Chain transfer polymerization

Chain transfer is an important consideration in solution polymerizations. Chain transfer to solvent may reduce the rate of polymerization as well as the molecular weight of the polymer. Other chain-transfer reactions may iatroduce dye sites, branching, chromophoric groups, and stmctural defects which reduce thermal stabiUty. Many of the solvents used for acrylonitrile polymerization are very active in chain transfer. DMAC and DME have chain-transfer constants of 4.95-5.1 x lO " and 2.7-2.8 x lO " respectively, very high when compared to a value of only 0.05 x lO " for acrylonitrile itself DMSO (0.1-0.8 X lO " ) and aqueous zinc chloride (0.006 x lO " ), in contrast, have relatively low transfer constants hence, the relative desirabiUty of these two solvents over the former. DME, however, is used by several acryhc fiber producers as a solvent for solution polymerization. 

Transfer and termination occur by the modes described previously for cyclic ether polymerizations. Chain transfer to polymer (both inter- and intramolecular) is facilitated in cyclic acetal polymerizations compared to cyclic ethers because acetal oxygens in the polymer chain are more basic than the corresponding ether oxygens [Penczek and Kubisa, 1989a,b]. Working at high monomer concentrations, especially bulk polymerizations, is used to depress cyclic oligomer formation. 

Hayes, R. A. Polymeric chain transfer reactions. Polymerization of some vinyl monomers in the presence of vinyl polymers. J. Polymer Sci. 11, 531 (1953). 

Hexenylsilane was introduced as a co-monomer into the organotitanium-mediated polymerization of ethylene to produce silane-terminated ethylene-5-hexenylsilane copolymers. High activities and narrow polydispersities were observed in the polymerization-chain transfer process. Ethylene-5-hexenylsilane copolymer molecular weights... 

The concept of PO macroinitiators centers on the introduction of an initiation moiety into an olefinic polymer chain for polymerization. The most effective route for preparing PO macroinitiators is by employing functional polyolefins containing hydroxyl groups or other reactive groups. These functional POs are prepared by copolymerization of olefins with functional monomers and post-polymerization reaction, as mentioned above. In the case where an initiation moiety was at the chain-end of the polyolefins, a block type copolymer is produced. It has been reported that thiol-terminated PP was used as polymeric chain transfer agent in styrene and styrene/acrylonitrile polymerization to form polypropylene-b/odc-polystyrene (PP-b-PS) and polypropylene-btock-poly(styrene-co-acrylonitrile) (PP-b-SAN) block copolymer [19]. On the other hand, polymer hybrids with block and graft structures can be produced if initiation moieties are in the polymer chain. 

Chiefari [1] prepared sulfur-containing chain transfer agents, (I) and (II), to control the polydispersity and multimodal molecular weight distribution of methyl methacrylate during free radical polymerization. The synthesis of a polymeric chain transfer agent analogue, (III), was also proposes by the author. 

Table 3 The Effect of Polymeric Chain Transfer Agent Composition on Branched Polymer Structure. All solution polymerisations were performed using MM A at 80° C, where PCTA is the polymeric chain transfer agent. fMMAJ = 40% (wlw), [initiator] = 0.4% (wlw), [toluene] = 47.6% (wlw) for samples 3 and 4, and 55.6% (wlw) for samples 5 and 6. Characterisation was performed using triple detector GPC a is the Mark-Houwink constant...

Dong, J.Y. Wang, Z.M. Han, H. Chung, T.C. Synthesis of isotactic polypropylene containing a terminal Cl, OH, and NH2 group via metallocene-mediated polymerization/chain transfer reaction. Macromolecules 2002, 35, 9352. 

Fig. 4.1G The direct synthesis of A-B block copolymers using the naked nickel catalyst with polymeric chain transfer agents.

Ignatz-Hoover, F., Petrukhin, R., Karelson, M. and Katritzky, A.R. (2001) QSRR correlation of free-radical polymerization chain-transfer constants for styrene. J. Chem. Inf. Comput. Sci., 41, 295-299. 

In the Soviet study110, the following elementary stages were taken into account in the kinetic scheme of vinyl acetate polymerization chain transfer to the monomer, solvent, and polymer, and chain termination caused by the disproportionation of radicals. It was assumed that long-chain branches could be formed by chain transfer both to the acetate group hydrogen atoms and to the main chain hydrogen. 

Controlled radical polymerization Chain transfer agent l,4-Diazabicydo[2,2,2]octane Dead-end polymerization D im ethylform amide Dimethyl sulfoxide 4,4,-Di-(5-nonyl)-2,2 bipyridine... 

Block Copolymers. Several methods have already been used for the synthesis of block copolymers. The most conventional method, that is, the addition of a second monomer to a living polymer, does not produce the same spectacular results as in anionic polymerization. Chain transfer to polymer limits the utility of this method. A recent example was afforded by Penczek et al. (136). The addition of the 1,3-dioxolane to the living bifunctional poly(l,3-dioxepane) leads to the formation of a block copolymer, but before the second monomer polymerizes completely, the transacetalization process (transfer to polymer) leads to the conversion of the internal homoblock to a more or less (depending on time) statistical copolymer. Thus, competition of homopropagation and transacetalization is not in favor of formation of the block copolymers with pure homoblocks, at least when the second block, being built on the already existing homoblock, is formed more slowly than the parent homoblock that is reshuffled by transacetalization. 

In control experiments (impurity-induced polymerizations) chain transfer is suppressed but not totally eliminated by increasing [ED] (see Sect. 4.1.2.1). Also with increasing [ED]s, the rate is reduced (Sect. 4.1.2.2) and the MWDs become narrower because of the increasing contribution of nondissociated living species to propagation (Sect. 4.1.2.3). 

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