Methylal chain transfer

Polymerization of trioxane with 1,3-dioxolane utilizing a BF3 initiator and a methylal chain transfer agent-designated POM. 

The molecular weight of a polymer can be controlled through the use of a chain-transfer agent, as well as by initiator concentration and type, monomer concentration, and solvent type and temperature. Chlorinated aUphatic compounds and thiols are particularly effective chain-transfer agents used for regulating the molecular weight of acryUc polymers (94). Chain-transfer constants (C at 60°C) for some typical agents for poly(methyl acrylate) are as follows (87) ... 

Chain transfer to solvent is an important factor in controlling the molecular weight of polymers prepared by this method. The chain-transfer constants for poly(methyl methacrylate) in various common solvents (C) and for various chain-transfer agents are Hsted in Table 10. 

Table 10. Chain-Transfer Constants for Methyl Methacrylate...

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. 

Hexachloroethane has been suggested as a degasifter in the manufacture of aluminum and magnesium metals. Hexachloroethane has been used as a chain-transfer agent in the radiochemical emulsion preparation of propylene tetrafluoroethylene copolymer (152). It has also been used as a chlorinating agent in the production of methyl chloride from methane (153). 

The molecular weight of the continuous phase is an important parameter that affects the mechanics and the melt flow of the end product. It can be controlled by the use of a suitable chain transfer agent (e.g., /er/-dodecyl mercaptan Ct = 4.0) or their combinations (e.g., primary mercaptans Or = 26.0 and dimeric a-methyl styrene Ct = 0.1) [132]. 

Free radicals, unlike carbocations, do not normally undergo isomerization by methyl or hydrogen migration. However, hydrogen transfer (chain transfer) occurs when a free radical reacts with other hydrocarbons. 

The proposal that PVAc also has non-hydrolyzable long chain branches stems from the finding that PVA also possesses long chain branches. No/akura et a/.171 "07 suggested, on the basis of kinetic measurements coupled with chemical analysis, that chain transfer to PVAc involves preferential abstraction of backbone (methine) hydrogens (ca 5 1 v,v the acetate methyl hydrogens at 60 °C). 

The polymerization of 2-methyl-2-oxazoline is a clean reaction, which is not disturbed by chain transfer and termination. In this polymerization, the propagating species having the structure of an oxazolinium salt is not fragile, which is conveniently utilized for syntheses of block copolymers and end-reactive polymers [28],... 

The above explanation of autoacceleration phenomena is supported by the manifold increase in the initial polymerization rate for methyl methacrylate which may be brought about by the addition of poly-(methyl methacrylate) or other polymers to the monomer.It finds further support in the suppression, or virtual elimination, of autoacceleration which has been observed when the molecular weight of the polymer is reduced by incorporating a chain transfer agent (see Sec. 2f), such as butyl mercaptan, with the monomer.Not only are the much shorter radical chains intrinsically more mobile, but the lower molecular weight of the polymer formed results in a viscosity at a given conversion which is lower by as much as several orders of magnitude. Both factors facilitate diffusion of the active centers and, hence, tend to eliminate the autoacceleration. Final and conclusive proof of the correctness of this explanation comes from measurements of the absolute values of individual rate constants (see p. 160), which show that the termination constant does indeed decrease a hundredfold or more in the autoacceleration phase of the polymerization, whereas kp remains constant within experimental error. 

The results of chain transfer studies with different polymer radicals are compared in Table XIV. Chain transfer constants with hydrocarbon solvents are consistently a little greater for methyl methacrylate radicals than for styrene radicals. The methyl methacrylate chain radical is far less effective in the removal of chlorine from chlorinated solvents, however. Vinyl acetate chains are much more susceptible to chain transfer than are either of the other two polymer radicals. As will appear later, the propagation constants kp for styrene, methyl methacrylate, and vinyl acetate are in the approximate ratio 1 2 20. It follows from the transfer constants with toluene, that the rate constants ktr,s for the removal of benzylic hydrogen by the respective chain radicals are in the ratio 1 3.5 6000. Chain transfer studies offer a convenient means for comparing radical reactivities, provided the absolute propagation constants also are known. 

If chain transfer of the radical center to a previously formed polymer molecule is followed ultimately by termination through coupling with another similarly transferred center, the net result of these two processes is the combination of a pair of previously independent polymer molecules. T. G. Fox (private communication of results as yet unpublished) has suggested this mechanism as one which may give rise to network structures in the polymerization of monovinyl compounds. His preliminary analysis of kinetic data indicates that proliferous polymerization of methyl acrylate may be triggered by networks thus generated. 

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