Photopolymerization of diacetylenes under hydrostatic pressure

Photopolymerization of diacetylene-bis (toluenesulfonate) (TS-6) under hydrostatic pressure up to 4 kbar has been studied employing timed-resolved absorption spectroscopy of the trimeric reaction intermediate. Formation of the biradical dimer is a non-thermal process occurring at a rate of 10⁷ s^?1 independent of both temperature and pressure. The rate constant for subsequent monomer addition increases exponentially with pressure delineating existence of a linear relationship between height of the energy barrier and reaction distance which is considered to be the key reaction parameter. Measurement of the photopolymerization yield in TS-6, TS-12 and 4-BCMU under pressure in conjunction with statistical considerations confirm (i) that formation of long chains is limited by preformed short chains and (ii) that in TS-6 and TS-12, yet not in 4-BCMU reaction-induced secondary-chain initiation is important.