Reactions of bifunctional addition-fragmentation chain transfer agents for synthesis of polymer bearing unsaturated moieties at both ends

The benzene solution polymerizations of ethyl methacrylate (EMA), cyclohexyl acrylate (CHA), and styrene (St) in the presence of ω-unsaturated methyl methacrylate oligomers (MMA-n; n = 2−4) and cyclohexyl methacrylate oligomers (CHMA-n; n = 2 and 3) as addition−fragmentation chain transfer (AFCT) agents have been studied to synthesize macromonomers efficiently by radical polymerization. The number of unsaturated end groups per chain (f) was used as an index to evaluate the effectiveness of the AFCT agents. In the EMA and St polymerizations, greater steric hindrance of the α-substituent of the AFCT agent and higher temperatures yielded polymers with higher f, and the level of retardation decreased with increasing f. CHA polymerization in the presence of MMA-n at 60 °C resulted in high-f polymer, although accompanied by marked retardation. Polymer structure analysis revealed that backward β-fragmentation of the EMA adduct radical, resulting in regeneration of the propagating radical and the AFCT agent, is a...

Synthesis of allyl sulfones bearing urethane groups as efficient addition-fragmentation chain transfer agents for the development of low-shrinkage composites

In radical polymerization of monofunctional monomers, addition fragmentation chain transfer (AFCT) agents are well known to regulate polymerization and yield polymers with lower molecular weights and narrower molecular weight distributions. Papers concerning bulk photopolymerization of monomer mixtures with AFCT agents are rarely found in literature. In this article, AFCT reagents based on β-allyl sulfones with different vinyl activating groups were synthesized and compared. The compounds were tested in mono- and difunctional monomer systems providing information about the influence on photoreactivity, molecular weight, as well as thermal and mechanical properties of the resultant polymers. Where more potent activating groups (-Ph, -CN) markedly influenced polymerization at lower concentrations, the AFCT reagent with an ester activating group reacted at a similar rate to the methacrylate monomer (CT ≈ 1) and provided the best overall performance. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016, 54, 1417-1427

Pyrazole‐based dithiocarbamates are versatile reversible addition fragmentation chain transfer (RAFT) agents that provide molar mass and dispersity ( Đ ) control over the radical polymerization of both more and less activated monomers (MAMs and LAMs). In this paper we report on theoretical and experimental findings demonstrating that their activity as RAFT agents can be significantly enhanced by introducing electron‐withdrawing substituents to the pyrazole ring. This enhancement is most noticeable in methyl methacrylate polymerization where product molar masses are more accurately predicted by the RAFT agent concentration, and significantly lower Đ values, with respect to those seen with the parent RAFT agent under similar conditions, are observed. Thus, use of 4‐chloro‐3,5‐dimethyl‐1 H ‐pyrazole‐1‐carbodithioate provides a poly(methyl methacrylate) with the anticipated molar mass and Đ as low as 1.3 at high monomer conversion. Good control is retained for monosubstituted MAMs, styrene, methyl acrylate and N , N ‐dimethylacrylamide. Low dispersities and less molar mass control are also achieved for homo‐ and copolymerizations with the LAM vinyl acetate, albeit with some retardation. © 2017 The Authors. Polymer International published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

ABSTRACTThe rapid and uncontrolled nature of network formation from di(meth)acrylate monomers produces high shrinkage stress and results in polymers with oftentimes brittle mechanical properties. Methods for regulating polymerization and network formation are sought. One option is the use of addition–fragmentation chain transfer (AFCT) agents, which are well known to control molecular weight and molecular weight distribution of monofunctional (meth)acrylates. A series of novel and previously described AFCT reagents were synthesized and screened with laser flash photolysis to determine reactivity. Well‐performing AFCT reagents were then tested in polymerizations with monofunctional and difunctional methacrylates. With monofunctional monomers, the molecular weight and polydispersity of the resultant linear polymers tend to decrease with the addition of AFCT agent. In copolymerization with dimethacrylate monomers, the AFCT agents were found to substantially lower and sharpen the glass transition. Sharpness of the glass transition is here indicative of a more regular and homogenous network. After coupling of the instruments, photorheology was performed simultaneously with real‐time IR to show an increase in monomer conversion at the time of gelation, which appears to have a positive effect on reducing shrinkage stress. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016, 54, 394–406

Abstractα‐(2‐Methyl‐2‐phenylpropyl)acrylate (RS‐2) was examined as a CC bond‐cleavage type addition–fragmentation chain transfer (AFCT) agent in the benzene solution polymerizations of styrene (St), ethyl methacrylate (EMA), and cyclohexyl acrylate (CHA) with the objective of achieving efficient macromonomer synthesis by radical polymerization. The AFCT efficiency was evaluated in terms of the decrease in the number‐average molecular weight (Mn) upon the addition of the AFCT agent and the number of unsaturated end groups introduced per chain (f). The AFCT efficiency was rationalized by the consideration of the relative importance of AFCT as an end‐forming event and the competition between ‐fragmentation and crosspropagation as adduct radical reaction pathways. In St and EMA polymerizations at 60 °C, RS‐2 resulted in higher f values and lower Mn values than methyl α‐(2‐methyl‐2‐carbomethoxypropyl)acrylate (MMA‐2), and this suggested the facilitation of ‐fragmentation due to the expulsion of the more stable cumyl radical from the RS‐2 adduct radical. Higher f values were observed for MMA‐2 than for RS‐2 in CHA polymerization because of unsaturated end group formation by ‐fragmentation of midchain radicals. However, RS‐2 resulted in lower Mn values for poly(CHA) than MMA‐2 because of a smaller contribution of crosspropagation. Retardation in the presence of the AFCT agents was affected by the balance between b‐fragmentation and crosspropagation and by the addition rate of the propagating radical to the AFCT agent. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 6021–6030, 2004

The design of a novel difunctional chain transfer agent (CTA) based on the propagating core (R group) approach is detailed. Successful synthesis of ABA block copolymers of poly(butyl methacrylate)-b-poly(methyl methacrylate)-b-poly(butyl methacrylate) and poly(butyl acrylate)-b-poly(methyl methacrylate)-b-poly(butyl acrylate) with targeted molecular weight and polydisperity indices in the range 1.2−1.4 is reported. The effect of free radical initiator concentration with respect to CTA concentration on block efficiency is investigated and lower concentrations were found to increase reaction time, but significantly reduce termination reactions. The consequence of difunctionality on homo and block extension polymerizations is discussed, with the polymerization of methyl and butyl methacrylate being more controlled than butyl acrylate.

Polymerization shrinkage and shrinkage stress development in ultra-rapid photo-polymerized bulk fill resin composites

The microstructure of acrylonitrile–butadiene rubber (NBR) was shown to be dependent on the polymerization conditions. The NBR investigated in the current study was obtained via radical copolymerization under azeotropic conditions (AN/BD = 38/62) in organic solution in the presence of either a conventional chain transfer agent or a reversible addition fragmentation chain transfer (RAFT) agent. The variation in the polymer microstructure was proven to originate from different radical environments during the polymerizations with initial radical initiator concentrations in the polymerizations studied ranging from 1.0 mM to 34.1 mM. The variation of the polymer microstructure was evidenced by triple SEC measurements, making use of the simultaneous determination of molecular weights with two independent methods, namely on-line viscometry and on-line light scattering. It is additionally evidenced that the microstructure shows a gradual variation during the course of a polymerization, a behaviour observed when the polymerizations were performed in the presence of an elevated initial radical initiator concentration. Furthermore, experimental evidence for the variation of the NBR microstructure during RAFT polymerizations is provided. At low conversions, a rather uniform polymer is obtained. With increasing conversion, a loss of the controlled character is observed and the microstructures approach those of nitrile rubber obtained by conventionally controlled free radical copolymerizations employing mercaptane transfer agents. Despite the differences in the polymer microstructure, it is possible to report a single set of MHKS parameters for the prepared NBR with azeotropic composition. A linear regression of the Mark–Houwink plots of the samples polymerized under different conditions gives values of K = (49.5 ± 5.5) × 10−5 dL g−1 and α = 0.689 ± 0.010 with a low error margin for SEC separation in THF at 25 °C. Weight average molecular weights of the investigated NBR samples were in the range of 40000 to 155000 g mol−1. The molecular weights of the copolymers determined via universal calibration with the MHKS parameters presented in the current study show a good agreement with molecular weights obtained from light scattering, underpinning the veracity of the obtained parameters.

Synthesis of core–shell polystyrene nanoparticles by surfactant free emulsion polymerization using macro-RAFT agent

The regulation of multifunctional methacrylates to yield tough photopolymers is a widely researched topic, whereby addition–fragmentation chain transfer (AFCT) agents represent one viable class of additives. Vinyl ethers have been described as potent AFCT agents in radical polymerization but are unexamined in network formation via photopolymerization. In this article, we present a sterically hindered vinyl ether as AFCT agent for methacrylate networks, which shows enhanced acid stability opposed to vinyl ethers described in the literature. After synthesis and confirmation of the efficient regulation in a monofunctional system, the reactivity and mechanical properties in a difunctional methacrylate were evaluated. An increase in double bond conversion, significantly lower shrinkage stress, and high toughness were assessed and substantiated the great potential of this compound in photopolymerizable resins.

Utility of propenyl groups in free radical polymerization: Effects of steric hindrance on formation and reaction behavior as versatile intermediates

6 - Chain Transfer

The dimerization of methyl methacrylate, ethyl methacrylate, methacrylonitrile, and α‐methylstyrene to 2‐substituted‐1‐allylic compounds [CH2C(X)CH2C(CH3)2X] (X = COOR, C6H5, or CN), and methyl α‐ethylacrylate to a 3‐substituted‐2‐allylic compound [CH3CHC(COOCH3)CH2C(CH3)(C2H5) COOCH3] was carried out by catalytic chain transfer using benzylbis (dimethylglyoximato) (pyridine) cobalt (III). These dimers were then used as addition‐fragmentation chain transfer agents in the polymerizations of methyl methacrylate and styrene at 800C or above. Cross‐dimers from methacrylic ester‐α‐methylstyrene and methacrylonitrile‐α‐methylstyrene mixtures were similarly prepared. Except for those from methyl α‐ethylacrylate and methacrylonitrile, all the dimers participated in the addition‐fragmentation and the copolymerization to different extents. The dimer of methyl α‐ethylacrylate was actually inactive during the styrene and methyl methacrylate polymerizations. The methacrylonitrile dimer was primarily incorporated in the polymer chain through copolymerization. Among the dimer and the cross‐dimers from α‐methylstyrene with the other monomers, those bearing the α‐methylstyrene moiety in the α‐substituent [CH2C(X)CH2C(CH3)2C6H5, XCOOCH3, COOC2H5, and CN] are noted as highly reactive chain transfer agents. © 1994 John Wiley & Sons, Inc.

Miniemulsion copolymerization of ethylene and butyl methacrylate (BMA) was carried out at 60 °C using 4-cyano-4-(phenylcarbonothioylthio)pentanoic acid as the reversible addition fragmentation chain transfer (RAFT) agent, sodium dodecyl sulphate as emulsifier and potassium per sulphate as free radical initiator under extremely low ethylene feed pressure of 5–20 bar. In the absence of a co-stabilizer, very stable emulsion was obtained, wherein the RAFT agent acted both as the chain transfer agent and as the co-stabilizer. FT-IR as well as NMR analysis established the formation of poly(ethylene-co-BMA) copolymer nanoparticles. The SEM analysis was used to study the morphological behaviour of poly(ethylene-co-BMA) copolymer nanoparticles. DLS analysis confirmed that the particle diameter Zavg of copolymer nanoparticles is in the range 100–140 nm and confirmed miniemulsion polymerization. In order to establish the stability of copolymers against thermal degradation, the kinetics and mechanism of thermal degradation of copolymers were investigated by thermogravimetric analysis. Thermogravimertic analysis established that thermal decomposition of poly(ethylene-co-BMA) copolymer nanoparticles occurred in the range 250–450 °C. This range is lower than the decomposition temperature of neat polyethylene (350–580 °C). Kinetic parameters were determined according to the model-free approaches of Friedman and Flynn–Wall–Ozawa. The multivariate nonlinear regression analysis was performed to determine the kinetic model and the corresponding kinetic triplets.