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Poly[l-(trimethylsilyl)propyne] (PTMSP) has the highest gas permeability of any known polymer.1 Understanding the structural origins of the exceedingly high gas permeability of PTMSP should facilitate the design of other highly gas permeable glassy polymers. Results of a systematic investigation of a homologous series of closely related polymers prompt us to offer a structure/property rationale to explain the high gas permeability of PTMSP. The following polymers were synthesized: poly[l-(n-alkyldimethylsilyl)propyne];A-E),poly [l-(isopropyldimethylsilyl)propyne];F),poly [l-(phenyldimethylsilyl)propyne];G),and poly[l-[p-(trimethylsilyl)phenyl]propyne]; H). Polymer conformation is herein discussed in relation to the steric requirement for main-chain conjugation, dynamic mechanical spectra, and a proposed mechanism of polymerization. The critical surface tension of pendant groups was used to qualitatively rationalize the intermolecular interactions for poly(silylpropynes). Gas permeability (P) was measured. For polymers A-E, as the chain length of the n-alkyl group decreases, the gas permeability increases. The large differences in the oxygen permeability coefficient, P(02), for poly[l-(isopropyldimethylsilyl)propyne] (F) and poly[l-(npropyldimethylsilybpropyne] (C) as well as those between polymer G and polymer H can be explained on the basis of the subtle but important London forces of side-chain association. Our conclusion is that the extraordinarily high gas permeability of PTMSP is a consequence of large excess free volume within the polymer matrix which is itself ascribed to (1) the rigid and irregular main-chain conformation and (2) the low interchain attraction force of the side chains which shield the main chain.

thiobenzophenone-derived cycloadducts were obtained relative to the amount resulting from a trap with dimethyl fumarate. a fellowship, K.S. and H.B. to UCSB for fellowships, and O.W. to the Swiss National Fund for financial support during postdoctoral studies conducted at UCSB.

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