Radical Termination Mechanisms in Bulk Polymerization

Abstract

-. I he termination mechanism in the bulk polymerization of styre~le and vinyl acetate initiated by ol,olf-azobisisobutyronitrile has been investigatetl. CM-Labelled initiators were employed and activities of salnples \yere determined by internal liquid scintillation c o~~~n t i~n g . Lleasurements of fractionated and tunfractionated polystyrene sho\ved an average of 1.94 and 2.04 tagged fragments per molecule, respectively, and indicated termination by combination a t 60" C. For polyvinyl acetate 1.06 fragments \\we found per molecule, indicating termination b3. disproportionatio~l a t 60' C.One of the features of vinyl addition reactions, which is still subject to coilsiderable uncertainty, is the inode of termination of gro\\~ing chains. I t is universally accepted that ter~nination reactions must occur as the result of the interaction of tu7o radicals, but the mechanism of this termination, whether by direct combination or by disproportionation, has only been investigated for a few systems, and has not been proved unequivocally even for these.Nevertheless the inode of termination is of primary in~portance in kinetic analysis of data, in the assignment of termination constants and the prediction of molecular weights. Unless the mode of termination is understood, these quantities are subject to an uncertainty of a factor of two.Two sensitive methods exist for the solution of this problem. Bamford and Jenltins (I) have described a technique of coupling preformed polymer possessing suitably reactive end groups. Alternatively with the use of radioactive initiators, it is possible to determine the number of initiator fragments present per polylner molecule (2, 3, 4). Since each initiator fragment found generated a chain, then in the final polymer the presence of t~v o initiator fragments per polymer nlolecule would indicate the exclusive occurrence of combination, and of one initiator fragment of disproportionation.Previous studies have not yielded completely conclusive results. For methyl methacrylate, the monomer subjected to the most intensive study, early work by Arnett and Peterson ( 5 ) using tagged initiators, indicated combination, whereas Bevington et al. (3) found a predominance of disproportionation. Ayrey and Moore (6) also found disproportionation and combination, but their results were not in exact agreement with those of Bevington.I t is reasonable to assume that for most monomers, termination reactions by colnbination and disproportionation can be considered as competitive reactions, each with a definite rate constant and activation energy. I t is our intention to investigate this problem in detail, and the present report represents our first work in this direction.The systems styrene and vinyl acetate, initiated with cr,crl-azobisisobut?;ronitrile (AIBN), were chosen for this preliminary study. E S P E R I M E S T A i L -1Iaterialscr,cr'-LAzobisisobutyronitrile (XIBN) dicyano C" was obtained from the Co~nmissariat 5 ~'~n e r~i e z4tomique, Saclay, France, and possessed a specific activity of 200 p...

Vinyl Alcohol Polymers

“…Termination reaction with vinyl acetate is nearly exclusively by disproportionation (180), although there are reports that recombination increases in importance at lower temperatures (181). Termination by disproportionation produces a double bond at the end of one molecule and a saturated bond at the other.…”

Scintillation Counting With Organic Phosphors

“…Frequently the sample is directly soluble in the scintillator solution. For example, in our work on C-14-labelled polystyrene i t was sufficient to dissolve the sample directly in the scintillator solution (8). For non-polar hydrocarbons surprisingly little quenching of the scintillations is found.…”

Interplay of Head, Tail, and Mid-Chain Radicals in Bulk Free-Radical and Reversible Degenerative Addition Fragmentation Chain-Transfer Polymerizations of Vinyl Acetate

“…Because of a resemblance with acrylate radical polymerization, recombination seems a logical choice. On the other hand, experiments using labeled initiators indicate disproportionation as the main termination mechanism, although chain transfer to monomer was not accounted for. , A more recent NMR study by Britton et al showed then again the absence of a significant amount of 1,2-disubstituted olefinic bonds, concluding that disproportionation is less likely. Notably, chain transfer to monomer can expect to be dominant so that radicals often participate in this reaction before terminating, masking the actual termination mechanism.…”