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

The controlled thermal degradation of some samples of isotactic and syndiot actic polypropylenes with molecular weights ranging from 3.8x10(4) to 1.1x1 0(5) in Mn was examined to develop an effective method for selective format ion of telechelic oligomers having two isopropenyl end groups, TVD. The pri ncipal conditions are at 370 degreesC for 30-90 min. The average number of TVD per molecule f(t) determined from the C-13 NMR spectrum of the nonvolat ile oligomers (M-n 3000-7000) isolated from the sample residues in the degr adation ranged from 1.5 to 1.9 with an increase in the initial molecular we ight of the original polymer, independent of the stereoregularity. The maxi mum f(t) value observed is 1.87, which indicates that about 88 mol% of the oligomer molecules are structurally symmetric telechelic oligomers having t wo TVDs. With an increase in the initial molecular weight M-n0, the yield o f residue decreases and the relative molecular weight (M-n/M-n0) of the non volatile oligomers decreases. These results strongly suggest that the inter molecular hydrogen abstraction of volatile radicals occurs more frequently than that of the terminal macroradicals, because almost all of the end doub le bonds of these oligomers are formed via the intermolecular hydrogen abst raction of various radicals followed by the B scission of on-chain macrorad ical and, thereby, the marked increase off, value is caused by a greater co ntribution of volatile radicals to the scission of main chains of polymer m olecules. Thus, it becomes clear from these results that the rates of eleme ntary reactions in the degradation process increases with an increase in in itial molecular weight owing to the diffusion-controlled mechanism of termi nation. (C) 2001 Elsevier Science Ltd. All rights reserved.