An effective method for selective formation of telechelic oligomers by controlled thermal degradation of polypropylenes

Abstract

The controlled thermal degradation of some samples of isotactic and syndiotactic polypropylenes with molecular weights ranging from 3.8×10 4 to 1.1×10 5 in Mn was examined to develop an effective method for selective formation of telechelic oligomers having two isopropenyl end groups, TVD. The principal conditions are at 370°C for 30–90 min. The average number of TVD per molecule f t determined from the 13C NMR spectrum of the nonvolatile oligomers ( M n 3000–7000) isolated from the sample residues in the degradation ranged from 1.5 to 1.9 with an increase in the initial molecular weight of the original polymer, independent of the stereoregularity. The maximum f t value observed is 1.87, which indicates that about 88 mol% of the oligomer molecules are structurally symmetric telechelic oligomers having two TVDs. With an increase in the initial molecular weight M n0, the yield of residue decreases and the relative molecular weight ( M n/ M n0) of the nonvolatile oligomers decreases. These results strongly suggest that the intermolecular hydrogen abstraction of volatile radicals occurs more frequently than that of the terminal macroradicals, because almost all of the end double bonds of these oligomers are formed via the intermolecular hydrogen abstraction of various radicals followed by the β scission of on-chain macroradical and, thereby, the marked increase of f t value is caused by a greater contribution of volatile radicals to the scission of main chains of polymer molecules. Thus, it becomes clear from these results that the rates of elementary reactions in the degradation process increases with an increase in initial molecular weight owing to the diffusion-controlled mechanism of termination.