Metallopolymers with transition metals in the side-chain by living and controlled polymerization techniques

Work on side-chain transition metal-containing polymers prepared by controlled and living polymerizations is summarized, including living anionic polymerization (LAP), ring-opening metathesis polymerization (ROMP) and controlled radical polymerization (CRP) such as atom transfer radical polymerization (ATRP), reversible addition-fragmentation chain transfer polymerization (RAFT), and nitroxide-mediated polymerization (NMP). These polymers include metallocene-containing polymers, ferrocenylsilane polymers with additional metal at the side chain, metal carbonyl complex polymers, and ligated metal complex polymers.     

Side-Chain Metallocene-Containing Polymers by Living and Controlled Polymerizations

This review summarizes recent work on side-chain metallocene-containing polymers prepared by controlled and living polymerizations, which include living anionic polymerization (LAP), ring-opening metathesis polymerization (ROMP) and controlled radical polymerization (CRP) such as atom transfer radical polymerization (ATRP), reversible addition fragmentation chain transfer polymerization (RAFT), and nitroxide-mediated polymerization (NMP). The majority of efforts in the field are focused on side-chain ferrocene-containing polymers, while cobaltocenium-containing polymers have recently started to draw attention. Future direction on the development of other metallocene-containing polymers is discussed.     

Block copolymerization of vinyl ester monomers via RAFT/MADIX under microwave irradiation

Well-defined homopolymers and block copolymers of vinyl esters were synthesized under microwave irradiation via reversible addition-fragmentation chain transfer (RAFT)/macromolecular design via interchange of xanthates (MADIX) polymerization without the significant inhibition or retardation often observed under conventional heating conditions. Poly(vinyl acetate) (PVAc) with molecular weights of 1000−10 000 g/mol was prepared in less than 15 min under microwave irradiation at an apparent temperature of 70 °C in the presence of the commercially available chain transfer agent ethylxanthogenacetic acid. The polymerizations were well-controlled, leading to polymers with narrow molecular weight distributions and excellent agreement between theoretical and experimental number average molecular weights. Despite the high rates of polymerization, the resulting PVAc homopolymers retained a high degree of thiocarbonylthio end group functionality that allowed the synthesis of block copolymers by chain extension with vinyl benzoate and vinyl pivalate. The rates of polymerization during the addition of the second blocks were also high, and the resulting block copolymers were obtained in good yield with excellent blocking efficiencies.     

Preparation of star polymers based on polystyrene or poly(styrene-b-N-isopropyl acrylamide) and divinylbenzene via reversible addition-fragmentation chain transfer polymerization

Star polymers based on styrene/divinyl benzene (St/DVB) and PSt-b-poly(N-isopropyl acrylamide) (NIPAAM)/DVB have been successively prepared by ‘arm-first’ method via reversible addition-fragmentation chain transfer (RAFT) polymerization. The linear macro RAFT agent PSt-SC(S)Ph was prepared by RAFT polymerization of St using benzyl dithiobenzoate and AIBN as RAFT agent and initiator. Successive RAFT polymerization of NIPAAM with PSt-SC(S)Ph as macro RAFT agent to afford diblock copolymer, PSt-b-PNIPAAM-SC(S)Ph. The coupling reactions of PSt-SC(S)Ph or PSt-b-PNIPAAM-SC(S)Ph in the presence of DVB produced the star copolymers, C(PSt)_n or C(PSt-b-PNIPAAM)_n. The molar ratio of DVB/PSt-SC(S)Ph and polymerization time influenced the yields, molecular weight and distribution of the star-shaped polymers, which was characterized by ¹H NMR and IR spectra, GPC measurements as well as DLS.     

Microwave assisted synthesis of high molecular weight polyvinylsilazane via RAFT process

The microwave-assisted synthesis of high molecular weight polyvinylsilazane (H-PVSZ) leads to a narrow molecular weight distribution at accelerated polymerization rates under controlled/living polymerization, while retaining the excellent controllability offered by reversible addition fragmentation chain transfer (RAFT) polymerization in a thermal reaction process. Under microwave irradiation, higher molecular weight H-PVSZ (6250 vs. 4370) was obtained with higher conversion (86.7 vs. 30.1%) than under conventional heating through a simple 3 h reaction of vinylcyclicsilazane with a molecular weight of 314 in the presence of dithiocarbamate derivatives (RAFT agent) and 2,2′-azobis-isobutyronitrile (AIBN, thermal initiator). H-PVSZ with a well-controlled high molecular weight is useful for synthesizing inorganic-organic diblock copolymer polyvinylsilazane-block-polystyrene (PVSZ-b-PS) with a higher molecular weight and lower polydispersity than by conventional heating of the PVSZ-derived product.     

Photomediated controlled radical polymerization

Photomediated controlled radical polymerization is a versatile method to prepare, under mild conditions, various well-defined polymers with complex architecture, such as block and graft copolymers, sequence-controlled polymers, or hybrid materials via surface-initiated polymerization. It also provides opportunity to manipulate the reaction through spatiotemporal control. This review presents a comprehensive account of the fundamentals and applications of various photomediated CRP techniques, including atom transfer radical polymerization (ATRP), reversible addition-fragmentation chain transfer (RAFT), nitroxide mediated polymerization (NMP) and other procedures. In addition, mechanistic aspects of other photomediated methods are discussed.     

Kinetics of UV‐initiated RAFT crosslinking polymerization of dimethacrylates

The UV‐initiated RAFT polymerizations of a series of poly(ethylene glycol) dimethacrylates (PEGDMA) were investigated using differential scanning photocalorimetry (DPC) at room temperature. The rate of the RAFT system was much lower than that of a conventional free radical polymerization. A mild autoacceleration occurred as the addition reaction became diffusion controlled. The influence of the spacer length (CH₂CH₂O)_x between the vinyl moieties of the dimethacrylates on the polymerization kinetics was examined. The polymerization rate of PEGDMA decreased with an increased x value from 4 to 9, but it increased with a further increased x value from 9 to 14. Mechanical properties of the resulting polymers were also examined by dynamic mechanical analysis (DMA). It was concluded that the presence of the RAFT agent during polymerization of multifunctional monomers did not have an effect on the heterogeneity of the polymer network. In comparison with three different PEGDMAs, the PEGDMA with the longest spacer formed the most homogeneous networks with a lower crosslinking density. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

ATRP with Alkyl Pseudohalides Acting as Initiators and Chain Transfer Agents: When ATRP and RAFT Polymerization Become One

All controlled radical polymerization (CRP) procedures rely on a dynamic and rapid equilibrium between dormant and active species. This equilibrium can be established through different mechanisms and all the resulting CRP processes have their own advantages and limitations. Therefore, it becomes interesting to investigate the possibility of combining CRP techniques to eliminate specific drawbacks of each individual procedure. Atom transfer radical polymerization (ATRP) with alkyl pseudohalides acting as initiators and chain transfer agents was developed for that purpose. The process relies on a dual mechanism involving both activation deactivation and reversible addition fragmentation chain transfer (RAFT). This peculiar characteristic of ATRP with alkyl pseudohalides acting as chain transfer agents made it possible to overcome some of the limitations typically associated with conventional ATRP and RAFT polymerization as well as to prepare new responsive materials.     

Alternate and random (co)polymers composed of anthracene and chloromethylstyrene units through controlled radical ring-opening polymerization: Synthesis,post-functionalization,and optical properties

A cyclic monomer containing the chloromethyl unit 10-methylene-9,10-dihydroanthryl-9-spiro(4′-chloromethylphenyl)cyclopropane (MDCMS) was polymerized using a controlled radical ring-opening polymerization via a reversible addition-fragmentation chain transfer (RAFT) process to afford a nonconjugated alternate polymer composed of anthracene and chloromethylstyrene (CMS) units. Well-defined random copolymers were obtained through the ring-opening RAFT copolymerization. Various functional groups were incorporated into the alternate polymer. The alternate polymer containing imidazole rings exhibited fluorescence quenching as a result of charge transfer. Fluorescence resonance energy transfer (FRET) was observed in the alternate polymers containing naphthalene and thiophene rings. The random copolymers obtained by copolymerization followed by post-functionalizations exhibited characteristic optoelectronic properties that differed from those of the alternate polymers.     

The direct polymerization of 2-methacryloxyethyl glucoside via aqueous reversible addition-fragmentation chain transfer (RAFT) polymerization

A preliminary study on the direct controlled radical polymerization of a glycomonomer, namely 2-methacryloxyethyl glucoside (MAGlu), under reversible addition-fragmentation chain transfer (RAFT) polymerization conditions in aqueous media has been conducted. This represents the first example detailing the direct polymerization of a sugar monomer via RAFT and, significantly, has been conducted without protecting group chemistry. 4-Cyano-4-methyl-4-thiobenzoylsulfanyl butyric acid (CTP) was employed as the RAFT chain transfer agent (CTA) due to its inherent water-solubility and its applicability for methacrylic monomers. The homopolymerization displays all the characteristics of a controlled/‘living’ polymerization—linear increase in M_n with conversion, pseudo-first order kinetics, the final polymers have narrow molecular distributions and novel block copolymers can be prepared.     

黄原酸酯调控的氯乙烯的可逆-加成断裂链转移聚合

以三种不同结构的黄原酸酯为调控剂,进行氯乙烯（VC）溶液和细乳液可逆加成-断裂链转移（RAFT）聚合,发现O-乙基黄原酸丙酸乙酯对VC聚合的调控效果良好,氯乙烯RAFT细乳液聚合速率明显大于溶液聚合,VC聚合12 h转化率大于90%,但聚氯乙烯（PVC）的分子量分布宽于溶液聚合产物。核磁共振和紫外可见吸收光谱分析证明合成的PVC具有黄原酸酯基端基结构,结构缺陷少。含黄原酸酯基PVC可进一步调控VC及醋酸乙烯酯聚合,进行扩链或得到嵌段共聚物。结合聚合动力学,说明黄原酸酯调控的氯乙烯聚合具有活性特征。     

Molecular composite materials formed from block copolymers containing a side-chain liquid crystalline segment and an amorphous styrene/maleic anhydride segment

Side-chain liquid crystalline block polymers containing a poly[6-[4-(4′-methoxyphenyl)phenoxy]hexyl methacrylate] (PMMA-LC) segment and a styrene-co-maleic anhydride segment (alternating structure) were prepared via reversible addition fragmentation chain transfer (RAFT) polymerization. PMMA-LC was initially prepared via RAFT polymerization mediated by 2-(2-cyanopropyl)dithiobenzoate (CPDB). The resulting polymer was subsequently isolated and used to re-initiate styrene/maleic anhydride alternating copolymerization. The block copolymerization proceeded to intermediate conversions with narrow polydispersities, however at higher conversions some molecular weight broadening was observed and this was attributed to radical-radical termination reactions. The resulting polymers were analyzed via size exclusion chromatography (SEC), differential scanning calorimetry (DSC) and polarizing optical microscopy (POM). Microporous honeycomb structured films were cast from solutions of the block copolymers to form porous molecular composites.     

RAFT-mediated synthesis of poly(N-(2-hydroxypropyl)methacrylamide-b-4-vinylpyridine) by conventional and microwave heating

We report the synthesis of N-(2-hydroxypropyl)methacrylamide (HPMA) macroCTA and HPMA-b-4-Vinylpyridine block copolymers via reversible addition-fragmentation chain transfer (RAFT) reaction. Polymerization was carried out in dimethylformamide (DMF) at 70 °C using 4-Cyano-4(thiobenzoylthio) pentanoic acid as chain transfer agent and AIBN as an initiator. Control over molecular weight and composition was achieved by altering the CTA, monomer and initiator feed ratio. The controlled living character of the polymerization was verified with pseudo-first-order kinetic plots, a linear increase of the molecular weight with conversion, and low polydispersities (PDIs ≤ 1.2). Effect of microwave heating on the homo- and copolymer formation was investigated and the rates were significantly higher than those observed under conventional heating conditions. These polymerization reactions were in controlled fashion resulting in polymers with low PDIs, too. These polymers have a great potential to be used in developing delivery vehicles and conjugates for further drug or gene delivery applications.     

The application of ionizing radiation in reversible addition-fragmentation chain transfer (RAFT) polymerization: Renaissance of a key synthetic and kinetic tool

Ionizing radiation, such as γ, ultraviolet, microwave and X-ray radiation, has long been used in polymer chemistry as a means of initiating polymerization, crosslinking gels and decomposing particular polymer components. More recently, ionizing radiation has found application in tandem with living radical polymerization to form novel polymeric materials with defined molecular weight and narrow molecular weight distribution. In particular, γ-rays and ultraviolet light both have shown promise as sources of initiation in reversible addition-fragmentation chain transfer (RAFT) polymerization. The ability to apply these sources of initiation at low temperatures is useful in applications where elevated temperature is likely to be detrimental to the system, for instance, in preparing protein-polymer conjugates. Similarly, the use of these initiating sources at low temperature is particularly suitable for some monomers, such as allyl compounds, which have not been synthesized using any other living radical approach. The current review examines the development of ionizing radiation as a tool in RAFT polymerization, with particular reference to the elucidation of the polymerization mechanism, the synthesis of high functionality polymers and probing the kinetic parameters of the RAFT process.     

New developments in polymer science and technology using combination of ionic liquids and microwave irradiation

The purpose of this review is to provide appropriate details concerning the application of ionic liquids (IL)s associated with microwave-assisted polymer chemistry. From the viewpoint of microwave chemistry, one of the key significant advantages of ILs is their high polarity, which is variable, depending on the cation and anion and therefore can effectively be tuned to a particular application. Hence, these liquids offer a great potential for the innovative application of microwaves for organic synthesis as well as for polymer science. ILs efficiently absorb microwave energy through an ionic conduction mechanism, and thus are employed as solvents and co-solvents, leading to a very high heating rate and a significantly shortened reaction time. Since an IL-based and microwave-accelerated procedure is efficient and environmentally benign, we believe that this method may have some potential applications in the synthesis of a wide variety of vinyl and non-vinyl polymers. This review describes application of combination of ILs with microwave irradiation as a modern tool for the addition and step-growth polymerization as well as modification of polymers and it was compared with ILs alone and conventional polymerization method.     

Using dilatometry in the reversible addition fragmentation transfer polymerization of N-(S)-(−)-α-methylbenzyl methacryloylamine

Optically active polymers were prepared using reversible addition-fragmentation chain transfer polymerization (RAFT) of N-(S)-α-methylbenzylmethacryloylamine (N-(S)-α-MBMA), a functional optically active monomer. RAFT polymerizations were carried out at 70 °C in ethanol using AIBN as a thermal initiator and benzyl or (1-phenyl)ethyl dithiobenzoate (BDB and PEDB, respectively) as the RAFT agents. The kinetic study was performed by dilatometry. Plots of conversion vs time indicated that the polymerizations followed first order kinetics. ¹H NMR, IR, and UV–vis spectrophotometric studies confirmed the presence of thiocarbonylthio moieties (−SCS-) in the polymer chains. The molecular weight distributions (MWDs) were moderately narrow with polydispersity indices between 1.3 and 1.6, which indicated that the control of the reaction was not completely achievement using BDB or PEDB as RAFT agents. The optical activity [α]_D²⁵ measurements of synthesized polymers by RAFT did not show a noticeably linear increase dependence with respect to molecular weight, as was previously reported for another controlled free radical polymerization (CRP) system.     

Controlled synthesis of vinyl-functionalized homopolymers and block copolymers by RAFT polymerization of vinyl methacrylate

Reversible addition-fragmentation chain transfer (RAFT) polymerization of an asymmetrical divinyl monomer, vinyl methacrylate (VMA), was investigated under various conditions. RAFT polymerization of VMA using a dithioester-type chain transfer agent (CTA) under suitable conditions afforded soluble polymers with a high content of pendant vinyl ester side chains in sufficient yields (>70%). The monomer concentration, the nature of the CTA, and the CTA/initiator ratio were found to affect the polymerization reaction and the structure of the resulting polymers; this behavior is attributed to the relative propensities for intermolecular propagating/cross-linking reactions and intramolecular cyclization. A kinetic study of the RAFT polymerization of VMA with the dithioester-type CTA 1 suggested that the propagation reaction of the methacryloyl group proceeded predominantly with a certain level of intramolecular cyclization during the early stage of the polymerization and intermolecular cross-linking during the final stage of the polymerization. Block copolymers with one segment featuring pendant vinyl functionality were synthesized by RAFT polymerization of VMA using poly(methyl methacrylate) as a macro-chain transfer agent (macro-CTA).     

Molecularly imprinted polymers via living radical polymerization: Relating increased structural homogeneity to improved template binding parameters

This work examined imprinted polymer networks prepared via controlled/living radical polymerization (LRP) and conventional radical polymerization (CRP) on chain growth, network formation, and efficiency of producing molecularly imprinted, macromolecular memory sites for the template molecule, diclofenac sodium. LRP extended the reaction-controlled regime of the polymerization reaction and formed more homogeneous polymer chains and networks with smaller mesh sizes. In addition, LRP negated the effect of the template on polymer chain growth resulting in polymers with a more consistent PDI independent of template concentration in the pre-polymerization solution. Improved network homogeneity within imprinted poly(HEMA-co-DEAEM-co-PEG200DMA) networks prepared via LRP resulted in a 38% increase in template binding affinity and 43% increase in the template binding over imprinted networks prepared via CRP and a 97% increase in affinity and 130% increase in capacity over non-imprinted networks prepared by LRP. By varying certain parameters, it was possible to create imprinted networks with even higher template binding affinities (155% over non-imprinted) and capacities (261% over non-imprinted). This work is the first to conclusively demonstrate that the observed improvement in binding parameters in weakly crosslinked, imprinted polymer networks could be explained by the more uniform molecular weight evolution associated with the LRP mechanism and the longer lifetime of an active polymer chain relative to the total polymerization time, which allowed for the formation of a more homogenous imprinted polymer network.     

RAFT‐mediated emulsion polymerization of chloroprene and impact of chain‐end structure on chloroprene rubbers

The reversible addition‐fragmentation chain transfer (RAFT) polymerization of chloroprene (CP) in an emulsion system using a dithiocarbamate‐type RAFT agent was studied. The controlled RAFT‐mediated emulsion polymerization was achieved by the appropriate combination of a RAFT agent and nonionic surfactant (polyoxyethylene phenyl ether) using a water‐soluble initiator (VA‐044) at 35 °C. An almost linear first‐order kinetic plot was observed until relatively high conversion (>80%) with molecular weights between 22,300 and 33,100 and relatively narrow molecular weight distributions (M_w/M_n ≦ 1.5) were achieved. The amount of the emulsifier used and the pH of the system were found to affect the controlled character, polymerization rate, and induction period, which are related to the size of the emulsion particles. Large‐scale RAFT‐mediated emulsion polymerization was also employed to afford industrially applicable poly(CP) (M_w > 25 × 10⁴, resulting product > 2300 g). The vulcanized CP rubber obtained from the RAFT‐synthesized poly(CP) exhibited better physical properties, particularly tensile modulus and compression set, which may be due to the presence of the reactive end groups and the absence of low‐molecular‐weight products. We also evaluated the impact of the chain‐end structure on the mechanical and physical properties of these industrially important CP rubbers with carbon black. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 46008.     

Synthesis of phenylphosphinic acid-containing amphiphilic homopolymers by reversible addition-fragmentation transfer (RAFT) polymerization and its aggregation in water

A novel amphiphilic phosphorus-containing polymer was prepared by RAFT polymerization of 3-[2-(acryloyloxy)ethoxy]-3-oxopropyl(phenyl) phosphinic acid (AOPA). The monomer was first synthesized by esterification of 3-[hydroxy(phenyl)phosphoryl]propanoic acid and 2-hydroxyethyl acrylate, and then the polymerizations were performed at 60 °C. The polymerization was well controlled, and the polymers with “well-defined” structures were successfully synthesized. The polymers can self-assemble to form the micelles in distilled water due to the special amphiphilic structure, and the shell of the micelles could be cross-linked by the coordination of phosphinic acid with cations. The property may promote the polymers to be used in the ionic exchange for the environment protection.