Grafting by transfer

Styrene-co-MA has been converted to a partial perester in acetone solution, using tert-huiyX hydroperoxide and pyridine catalyst.The partial peresters were used to initiate methyl methacrylate polymerization by a thermal mechanism in a homogeneous acetone solution. The method provided both homopolymer and graft copolymer. Grafting of methyl methacrylate or styrene-co-MA was also achieved with tert-h xiy hydroperoxide initiator. However, grafting by transfer was substantially less than grafting by the perester technique. Partially neutralized styrene-co-MA has also grafted with acrylonitrile in aqueous solution by a redox initiation procedure. 

Grafting by chain transfer other than redox system. 

R may be a radical formed by the decomposition of an initiator or a growing radical chain. Similarly, grafting by the chain-transfer mechanism occurs when the branched part consists of another monomer. Since cellulose is a poor transfer agent [8], the efficiency of grafting is quite poor. Incorporation of—SH groups into cellulose enhances the probability of chain transfer. This can be achieved as follows ... 

In the middle range of styrene concentrations, a compromise is attained where there is sufficient styrene to scavenge all excess methanol radicals not involved in activation of the trunk polymer, yet an excess of monomer remains for grafting by the charge-transfer mechanism proposed by Dilli and Garnett (12) originally for copolymerisation to cellulose (4) and subsequently extended to wool (3), polyolefin (2,5) and PVC (13) systems. The data in Table V are consistent with this interpretation. 

Graft polymers were obtained by mastication of a 60/40 blend of natural rubber and chlorosulphonated polyethylene (/). From 10-55% natural rubber was obtained as side chains. Grafting presumably proceeds by transfer of chlorine atoms to the rubber radicals to give grafting sites for combination with rubber radicals. Soluble linear polymers were also obtained by mastication for 50-180 min under nitrogen for a blend 50/50 of natural rubber and a polyurethane rubber (Vulcaprene A) (/). 

Table 1. Influence of the initiator on grafting by chain transfer...

Grafting by chain transfer has permitted to combine sequences of two monomers which would not copolymerize directly with each other. Thus Smets and coworkers, by polymerizing vinyl acetate in the presence of polyethyl a-chloroacrylate, obtained, besides some insoluble products, very appreciable amounts of graft copolymers, in which the vinyl acetate content ranged from 40 to 86% (204). 

Grafting by chain transfer initiation has been carried out not only in homogenous medium but also by emulsion polymerization techniques, where the monomer and the catalyst are added to a latex containing the original backbone polymer (99). The efficiency of grafting increases with an increase of temperature of polymerization and with an increase of initiator concentration (generally potassium persulfate) these results indicate not only that the chain transfer reaction has a higher activation... 

The amount of grafted rubber is influenced only slightly by the type of initiator used this suggests that grafting probably takes place on the active centers formed on the rubber by transfer reactions between the growing chains of PVC and the elastomer chains. 

Qin, L., Rahud, D.R., Ding, Y., Bielinska, A.U., Kukowska-Latallo, J.F., Baker, Jr., J.R. and Bromberg, J.S. (1998) Efficient transfer of genes into murine cardiac grafts by starburst polyamidoamine dendrimers. Hum. Gene Ther., 9, 553-560. 

The authors demonstrated that polyvinyl chloride segments of these graft and block copolymers are more stable than PVC homopolymers prepared according to traditional ways. These last ways lead to disproportionation reactions giving unsaturations at the ends of polymers and these ends are responsible for the poor thermal stability of the polymers. In the present method, termination occurring essentially by transfer reaction, no unsaturations are observed and the authors showed an improvement in stability of the PVC segments of about 20 °C (Scheme 39). 

Radical transfer reactions involving poly ethers of the poly (ethylene oxide) type are well known (7). Heating polyethylene glycols of various molecular weights at 140 °C. with dicumyl peroxide for 2.5 hours has resulted in a gel fraction explained by transfer at the a-carbon followed by combination of the polymer radicals. Further, poly (ethylene oxide) dissolved in MMA and heated in the presence of benzoyl peroxide results in grafted copolymer. 

To survey as completely as possible the grafting behavior of EVA copolymers toward various vinyl compounds, our investigations covered the grafting of vinyl acetate, vinylidene chloride, and acrylic and meth-acrylic esters. As polymerization processes, at first we preferred suspension polymerization to exclude the influence of solvents by terminating or transfer reactions during polymerization. Grafting by emulsion polymerization, in which the EVA copolymer was dissolved in the monomer before polymerization, was difficult because coagulate was formed as polymerization proceeded. 

NR and synthetic diene based elastomers are a very versatile material for grafting. Chain transfer grafting may be carried out by means of a functionalized monomer-initiator system in a rubber solution, in the latex phase, in swollen rubber or during vulcanization. The chain transfer can be accompanied in diene-based polymers by addition reactions involving both the starting radicals and growing polymer radicals [217]. 

Kennedy and coworkers improved considerably the degree of grafting by using di-ethylaluminium chloride at low temperature and claimed that this achievement was due to the essential absence of transfer reactions in the branching pdymerisation. The work of this prolific group has recently been the object of an entire issue of Applied Polymer Symposium and the interested reader will find in those papers a general introduction to cationic grafting and the ecific contributions and applications achieved at Akron. 

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