic PIB ( 7b ), and (c) (α)‐phosphinoxide‐(ω)‐thymine‐telechelic PIB ( 13 ) with molecular weights M_n ～ 4000 g mol^?1 and low polydispersities (M_w/M_n = 1.3). The chemical identity of the final structures was proven by extensive ¹H NMR investigations and matrix‐assisted laser desorption/ionization‐mass spectroscopy (MALDI). The presented method for the first time offers a simple and highly versatile approach toward supramolecular nonsymmetric α‐ω‐functionalized PIB. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011     

Photo‐induced thiol‐ene polysulfide‐crosslinked materials with tunable thermal and mechanical properties

Photo‐induced thiol‐ene crosslinked polymeric networks have been extensively explored in constructing a variety of new materials with enhanced mechanical properties for optical, biomedical, and sensing applications. Toward the broad applications, however, tunable mechanical properties are greatly desired. Here, an effective approach utilizing high‐molecular‐weight methacrylate copolymers having pendant thiol and vinyl groups (MCPsh and MCPenes) to modulate thermal and mechanical properties of photo‐induced thiol‐ene crosslinked materials is reported. The MCP copolymers are synthesized by an industrially friendly polymerization method, followed by post‐modification including either a facile coupling reaction or reductive cleavage. Upon UV irradiation, thiol‐ene reactive blends of MCPsh and MCPenes yield highly crosslinked materials through the formation of flexible sulfide linkages. These polysulfide‐crosslinked materials based on rigid MCP backbones exhibit enhanced mechanical properties. Further, their thermal and mechanical properties are tuned by modulating monomer compositions of MCPs as well as varying numbers of pendant SH or vinyl groups (i.e., extent of crosslinking densities). This approach is versatile and effective for development of high performance polymeric materials. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014 , 52, 3060–3068     

Sequential thiol‐ene/thiol‐ene and thiol‐ene/thiol‐yne reactions as a route to well‐defined mono and bis end‐functionalized poly(N‐isopropylacrylamide)

Sequential thiol‐ene/thiol‐ene and thiol‐ene/thiol‐yne reactions have been used as a facile and quantitative method for modifying end‐groups on an N‐isopropylacrylamide (NIPAm) homopolymer. A well‐defined precursor of polyNIPAm (PNIPAm) was prepared via reversible addition‐fragmentation chain transfer (RAFT) polymerization in DMF at 70 °C using the 1‐cyano‐1‐methylethyl dithiobenzoate/2,2′‐azobis(2‐methylpropionitrile) chain transfer agent/initiator combination yielding a homopolymer with an absolute molecular weight of 5880 and polydispersity index of 1.18. The dithiobenzoate end‐groups were modified in a one‐pot process via primary amine cleavage followed by phosphine‐mediated nucleophilic thiol‐ene click reactions with either allyl methacrylate or propargyl acrylate yielding ene and yne terminal PNIPAm homopolymers quantitatively. The ene and yne groups were then modified, quantitatively as determined by ¹H NMR spectroscopy, via radical thiol‐ene and radical thiol‐yne reactions with three representative commercially available thiols yielding the mono and bis end functional NIPAm homopolymers. This is the first time such sequential thiol‐ene/thiol‐ene and thiol‐ene/thiol‐yne reactions have been used in polymer synthesis/end‐group modification. The lower critical solution temperatures (LCST) were then determined for all PNIPAm homopolymers using a combination of optical measurements and dynamic light scattering. It is shown that the LCST varies depending on the chemical nature of the end‐groups with measured values lying in the range 26–35 °C. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 3544–3557, 2009     

Polyisobutylene‐based polyurethanes. VIII. Polyurethanes with ‐O‐S‐PIB‐S‐O‐ soft segments

We describe the synthesis, characterization, and select properties of a novel polyurethane (PU) prepared using a new polyisobutylene diol, HO‐CH₂CH₂‐S‐PIB‐S‐CH₂CH₂‐OH, soft segment and conventional hard segments. The diol is synthesized by terminal functionalization of ally‐telechelic PIB followed by low‐cost thiol‐ene click chemistry. Properties of ‐S‐ containing PU (PIB_S‐PU) containing 72.5% PIB were investigated and compared to similar PUs made with HO‐PIB‐OH (PIB_O‐PU). Hydrolytic resistance was studied by contact with phosphate‐buffered saline, oxidative resistance by immersing in concentrated HNO₃, and metal ion oxidation resistance by exposure to CoCl₂/H₂O₂. Hydrolytic and oxidative resistances of PIB_S‐PU and PIB_O‐PU are similar and superior to a commercial PDMS‐based PU, Elast‐Eon? E2A. According to ¹H NMR spectroscopy the ‐S‐ in PIB_S‐PUs remained unchanged upon treatment with HNO₃, however, oxidized mainly to ‐SO₂‐ by CoCl₂/H₂O_2. Static mechanical properties of PIB_S‐PU and PIB_O‐PU are similar, except creep resistance of PIB_S‐PU is surprisingly superior. The thermal stability of PIB_S‐PUs is ～15 °C higher than that of PIB_O‐PU. FTIR spectroscopy indicates H bonded S atoms (N‐H…S) between soft and hard segments, which noticeably affect properties. DSC and XRD studies suggest random low‐periodicity crystals dispersed within a soft matrix. Energy dispersive X‐ray spectroscopy–scanning electron microscopy indicates homogeneous distribution of S atoms on PIB_S‐PU surfaces. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016 , 54, 1119–1131     

Orthogonal multifunctionalization of aliphatic polycarbonate via sequential Michael addition and radical‐thiol‐ene click reactions

Aliphatic polycarbonate (PC) copolymer is synthesized by ring opening copolymerization of acrylate‐ and allyl‐functional cyclic carbonate monomers. The post‐polymerization functionalization of the resulting copolymer is performed quantitatively using a variety of thiol compounds via sequential Michael addition and photo‐induced radical thiol‐ene click reactions within relatively short reaction time at ambient temperature. This metal‐free click chemistry methodology affords the synthesis of biocompatible PC copolymer with multifunctional groups. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014 , 52, 1581–1587     

Poly(ether ketone)–polyisobutylene block copolymers: Synthesis and phase behavior

Telechelic poly(ether ketone)s (PEKs) and polyisobutylenes (PIBs) were combined to form PIB? PEK? PIB triblock copolymers and (PIB? PEK)_n multiblock copolymers via the formation of urea linkages. Monovalent and bivalent amino telechelic PIBs were prepared quantitatively from allyl telechelic PIBs by a newly developed reaction sequence featuring nucleophilic reaction steps. Telechelic PEK? NCO polymers were prepared from the corresponding amino telechelic PEKs via a reaction with diphosgene. The highly reactive PEK? NCO and PIB? NH₂ telechelics formed PEK? PIB block copolymers only quantitatively when appropriately reactive primary amino groups were present on the amino telechelic PIBs. The obtained block copolymers were microphase‐separated and featured mostly lamellar structures, as determined by small‐angle X‐ray scattering (SAXS). Temperature‐dependent SAXS measurements revealed ordered polymers in the melt up to 210 °C. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 188–202, 2005     

Synthesis and characterization of novel amphiphilic block copolymers di‐, tri‐, multi‐, and star blocks of PEG and PIB

A simple but efficient strategy has been developed for the synthesis of novel di‐, tri‐, multi‐, and star‐block copolymers comprising poly(ethylene glycol) (PEG) and polyisobutylene (PIB) blocks. The synthesis principle involves the coupling of appropriately terminally functionalized PEG and PIB sequences, specifically the hydrosilation of mono‐, di‐, and tetra‐allyl‐telechelic PEGs (PEG‐allyl, allyl‐PEG‐allyl, and C(‐PEG‐allyl)₄ by mono‐ and di‐Si(CH₃)₂H telechelic PIBs (PIB‐SiH and HiS‐PIB‐SiH). Representative block copolymers, for example, PEG‐PIB, PIB‐PEG‐PIB, (‐PIB‐PEG‐)_n, and C(‐PEG‐PIB)₄ have been assembled and their structures determined by ¹H and ¹³C NMR spectroscopy. The bulk and surface morphology of select triblocks have been investigated by DSC and AFM and the findings interpreted in terms of phase‐separated PEG and PIB microdomains. The swelling behavior in water of various block copolymers also has been studied. Block copolymers containing 50–70 wt % PIB produce hydrogels, the integrity of which is maintained by physical crosslinks by PIB segments. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 3200–3209, 2000     

Postfunctionalization of polyoxanorbornene via sequential Michael addition and radical thiol‐ene click reactions

We demonstrated the successful postfunctionalization of poly(oxanorbornene imide) (PONB) with two types of double bonds using sequential orthogonal reactions, nucleophilic thiol‐ene coupling via Michael addition and radical thiol‐ene click reactions. First, the synthesis of PONB with side chain acrylate groups is carried out via ring‐opening metathesis polymerization and nitroxide radical coupling reaction, respectively. Subsequently, the resulting polymer having two different orthogonal functionalities, main chain vinyl and side chain acrylate, is selectively modified via two sequential thiol‐ene click reactions, nucleophilic thiol‐ene coupling via Michael addition and photoinduced radical thiol‐ene. The orthogonal reactivity of two diverse double bonds, vinyl and acrylate functionalities, for the abovementioned consecutive thiol‐ene click reactions was first demonstrated on the model compound. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012     

Photo‐initiated thiol–ene “click” hydrogels from RAFT‐synthesized poly(N‐isopropylacrylamide)

Despite the efficiency and robustness of the widely used copper‐catalyzed 1,3‐dipolar cycloaddition reaction, the use of copper as a catalyst is often not attractive, particularly for materials intended for biological systems. The use of photo‐initiated thiol‐ene as an alternative “click” reaction to synthesize “model networks” is investigated here. Poly(N‐isopropylacrylamide) precursors were synthesized by reversible addition fragmentation chain transfer (RAFT) polymerization and were designed to have trithiocarbonate moieties as end groups. This structure design provides opportunity for subsequent end‐group modifications in preparation for thiol‐ene “click.” Two reaction routes have been proposed and studied to yield thiol and ene moieties. The advantages and disadvantages of each reaction path were investigated to propose a simple but efficient route to prepare copper‐free “click” hydrogels. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013 , 51, 4626–4636     

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     

Rubbery wound closure adhesives. I. design,synthesis, characterization,and testing of polyisobutylene‐based cyanoacrylate homo‐ and co‐networks

Novel rubbery wound closures containing various proportions and molecular weights of polyisobutylene (PIB) and poly(2‐octyl cyanoacrylate) [P(OctCA)] for potential clinical use were designed, synthesized, characterized, and tested. Homo‐networks were prepared by crosslinking 3‐arm star‐shaped PIBs fitted with terminal cyanoacrylate groups, [Ø(PIB‐CA)₃], and co‐networks by copolymerizing Ø(PIB‐CA)₃ with OctCA using N‐dimethyl‐p‐toluidine (DMT). Neat Ø(PIB‐CA)₃, and Ø(PIB‐CA)₃/OctCA blends, upon contact with initiator, polymerize within seconds to optically transparent strong rubbery co‐networks, Ø(PIB‐CA)₃‐co‐P(OctCA). Homo‐ and co‐network formation was demonstrated by sol/gel studies, and structures and properties were characterized by a battery of techniques. The T_g of P(OctCA) is 58 °C by DSC, and 75 °C by DMTA. Co‐networks comprising 25% Ø(PIB‐CA)₃ (M_n = 2400 g/mol) and 75% P(OctCA) are stronger and more extensible than skin. Short and long term creep studies show co‐networks exhibit high dimensional stability and <6% creep strain at high loading. When deposited on porcine skin co‐networks yield hermetically‐adhering clear rubbery coatings. Strips of porcine skin coated with co‐networks could be stretched and twisted without compromising membrane integrity. The co‐network is nontoxic to L‐929 mouse fibroblasts. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2015 , 53, 1640–1651     

Polymerizability,copolymerizability, and properties of cyanoacrylate‐telechelic polyisobutylenes I: three‐arm star cyanoacrylate‐telechelic polyisobutylene

This series of papers concern new materials for possible biological applications created by combining the chemistry of highly reactive cyanoacrylates (CAs) with polyisobutylene (PIB) rubbers. First, a new strategy for the synthesis of CA–telechelic PIBs is described. Subsequently, the strategy is employed for the synthesis of low viscosity (syringible) CA–telechelic three‐arm star PIB [Ø(PIB–CA)₃]. The intermediates of the synthesis route are characterized by ¹H NMR spectroscopy. Injecting liquid Ø(PIB–CA)₃ into living tissue (fresh chicken egg) produces a bolus of crosslinked PIB rubber. The spectacular oxidative resistance of this rubber is documented by its resistance to concentrated HNO₃. A structural model of the crosslinked rubber obtained upon contacting Ø(PIB–CA)₃ with proteinaceous tissue is proposed. Copyright © 2007 John Wiley & Sons, Ltd.     

ABC type miktoarm star copolymers through combination of controlled polymerization techniques with thiol‐ene and azide‐alkyne click reactions

ABC type miktoarm star copolymer with polystyrene (PS), poly(ε‐caprolactone) (PCL) and poly(ethylene glycol) (PEG) arms was synthesized using controlled polymerization techniques in combination with thiol‐ene and copper catalyzed azide‐alyne “click” reactions (CuAAC) and characterized. For this purpose, 1‐(allyloxy)‐3‐azidopropan‐2‐ol was synthesized as the core component in a one‐step reaction with high yields (96%). Independently, ω‐thiol functionalized polystyrene (PS‐SH) was synthesized in a two‐step protocol with a very narrow molecular weight distribution. The bromo end function of PS obtained by atom transfer radical polymerization was first converted to xanthate function and then reacted with 1, 2‐ethandithiol to yield desired thiol functional polymer (PS‐SH). The obtained polymer was grafted onto the core by thiol‐ene click chemistry. In the following stage, ε‐caprolactone monomer was polymerized from the core by ring opening polymerization (ROP) using tin octoate as catalyst through hydroxyl groups to form the second arm. Finally, PEG‐acetylene, which was simply synthesized by the esterification of Me‐PEG and 5‐pentynoic acid, was clicked onto the core through azide groups present in the structure. The intermediates at various stages and the final miktoarm star copolymer were characterized by ¹H NMR, FTIR, and GPC measurements. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011     

Hairy polymeric nanocapsules with ph‐responsive shell and thermoresponsive brushes: Tunable permeability for controlled release of water‐soluble drugs

The hairy poly(methacrylic acid‐co‐divinylbenzene)‐g‐poly(N‐isopropylacrylamide) (P(MAA‐co‐DVB)‐g‐PNIPAm) nanocapsules with pH‐responsive P(MAA‐co‐DVB) inner shell and temperature‐responsive PNIPAm brushes were prepared by combined distillation–precipitation copolymerization and surface thiol‐ene click grafting reaction using 3‐(trimethoxysilyl)propyl methacrylate‐modified silica (SiO₂‐MPS) nanospheres as a sacrificial core material. The well‐defined PNIPAm was synthesized by a reversible addition fragmentation chain transfer (RAFT) polymerization. The chain end was converted to a thiol by chemical reduction. The PNIPAm was integrated into the nanocapsules via thiol‐ene click reaction. The surface thiol‐ene click reaction conduced to tunable grafting density of PNIPAm brushes. The grafting densities decreased from 0.70 chains nm^?2 to 0.15 chains nm^?2 with increasing the molecular weight of grafted PNIPAm chains. Using water soluble doxorubicin hydrochloride (DOX·HCl) as a model molecular, the tunable shell permeability of the nanocapsule was investigated in detail. The permeability constant can be tuned by controlling the thickness of the P(MAA‐co‐DVB) inner shell, the grafting density of PNIPAm brushes, and the environmental pH and temperature. The tunable shell permeability of these nanocapsules results in the release of the loaded guest molecules with manipulable releasing kinetics. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014 , 52, 2202–2216     

Stable Covalently Photo‐Crosslinked Poly(Ionic Liquid) Membrane with Gradient Pore Size

Porous polyelectrolyte membranes stable in a highly ionic environment are obtained by covalent crosslinking of an imidazolium‐based poly(ionic liquid). The crosslinking reaction involves the UV light‐induced thiol–ene (click) chemistry, and the phase separation, occurring during the crosslinking step, generates a fully interconnected porous structure in the membrane. The porosity is on the order of the micrometer scale and the membrane shows a gradient of pore size across the membrane cross‐section. The membrane can separate polystyrene latex particles of different size and undergoes actuation in contact with acetone due to the asymmetric porous structure.     

Networks and conetworks of PIB‐based cyanoacrylate‐telechelic prepolymers: Synthesis,characterization, and properties

Novel networks and conetworks were prepared by the instantaneous polymerization of cyanoacrylate‐telechelic three‐arm star polyisobutylene [Ø(PIB‐CA)₃] and the copolymerization of Ø(PIB‐CA)₃/2,4,4‐trimethylpentane cyanoacrylate (TMP‐CA) mixtures, respectively, by means of strong nucleophiles, such as NEt₂‐telechleic linear or three‐arm star PIBs [Ø(PIB‐NEt₂)_{2 or 3}]. The reactants were combined in the bulk in double‐barrel syringes by reaction injection molding (RIM). The chemical and mechanical properties of products prepared in the bulk and by conventional solution techniques were characterized by various techniques (swelling, extractables, FTIR and NMR spectroscopies, Instron, microhardness, oxidative resistance, DSC, DMTA). Conditions for the preparation of rubbery materials for possible biomedical applications were developed. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 2612–2623, 2008     

New photo‐induced thiol‐ene crosslinked films based on linear methacrylate copolymer polythiols

Thiol‐ene radical addition by photolysis is a highly efficient click reaction of sufhydryl groups with reactive enes that has been extensively explored as a promising means to construct multifunctional materials. Here, photo‐induced thiol‐ene crosslinked films composed of linear methacrylate copolymer polythiols (MCPsh) are reported. Well‐defined MCPsh copolymers were prepared by thiol‐responsive cleavage of pendant disulfide linkages positioned in the corresponding methacrylate copolymers with narrow molecular weight distribution which were synthesized by a controlled radical polymerization method. With a commercially available multifunctional acrylate as a model ene, photo‐induced thiol‐ene radical polyaddition of these polythiols is competitive to free‐radical homopolymerization of acrylates, yielding crosslinked films exhibiting rapid cure, uniform network, and enhanced mechanical properties; these properties are required for high performance coating materials. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013, 51, 2860–2868.