Synthesis and characterization of well-defined comb-like branched polymers

Well-defined comb-like branched polymers having one branch in each repeating unit have been successfully synthesized by the coupling reaction of living polystyrene (PS) and polyisoprene (PI) anions with 1, 1-diphenylethenyl (DPE) groups along PS backbone prepared via atom transfer radical polymerization (ATRP) of 4-vinylbenzyloxy benzophenone (Sc) followed by Wittig reaction. The resulting comb-like branched polymers were characterized by IR, ¹H-NMR, gel permeation chromatography (GPC) and static light scattering (SLS) in detail. The effect of living chains and DPE group molar ratio on grafting efficiency was discussed. The results show the coupling reaction of living chains and DPE groups was highly effective, and the coupling efficiency can be controlled via the feed molar ratios of living chains and DPE groups. Moreover, the effect of molecular weights of backbone (PSe) and PSLi or PILi on grafting efficiency was also discussed. The results show that when excess living polymers were used, the almost quantitative grafting efficiency could be achieved.     

Novel synthesis of highly branched comb-like polymers by coupling reaction of polystyryllithium with 1,1-diphenylethenyl-functionalized polystyrene

Well-defined highly branched comb-like polystyrenes (PS) having one branch in each repeating unit have been successfully synthesized by the coupling reaction of living PS anions with 1,1-diphenylethenyl (DPE) groups along PS backbone prepared via atom transfer radical polymerization. When excess polystyryllithium (PSLi) was used, the quantitative grafting efficiency was achieved. The resulting polymers were characterized by NMR, IR, GPC, and SLS in detail.     

Synthesis and characterization of graft copolymers with poly(epichlorohydrin-co-ethylene oxide) as backbone by combination of ring-opening polymerization with living anionic polymerization

Series of graft copolymers with [Poly(epichlorohydrin-co-ethylene oxide)] [Poly(ECH-co-EO)] as backbone and polystyrene (PS), poly(isoprene) (PI) or their block copolymers as side chains were successfully synthesized by combination of ring-opening polymerization (ROP) with living anionic polymerization. The Poly(ECH-co-EO) with high molecular weight (M_n = 3.3 × 10⁴ g/mol) and low polydispersity index (PDI = 1.34) was firstly synthesized by ring-ROP using ethylene glycol potassium as initiator and triisobutylaluminium (i-Bu₃Al) as activator. Subsequently, by “grafting onto” strategy, the graft copolymers Poly(ECH-co-EO)-g-PI, Poly(ECH-co-EO)-g-PS and Poly(ECH-co-EO)-g-(PI-b-PS) were obtained using the coupling reaction between living PI⁻Li⁺, PS⁻Li⁺ or PS-b-PI⁻Li⁺ species capped with or without 1,1-diphenylethylene (DPE) agent and chloromethyl groups on poly(ECH-co-EO). By model experiment, the addition of DPE agent was confirmed to have an important effect on the grafting efficiency at room temperature. Finally, the target graft copolymers and intermediates were characterized by SEC, ¹H NMR, MALLS and FTIR in detail, and thermal behaviours of the graft copolymers were also investigated by DSC measurement.     

Side-chain crystallization and phase transition of poly[styrene-co-(maleic anhydride)]-g-alkylamine comb-like polymers

Taking ? NHCO? as the chemical spacer, poly[styrene-co-(maleic anhydride)]-g-alkylamine (SMACnN) comb-like polymers are prepared through grafting reaction between poly[styrene-co-(maleic anhydride)] and n-alkylamine with n changing from 12 to 18. SMACnN comb-like polymers present an obvious crystallization behavior owing to the introduced alkyl side chains, and the melting temperature and enthalpy of the side-chain crystallites increase from 9.2 to 41.6 °C and from 6.0 to 17.6 kJ mol^?1, respectively, showing a variation with the side-chain length. At least 12 side-chain carbon atoms are required to pack into a hexagonal crystal structure for SMACnN comb-like polymers. Fourier transform infrared and X-ray analysis results demonstrate the phase transition from hexagonal to an amorphous state, and the conformationally disordered alkyl chains affect the packing manner of side-chain crystallites. Including the side-chain length and polymer backbone, the chemical spacer between the side chain and the main chain influences the formation of side-chain crystallites and the phase transition process. So, the study of SMACnN further strengthens the understanding of the phase transition behavior and side-chain crystallization of comb-like polymers triggered by the side-chain length and the microstructure differences. © 2021 Society of Industrial Chemistry.     

A new approach to controlled/living radical polymerization by DPE method

Controlled free radical polymerizations of methyl methacrylate and styrene in bulk by 1,1-diphenylethene (DPE) were demonstrated in a two-step process, preheating treatment of initiators followed by a living polymerization of monomers. Over the course of polymerization, continuous growing of polymers with unimodal molecular weight distribution and a relatively small polydispersity index (around 1.5 even in the range of Mn ∼ 10⁵ g/mol) on GPC diagrams was observed. In our previous study, the DPE controlled radical polymerization with constant molecular weight throughout the polymerization was caused by the intrinsically low reactivation rate constant (k₂) of DPE capped dormant chains. To raise the reaction temperature in order to increase k₂, a continuous molecular weight growing but broader or bimodal molecular weight distribution was obtained if the living polymerization was conducted in a one-step process. In this work, a two-step polymerization process was proposed. In the first step, the initiator 2,2′-azobisisobutyronitrile (AIBN), control agent DPE, and small amount of monomer were mixed and heated for a specific time period. Then a living polymerization of monomers was conducted in the second step of polymerization. This two-step new approach had minimized the imperfections of the DPE system; thus the polymerization showed better living characters and revealed its enhanced control abilities.     

Synthesis of hyperbranched polymers via a facile self-condensing vinyl polymerization system - Glycidyl methacrylate/Cp2TiCl2/Zn

A facile self-condensing vinyl polymerization (SCVP) system, the combination of glycidyl methacrylate, Cp₂TiCl₂ and Zn, has been firstly used to prepare novel hyperbranched polymers, consisting of vinyl polymers as the backbone, and cyclic ester polymers (poly(?-caprolactone) or poly(l-lactide)) as the side chains. The polymerizations are initiated by the epoxide radical ring-opening catalyzed by Cp₂Ti(III)Cl which is generated in situ via the reaction of Cp₂TiCl₂ with Zn. The key to success is that the polymerizations can proceed concurrently via two dissimilar chemistries possessing the opposite active initiating species, including ring-opening polymerization (ROP) and controlled/living radical polymerization (CRP). We have demonstrated that this facile one-step polymerization technique can be applied successfully to prepare highly branched polymers with a multiplicity of end reactive functionalities including Ti alkoxide, hydroxyl and vinyl functional groups.     

Strain-promoted azide-alkyne cycloaddition “click” as a conjugation tool for building topological polymers

Strain-promoted azide-alkyne cycloaddition “click” reaction (SPAAC) was successfully used as a tool in synthesis of star polymers by grafting onto approach. The application of SPAAC method in star polymer synthesis was investigated for coupling reaction of the dibenzocyclooctyne (DIBO) end group of polystyrene (PS) and poly(ethylene glycol) (PEG) with coupling agents bearing 2, 3, or 4 azido groups. Firstly, well-defined linear DIBO-terminated PS was obtained by atom transfer radical polymerization (ATRP) of styrene using a DIBO containing ATRP initiator and linear DIBO-terminated PEG was obtained by terminal functionalization of PEG monomethyl ether (PEG-OH). Then a series of star PS and PEG bearing two, three and four arms were prepared respectively by subjecting SPAAC coupling reaction between the linear polymer-DIBO and the azido tethered core molecules at 30 °C without catalyst. The obtained star PS showed a well-defined structure after fractional precipitation to remove slightly excess linear polymers, and all the star polymers were characterized via Fourier transform infrared spectroscopy (FTIR), ¹H nuclear magnetic resonance spectroscopy (¹H NMR), size exclusion chromatography (SEC) and matrix-assisted laser desorption ionization-time-of-flight mass spectrometry (MALDI-TOF MS).     

Highly efficient synthesis of cylindrical polymer brushes with various side chains via click grafting-onto approach

Though much attention has been paid to synthesis of cylindrical polymer brushes, it is still not easy to prepare well-defined brushes by a general approach. Herein, well-defined cylindrical polymer brushes with various side chains were synthesized via grafting-onto approach by CuAAC click chemistry. Narrowly dispersed polymer backbones functionalized with azide groups were obtained by post-modification of poly(glycidyl methacrylate) (PGMA) which was prepared by reversible addition-fragmentation chain transfer (RAFT) mediated radical polymerization. The alkyne-terminated side chains, polystyrene, polyacrylates, polymethacrylates and poly(N-alkyl acrylamide)s, were synthesized by RAFT mediated radical polymerization with alkyne-containing chain transfer agents (CTAs). The CuAAC reactions between the backbone and side chain polymers were conducted with an equivalent feed of alkyne-terminated side chains and azide groups under mild conditions. Influences of reaction conditions and chemical composition of polymer side chains on grafting efficiency and molecular weight distribution of the polymer brushes were investigated. It is demonstrated that the side chains of polystyrene, polyacrylates and poly(N-alkyl acylamide)s were grafted at a density above 85% while that of polymethacrylates decreased to ca. 50%. The polymer brushes synthesized under the optimized reaction conditions had well-defined chemical composition and narrow distribution of molecular weight, and their wormlike morphology was visualized by atomic force microscopy (AFM).     

One-pot synthesis of comb-like copolymer bearing side chains composed of branched and linear polylactides

Ethyl cellulose (EC), lactide (LA) and branching comonomer 2,2-bis(hydroxymethyl) butyric acid (BHB) were copolymerized in xylene using Sn(Oct)₂ as catalyst. Catalyst amount and polymerization temperature were optimized for the effective introduction of certain amount of branched polylactide (PLA) into the side chains of comb-like copolymers having EC backbone. The characterization of the obtained copolymers by GPC and ¹H NMR demonstrated that the influence of polymerization temperature in the range of 110–150 °C on the efficient incorporation of branching units was not pronounced, whereas high amount of catalyst was the key point. The content of branching units in the copolymers could be enhanced by increasing the feed amount of BHB monomer. A plausible mechanism for the polymerization was proposed according to the model experiments. Compared with the comb-like copolymers with merely linear PLA side chains, the ones bearing both branched and linear PLA side chains had the following characters: (1) better solubility in the polar solvent of ethanol; (2) much faster hydrolysis degradation. These characters became more pronounced when the content of branching units in the side chains increased.     

Synthesis and characterization of hyperbranched polymers via Cp2TiCl-catalyzed self-condensing vinyl polymerization using glycidyl methacrylate as inimer

Hyperbranched polymers were produced using glycidyl methacrylate (GMA)/Cp₂TiCl₂/Zn as self-condensing vinyl polymerization (SCVP) system. The polymerization is firstly initiated by the epoxide radical ring opening catalyzed by Cp₂Ti(III)Cl generated in situ via the reaction of Cp₂TiCl₂ with Zn. By optimizing the molar ratio of the SCVP inimer (GMA) to the mediator (Cp₂Ti(III)Cl), the active propagation chains are reversibly transformed to the dormant species and the cross-linking does not occur until a higher level of monomer conversion (ca. 80%). We detail this facile one-step polymerization technique to prepare highly branched polymers with a multiplicity of particular end reactive functionalities including Ti alkoxide, hydroxyl and vinyl functional groups, which differs from most previously reported SCVP systems.     

Synthesis and properties of novel poly(aryl ether ketone)s containing imide linkages in the main chains

A new monomer containing imide linkages, bis[4-(p-phenoxybenzoyl)-1,2-benzenedioyl]-N,N,N′,N′-4,4′-diaminodiphenyl ether (BPBDADPE), was prepared by the Friedel–Crafts reaction of bis(4-chloroformyl-1,2-benzenedioyl)-N,N,N′,N′-4,4′-diaminodiphenyl ether (BCBDADPE) with diphenyl ether (DPE). Novel poly(aryl ether ketone)s containing imide linkages in the main chains (PEK-I) were synthesized by electrophilic Friedel–Crafts solution copolycondensation of terephthaloyl chloride (TPC) with a mixture of DPE and BPBDADPE. The polymers were characterized by different physico-chemical techniques. The polymers with 10–40 mol% BPBDADPE are semicrystalline and had increased T _gs over commercially available poly(ether ether ketone) (PEEK) and poly(ether ketone ketone) (PEKK) (70/30) due to the incorporation of imide linkages in the main chains. The polymers IV and V with 30–40 mol% BPBDADPE had not only high T _gs of 182–183 °C, but also moderate T _ms of 341–343 °C, having good potential for melt processing and exhibited high thermal stability and good resistance to common organic solvents.     

Synthesis of fluorinated polymer electrolyte membranes by radiation grafting and atom transfer radical polymerization techniques

A novel polymer electrolyte membrane was synthesized by radiation-induced grafting and consequent atom transfer radical polymerization (ATRP). First, bromine-containing perfluorinated grafts were prepared by radiation grafting of 2-bromotetrafluoroethyl trifluorovinyl ether (BrTFF) into a poly(ethylene-co-tetrafluoroethylene) (ETFE) film. Then, the bromine atoms in the ETFE-g-PBrTFF grafted films were acted as initiators, and the films were treated with Cu(I)-based catalytic system of a CuBr and 2,2′-bipyridyl (bpy) for the ATRP. By adjusting the molar ratio of initiator/CuBr/bpy and the reaction temperature, branched poly(styrene) with a grafting yield of above 100% on the poly(BrTFF) main chains was constructed in ETFE-g-PBrTFF films. Thermal analysis revealed that the perfluorinated poly(BrTFF) main chains were miscible to ETFE, whereas the hydrocarbon poly(styrene) branches were phase-separated from the ETFE-g-PBrTFF film. Sulfonic groups could be further introduced into the poly(styrene) grafts of ETFE-g-PBrTFF-g-PS films with homogeneous distribution in a perpendicular direction to the membrane surface. The resulting membrane with a styrene grafting yield of 15% exhibited higher proton conductivity than commercial Nafion 117 membrane. Likewise, it had better chemical stability than ETFE-g-PSSA membrane prepared by conventional radiation-induced grafting.     

Precise synthesis of polymers containing functional end groups by living ring-opening metathesis polymerization (ROMP): Efficient tools for synthesis of block/graft copolymers

This article summarizes recent examples for precise synthesis of (co)polymers containing functional end groups prepared by living ring-opening metathesis polymerization (ROMP) using molybdenum, ruthenium complex catalysts. In particular, this article reviews recent examples for synthesis of amphiphilic block/graft copolymers by adopting transition metal-catalyzed living ROMP technique. Unique characteristics of the living ROMP initiated by the molybdenum alkylidene complexes (so-called Schrock type catalyst), which accomplish precise control of the block segment (hydrophilic and hydrophobic) as well as exclusive introduction of functionalities at the polymer chain end, enable us to provide the synthesis of block copolymers varying different backbones by adopting the “grafting to” or the “grafting from” approach as well as “soluble” star shape polymers with controlled manner. The “grafting through” approach (polymerization of macromonomers) by the repetitive ROMP technique offers precise control of the amphiphilic block segments.     

Continuous Flow Fabrication of Block Copolymer–Grafted Silica Micro‐Particles in Environmentally Friendly Water/Ethanol Media

Polymer‐grafted inorganic particles (PGIPs) are attractive building blocks for numerous chemical and material applications. Surface‐initiated controlled radical polymerization (SI‐CRP) is the most feasible method to fabricate PGIPs. However, a conventional in‐batch reaction still suffers from several disadvantages, including time‐consuming purification processes, low grafting efficiency, and possible gelation problems. Herein, a facile method is demonstrated to synthesize block copolymer–grafted inorganic particles, that is, poly(poly(ethylene glycol) methyl ether methacrylate) (PPEGMEMA)‐b‐poly(N‐isopropylacrylamide) (PNIPAM)–grafted silica micro‐particles using continuous flow chemistry in an environmentally friendly aqueous media. Immobilizing the chain transfer agent and subsequent SI‐CRP can be accomplished sequentially in a continuous flow system, avoiding multi‐step purification processes in between. The chain length (MW) of the grafted polymers is tunable by adjusting the flow time or monomer concentration, and the narrower molar mass dispersity (Р< 1.4) of the grafted polymers reveals the uniform polymer chains on the particles. Moreover, compared with the in‐batch reaction at the same condition, the continuous system also suppresses possible gelation problems.     

Ring opening reactions of glycidyl methacrylate copolymers to introduce bulky organosilicon side chain substituents

Summary Glycidyl methacrylate (GMA) random copolymers with methyl acrylate (MA), ethyl acrylate (EA), n-butyl acrylate (BA), methyl methacrylate (MMA), ethyl methacrylate (EMA) and n-butyl methacrylate (BMA) were synthesized by solution free radical polymerizations, at 70±1 °C using α,α’-azobis(isobutyronitrile) as an initiator to give the copolymers I – VI in good yields. The copolymer compositions were obtained using related ¹H NMR spectra and the polydispersity indices of the copolymers determined using gel permeation chromatography (GPC). Tris(trimethylsilyl)methyl (Tsi=trisyl) groups were then covalently attached to the obtained copolymers as side chains by ring opening reaction between excess of TsiLi and expoxide groups of GMA units to give the copolymers I_Tsi – VI_Tsi in good yields. In the coupling reaction, the TsiLi reacted selectively with the epoxy groups of the backbone polymer rather than with the carbonyl groups of the backbone. This method of preparing functionalized silanes is limited by the readiness with which TsiLi abstracts a proton, if one is available, rather than attacks at carbon. In addition in the reaction with epoxides, the product alkoxide can transfer a silyl group from carbon to oxygen or ring opening polymerization. However these were shown not to occur at the conditions of interest here. The epoxy group possesses a higher reactivity for the TsiLi than the ester and chloromethyl groups. The ring opening reaction between the epoxy group and the TsiLi is simple and fast. All the resulted polymers were characterized by FT-IR and ¹H NMR spectroscopic techniques. The glass transition temperature (Tg) of all copolymers was determined by differential scanning calorimetry (DSC) apparatus. All the polymers containing trisyl groups showed a high glass transition temperature in comparison with unmodified copolymers (I – VI). Attaching the tris(trimethylsilyl)methyl group to macromolecular chain should lead to important modifications of polymer properties such as gas permeability and perm selectivity parameters.     

Anionic Graft Polymerisation of Styrene from Polyisoprene. Dependence of Grafting Efficiency on Polyisoprene Structure

Styrene was grafted from polyisoprene forming poly(isoprene-g-styrene) graft polymers. N,N,N',N'-Tetramethylene diamine and s-butyllithium were used to create active centres on the polyisoprene backbone in cyclohexane under anionic living polymerisation conditions. The stereoregularity of polyisoprene is shown to influence the grafting efficiency. Addition of small amounts of tetrahydrofuran increased grafting efficiency and decreased polydispersity of the graft polymers obtained. Polystyrene branches are shown to be syndiotactic rich.     

Preparation and characterization of grafting styrene onto LLDPE by cyclohexane compatibilization in suspension polymerization

A novel method of grafting styrene onto linear low‐density polyethylene (LLDPE) by suspension polymerization was systematically evaluated. Cyclohexane as a compatibilizer was introduced to swell and activate the surface of LLDPE molecular chain for amplifying the contact point of styrene monomer with LLDPE. A series of copolymer of grafting polystyrene (PS) onto LLDPE, known as LLDPE‐g‐PS, were prepared with different ratios of cyclohexane/styrene monomer and various LLDPE dosages. FTIR and ¹H NMR techniques both confirmed successful PS grafting onto the LLDPE chains. In addition, SEM images of LLDPE‐g‐PS particles showed that the cross‐section morphology becomes smooth and dense with suitable cyclohexane dosages, indicating a better compatibility between LLDPE and PS. The highest grafting efficiency was 28.4% at 10 mL/g cyclohexane and styrene monomer when 8% LLDPE was added. In these conditions, the LLDPE‐g‐PS elongation at break increased by about 30 times compared with PS. Moreover, thermal gravimetric analysis (TGA) demonstrated that LLDPE‐g‐PS possesses much higher thermal stability than pure PS. Therefore, the optimal amount of cyclohexane as compatibilizer could increase the grafting efficiency and improve the toughness of PS. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 41671.     

High molecular arborescent polyoxyethylene with hydroxyl containing shell

Arborescent polyoxyethylene of high molar mass (2×10⁵ g/mol) and narrow molar mass distribution was synthesized in a three-stage process. In the first stage a triblock copolymer of ethylene oxide (central block, DP ca. 90) and 2,3-epoxypropanol-1 (short flanking blocks, DP ca. 5) was synthesized. The potassium alcoholate derived from this copolymer was used to initiate the polymerization of ethylene oxide and the subsequent addition of protected glycidol (1-etoxyethyl glycidyl ether). After deprotection the short polyglycidol blocks were used as branching units for the next generation. Repeated step by step process leads to the ‘pom-pom like’ branched polyoxyethylene macromolecules enriched with the reactive hydroxyl groups in the outer shell. The branched structure of the obtained polymers was evidenced by the size exclusion chromatography and NMR spectroscopy.     

Controlled radical polymerization of p-(iodomethyl)styrene—a route to branched and star-like structures

p-(Iodomethyl)styrene was polymerized under the action of a radical initiator (AIBN). The polymerization proceeds with degenerative chain transfer and leads to well defined branched polymers with functional primary and secondary iodomethyl groups as revealed by NMR studies. The obtained polymer can be further used as macroinitiator for radical polymerization of styrene. This polymerization proceeds in controlled way to polystyrene star polymers with reactive groups at the end of their arms. The characterization of branched and star structures was performed by NMR and GPC with absolute molar mass detection (MALLS).     

Photo-induced living/controlled surface radical grafting polymerization and its application in fabricating 3-D micro-architectures on the surface of flat/particulate organic substrates

This feature article covers the fundamental chemistry and applications of photo-induced living surface grafting polymerization. The mechanism of activation of inert alkyl C-H bonds of polymer substrates, the structures of the active free radical and reversible dormant species, the modes of the grafting chain growth (including linear, branched and cross-linked variants), and the role of spatial effect are discussed. Two technologies, i.e., 1-step and 2-step processes, their features and applications in fabricating polymer brushes with precisely controlled patterns, desired functions, branched and block grafting chains on planar substrates, and polymer lamination are presented. The fabrication of 3-dimensional covalently bonded polymer particles, such as nano-sized polymer particle monolayers (with uniform and bimodal distributions), discrete solid and hollow polymer particles of micrometer size, and multilayer polymer particles on polymeric substrates are also introduced. In the last part, the application of photo-induced living surface grafting polymerization in non-planar surface modifications, such as the preparation of core-shell polymer particles, Janus particles and cross-linked hydrogels with hairy polymer chains is summarized.