Poly chain-transfer branching

Investigation has shown that chain transfer to polymer occurs predominantly on the acetate methyl group in preference to the chain backbone one estimate of the magnitude of the predominance is 40-fold (92,93). The number of branches per molecule of poly(vinyl acetate) polymerised at 60°C is ca 3, at 80% conversion. It rises rapidly thereafter and is ca 15 at 95% conversion and 1-2 x lO" number-average degrees of polymerisation. 

In radical template polymerization, when only weak interaction exists between monomer and template and pick-up mechanism is commonly accepted, the reaction partially proceeds outside the template. If macroradical terminates by recombination with another macroradical or primary radical, some macromolecules are produced without any contact with the template. In fact, such process can be treated as a secondary reaction. Another very common process - chain transfer - proceeds simultaneously with many template polymerizations. As a result of chain transfer to polymer (both daughter and template) branched polymers appear in the product. The existence of such secondary reactions is indicated by the difficulty in separating the daughter polymer from the template as described in many papers. For instance, template polymerization of N-4-vi-nyl pyridine is followed, according to Kabanov et aZ., by the reaction of poly(4-vinylpyridine) with proper ions. The reaction leads to the branched structure of the product ... 

The group X transferred may be H or halogen. Attack on the CH2 groups in the polymer, or on pendent groups, such as — COO CH3 in poly (vinyl acetate), would produce similar effects in the case of poly (vinyl acetate) branches attached via the acetate group can be removed by hydrolysis, unlike those formed by attack on hydrogen atoms linked to main chain carbons. 

It has been known for some time [see Ref. (176) for earlier work] that if poly(vinyl alcohol), produced by hydrolysis of poly(vinyl acetate) is reacetylated, the PVAc so obtained has a lower MW than the original PVAc prior to hydrolysis, though the MW of the material is not lowered any further by subsequent cycles of hydrolysis and reacetylation. Various explanations had been advanced for this phenomenon Wheeler explained it as a consequence of the presence of branches joined to the main chain through ester linkages which would be broken on hydrolysis and not re-formed on reacetylation. These branches were ascribed to chain transfer reactions with acetate groups, either in the polymer, or in monomer molecule subsequently polymerized at their double bonds. Transfer reactions by attack on hydrogen atoms other than those in... 

It was known that polystyrene and poly- >-methoxystyrene initiated by tin tetrachloride have a branched structure, due to aromatic substitution in the course of the polymerization (186). Haas, Kamath and Schuler (93, 124) studied the ionic chain transfer reaction between a polystyrene carbonium chain and poly-/>-methoxystyrene. They were able to separate the homopolymers from the graft copolymers by extraction with methylcyclohexane. 

In ionic polymerization a hydride (H-) transfer or a proton transfer are the analogues of the hydrogen atom transfer in radical polymerization. A hydride (H-) ion transfer is observed in many isomerizations and dimerizations of hydrocarbons which proceed via carbonium-ion mechanism. A similar process is responsible for chain transfer ip some carbonium-ion polymerizations. The transfer of negative ions like Cl- is also common, e.g. triphenyl methyl chloride is an efficient transfer agent in such a polymerization. Transfer of a proton is, on the other hand, a very common mode of termination of anionic polymerization. Indeed, this mode of termination was discussed previously in connection with branching reactions, and it was postulated in the earliest studies of anionic poly-... 

When 1,3-dioxolane was added to the solution of living (nontermi-nated) poly(l,3-dioxepane) or vice versa, further polymerization ensued and the increase of molecular weight indicated that polymerization of added monomer proceeded exclusively on living active species of the former monomer. The isolated copolymer was analyzed by l3C NMR spectroscopy and it was found that, instead of a block copolymer, the copolymer with nearly statistical distribution of DXL and DXP units was formed practically from the beginning of the process. This is a clear indication that chain transfer to polymer leads to branched oxonium ions, which participate in further reactions with a rate comparable to the rate of propagation. 

The molecular weight, in general, was found to be insensitive to dose rate and emulsifier concentration, consistent with the determining factor being chain transfer to monomer. The radiation-produced poly(vinyl acetate) had a somewhat higher molecular weight even at the same temperature, perhaps due to branching initiated by direct radiolysis of the polymer chains. 

Another typical process involving branched radical polymerization is the production of poly(vinyl acetate). In the experiments of Stein105,106, the method of mathematical simulation has been used to evaluate the effect of longchain branches on the width of MWD. The reactions of chain transfer to the polymer and polymerization by the terminal double bonds of the polymer were examined separately. A comparison of the calculated and experimental Pw/Pn - f ( ) dependencies yielded the values of Cp = kf/kp and K = k p/kp. 

Aziridines Ethylenimine is the simplest aziridine and its CROP is already known since 1941 [126]. Currently, poly(ethyleneimine) is still produced on an industrial scale via CROP. However, the CROP of ethylenimine, that is, unsubstituted aziridine, produces a highly branched poly(ethylenimine) because of the occurrence of proton transfer reactions, chain transfer reactions as well as various termination reactions resulting in a polymer that contains a mixture of primary, secondary, and tertiary amine groups. This extensive occurrence of transfer reactions is caused by the high nucleophilicity of the secondary amine groups in the polymer that strongly compete with the monomer. The CROP of 2-methylaziridine and 2-phenylaziridine have also been reported, but are even... 

S.3.2.2 Azetidines The CROP of azetidines without the N-substituent is very similar to the CROP of ethylenimine as discussed in the previous section. As such, the CROP of azetidine is accompanied by a large number of hydrogen transfer, chain transfer, and termination reactions resulting in the formation of branched poly(propylenimine) comprising a mixture of primary, secondary, and tertiary amines [141]. 

In a study of chain-transfer constants of the monomeric vinyl acetate it was found that the formation of nonhydrolyzable branches is virtually negligible while hydrolyzable branches are formed at position 1 of Structure 1 by a terminal double-bond reaction rather than by a polymer-transfer reaction. The long nonhydrolyzable branches in poly(vinyl alcohol) are, presumably formed almost exclusively by a polymer transfer mechanism [35]. 

Wolf, C., Burchard, W. Branching in free radical polymerization due to chain transfer, application to poly(vinyl acetate). Makromol. Chem. 177, 2519-2538 (1976)... 

Mw=1.2xl0 with broad molecular weight distributions (PDI 6) has been reported by the free radical polymerization of vinylruthenocene in benzene with AIBN as the initiator 43]. The polymers were proposed to possess a branched structure as a result of chain-transfer steps. Copolymers with other organic vinyl monomers have also been prepared [43]. Poly(vinylruthenocene) and poly(vinylosmocene) have been tested as preheat shields for targets in inertial[Pg.48]

The branches of poly(vinyl acetate) that form during polymerization as a result of chain transferring to the acetate groups cleave during transesterification. As a result, poly(vinyl alcohol) is lower in molecular weight than its parent material. 

In vinyl acetate (VA) bulk or solution polymerization systems, side reactions (e.g., chain transfer or termination) will inevitably occur to produce highly branched poly(vinyl acetate) (PVA), based on the nonconjugated nature of the propagating radical. However, the polymerization of VA in nanochannels of [Cu2(terephthalate)2ted] effectively suppresses chain branching during the polymerization, and this results in a constrained chain growth in the narrow 1-D nanochannels [26]. 

An example of the macromonomer method is the preparation of graft copolymers of PEO by the free radical polymerization of vinyl acetate in the presence of PEO. The growing vinyl acetate radical would abstract a hydrogen atom from the PEO chain, creating a radical at this site. The newly created radical would then polymerize vinyl acetate to form a branch on the chain. The rather randomly occurring chain transfer reaction would form a graft copolymer of PEO and poly(vinyl acetate). 

Detailed structural analysis of poly(vinyl acetate) has revealed that hydrogen abstraction from the acetyl group is the predominant chain transfer mechanism (285). Abstraction from the methine hydrogen of the main chain leads to a small amoimt of branching (286,287). 

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