Radical and Sulfur Curing of Isobutene-Trienes Copolymers

Previous investigations have shown that polyisobutene and poly(isobutene-co-iso-prene), i. e. butyl rubber, are unable to cross-link in the presence of free radicals, since extensive chain scissions occur and thus low molecular weight products are formed The degradation mechanism proposed by Loan involves, in the case of polyisobutene, the H abstraction from methyl groups followed by chain scission. Apparently, the formation of secondary alkyl radicals, which are believed to be responsible of polyolefin radical curing is prevented for steric reasons by the presence of two adjacent dimethyl substituted carbon atoms and hence j3 scission reactions prevail. 

This situation is substantially undian in the case of butyl rubber, where a few percent (0.6—3 mol %) of trans-l, 4-isoprene units, i. e. C-CH2 -CH=C—CH2 -CH2 —C, are present and each methjdene group is adjacent to at l t one methyl substituted carbon atom. In fact. Loan has shmvn that the unsaturations of butyl rubber react more easily with the fragn nts arising frcan the dicung l peroxide decomposition than the H atoms of isobutene methyl groups. The ratio of the two rate constants is 

Conditions Dicumyl peroxide = 3 phr T = 153 °C time = 60 Abbreviations as in Table 6. 

Residue to extraction with boiling cyclohexane (24 hours). 

M = modulus TS = tensile strength EB = elongation at break. Degradated sample. 

Attempts were made to overcome the difficulties above mentioned by ter-polymerizing isobutene, isoprene and divinylbenzene, to partially cross-linked products, which can give further radical curing69.  

The presence in polyisobutene chains of conjugated unsaturations highly reactive toward free radicals, as it occurs in tire case of isobutene-triene copolymers, induced us to investigate radical cross-linking of these copolymers. 

The results obtained, when dicumyl peroxide is used as curing agent (Table 19), indicate that 1,3,5-hexatriene (HT1) and 2,4,6-octatriene (OTI)-based copolymers can be easily cross-linked. 

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