Styrene reversible chain transfer

The kinetic model developed in this research allows an adequate description of molecular-mass properties of polystyrene, obtained by controlled radical polymerization, which proceeds by reversible chain transfer mechanism and accompanied by addition-fragmentation. This means, that the model can be used for development of technological applications of styrene RAFT-polymerization in the presence of trithiocarbonates. 

Several studies reported the successful application of reversible chain transfer techniques in water-borne systems. All of these studies apply RCTA species with low Qx constants to control the polymerisation. The alkyl iodides (degenerative transfer) used by several groups (Lansalot et aL, 1999 Butte et al., 2000) have a transfer constant only shghtly higher than unity. The ab initio emulsion polymerisation of styrene using CsFbI was carried out at 70°C (Lansalot et aL, 1999). It was found that the rate of polymerisation was not affected by the presence of CeFial. However, the evolution of Mn with conversion was not in accord with the Qx value. The authors postulated that due to the hydrophobic character of CsFbI, its transfer from droplets to particles was slower than the rate of consumption of CeF I within the particles. To overcome slow diffusion of CsFbI to the particles, the authors carried out miniemulsion (essentially Interval III kinetics), in which polymerisation takes... 

The controlled emulsion polymerization of styrene using nitroxide-mediated polymerization (NMP), reversible addition-fragmentation transfer polymerization (RAFT), stable free radical polymerization (SFR), and atom transfer radical polymerization (ATRP) methods is described. The chain transfer agent associated with each process was phenyl-t-butylnitrone, nitric oxide, dibenzyl trithiocarbonate, 1,1-diphenylethylene, and ethyl 2-bromo-isobutyrate, respectively. Polydispersities between 1.17 and 1.80 were observed. 

To give a specific example, the advantages of styrene as a substrate for peroxyl radical trapping antioxidants are well known" (i) Its rate constant, kp, for chain propagation is comparatively large (41 M s at 30 °C) so that oxidation occurs at a measurable, suppressed rate during the inhibition period and the inhibition relationship (equation 14) is applicable (ii) styrene contains no easily abstractable H-atom so it forms a polyper-oxyl radical instead of a hydroperoxide, so that the reverse reaction (equation 21), which complicates kinetic studies with many substrates, is avoided and (iii) the chain transfer reaction (pro-oxidant effect, equation 20) is not important with styrene since the mechanism is one involving radical addition of peroxyls to styrene. 

Conversely it is possible to produce low-molar-mass oligomers or telomers by deliberately choosing an agent with a large value of Ca (e.g. methyl mercaptan, Ca 2x 10 in styrene), so that DP is reduced to 5 for a concentration of 0.001%. Further particular examples of chain transfer (e.g. to polymer to form branches) will be discussed later, together with the use of reversible-addition fragmentation transfer (RAFT) and other radical-mediated synthetic strategies. 

The bifunctional initiator approach using reversible addition fragmentation chain-transfer polymerization (RAFT) as the free-radical controlling mechanism was soon to follow and block copolymers of styrene and caprolactone ensued [58]. In this case, a trithiocarbonate species having a terminal primary hydroxyl group provided the dual initiation (Figure 13.3). The resultant polymer was terminated with a trithiocarbonate reduction of the trithiocarbonate to a thiol allows synthesis of a-hydroxyl-co-thiol polymers which are of particular interest in biopolymer applications. 

Using NMP [114, 115] or reversible addition-fragmentation chain transfer (RAFT) [ 119,120,127], agents with ammonium groups for the ion exchange allowed the attachment of initiators on the clay surface for controlled radical polymerizations (NMP, RAFT). Samakande et al. investigated the kinetics of RAFT-mediated living polymerization of styrene [120] and styrene/BA [119] mixtures in miniemulsion. 

Thus the first step in the initiation reaction (Eq. (2.64)) involves a reversible electron transfer reaction from the alkali metal to the styrene monomer to form the styryl radical anion in a rapid subsequent reaction, two radical anions couple to form a di-anion which can grow a polymer chain at both ends. In the case of the soluble alkali metal aromatic complexes, the overall initiation reaction is extremely fast, due to the high concentrations of radical anion M) and... 

Barner, L. Li, C.E. Hao, X. Stenzel, M.H. Barner-Kowollik, C. Davis, T.P. Synthesis of core-shell poly(divinylbenzene) microspheres via reversible addition fragmentation chain transfer graft polymerization of styrene. J. Polym. Sci. A 2004,42 (20), 5067-5076. 

The catalytic system used to make OBCs uses a chain-shuttling agent (CSA) to shuttle or transfer growing chains between two distinct catalysts with different comonomer (alpha-olefm) selectivity." This is shown in Figure 9. Synthesis of olefin block polymer via chain shuttling requires the chain transfer to be reversible. OBCs are produced in a continuous solution polymerization process more economically favorable than the batch processes employed to make styrenic block copolymers. 

Alternatively, Whittaker et al. utilized the reversible oxidation/reduction of a thiol-terminated linear polymer as a homocoupling reaction to access macrocycles that could be reversibly cyclized and cleaved (Scheme 12.5) [29]. The linear precursors were prepared using reversible addition-fragmentation chain transfer (RAFT) polymerization of styrene from a bifunctional initiator (16). The desired polystyrene with thiol end groups could be isolated in near-quantitative yields by aminolysis of the polymer with terminal dithioester groups (17). The linear dithiols... 

Thus the first step in the initiation reaction [Eq. (64)] involves a reversible electron transfer reaction from the alkali metal to the styrene monomer to form the styryl radical anion in a rapid subsequent reaction, two radical anions couple to form a di-anion which can grow a polymer chain at both ends. In the case of the soluble alkah metal aromatic complexes, the overall initiation reaction is extremely fast, due to the high concentrations of radical anion ( 10 M) and monomer (-1M), and so is the subsequent propagation reaction. However, in the case of the alkah metal initiators, the electron transfer step [Eq. (64)] is very much slower, due to the heterogeneous nature of the reaction, so that the buildup of radical anions is much slower. In fact, there is evidence [153] that, in such cases, a second electron transfer step can occur between the metal and the radical anion to form a di-anion, rather than coupling of the radical anions. In either case, the final result is a di-anion, i.e., a difunctional growing chain. 

Lacroix and coworkers reported a reverse iodine transfer pol5mierization (RITP), where elemental iodine is used as a control agent in living radical polymerization [288]. Styrene, butyl acrylate, methyl acrylate, and butyl ot-fluoroacrylate were homopolymerized, using a radical catalyst and I2 as a chain transfer agent. Methyl acrylate was also copolymerized with vinyUdene chloride using this process. 

The kinetics of the free radical emulsion polymerization of a-meth-ylene-y-valerolactone has been investigated (58). Stable polymer latices could be prepared. A homogeneous nucleation is the dominant path for particle formation. Also, the miniemulsion copolymerization with styrene as comonomer has been investigated. Both the reversible addition-fragmentation chain transfer (RAFT) miniemulsion polymerization and the RAFT bulk polymerization are weU controlled and copolymers with a narrow polydispersity are formed. 

Brouwer, H. De, Schellekens, M. A. Klumperman, B., Monteiro, M. J., and German, A. L. 2000. Controlled radical copolymerization of styrene and maleic anhydride and the synthesis of novel polyolefin-based block copolymers by reversible addition-fragmentation chain-transfer (RAFT) polymerization. Journal of Polymer Science, Part A Polymer Chemistry 38 3596-3603. 

Reversible Addition Fragmentation Chain Transfer Mediated Dispersion Polymerization of Styrene Prakash J. Saikia, Jung Min Lee, Byung H. Lee, Soonja Choe ... 

Reversible Addition Fragmentation Chain Transfer Mediated Dispersion Polymerization of Styrene... 

A comprehensive kinetic mechanism is proposed to describe the combined chemical and thermal free-radical polymerization of styrene. Thus, besides the commonly employed reactions (e.g., chemical initiation, propagation and termination), thermal initiation and chain transfer to monomer and to Diels-Alder adduct reactions are included. In particular, the so-called AH thermal initiation mechanism of Mayo comprises a reversible Diels-Alder dimerization of styrene to form l-phenyl-1,2,3,9-tetrahydronaphtalene (AH), the formation of a styryl (m) and a 1-phenyltetralyl radical... 

Monteiro, M. J., and de Barbeyrac, J. (2001). Free-radical polymerization of styrene in emulsion using a reversible addition-fragmentation chain transfer agent with a low transfer constant effect on rate, particle size, and molecular weight. Macromolecules, 54(13) 4416-4423. 

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