Singlet Oxygen Initiation of Polymer Photooxidation: Photolysis of cis-1,4-Poly(isoprene hydroperoxide)

Abstract

Reaction of singlet oxygen with cts-l,4-polyisoprene gives cis-l,4-poly(isoprene hydroperoxide). The kinetics and mechanism of the photolysis of this polymer-singlet oxygen adduct have been studied at 313 nm in solution in the absence of oxygen. The primary quantum yield for photolysis of the polymer hydroperoxide was 0.8. The subsequent radical-induced chain decomposition of the hydroperoxide results in a high overall quantum yield for hydroperoxide decomposition ( )• The chain propagation reaction rate constant is estimated as 9 X 104 M"1 s"1. The polymer degradation kinetics was studied and the rate constant for the ß scission of the polymeric alkoxy radical (generated from primary hydroperoxide decomposition) is estimated as 2.7 X 104 s_1. The quantum yield of polymer chain scission ( §) is of the order of 0.1. From the experimental values of and 8, the ratio 8/ can be approximated as a constant of 0.014. This agrees remarkably well with the theoretical value of 0.01 derived from the mechanism proposed in this study.

Photosensitized oxidation of unsaturated polymers

“…However, the comparison breaks down with regard to the corresponding structures and a (or 4 and fl, and this is perhaps a reflection of the different solvents used for the MO and PIP studies. On the other hand, the distribution obtained by Ng and Guillet (7,16) for the singlet oxygenation of MO with unspecified cis-trans content is almost identical to that obtained by us (10) for trans-PIP, but quite different from the distribution obtained by Tanielian and Chaineaux. It should be added that the latter workers (15) found the relative reactivities of trans-and cis-MO towards 102 to be "d.7:l.0, which agrees very well with the corresponding ratio of 'l.5:l.0 obtained by us for trans-and cis-PIP (10).…”

“…This reinforces the view developed on spectroscopic grounds that the mechanisms for singlet oxygenation are fundamentally the same for the two classes of compounds. Where differences arise, these must be due to secondary processes occurring in the hydroperoxidized polymers or to interactions between polymer and sensitizer, to reaction 1, was found to cause sharp decreases in viscosity or molecular weight of PBD (1,4,12,32,33), butadiene-styrene copolymers (1,12) and PIP (16). On the other hand, hydroperoxidation of cis-and trans-PBD in solution, with 102 produced in situ from thermal decomposition of triphenylphosphite ozonide, gave rise to gelled polymers, more so in cisthan in trans-PBD, but in neither polymer was there chain scission (22,29).…”

“…In contrast to singlet oxygenation in solution, thin films of PBD (22,30,32,33) and PIP (6,7,16) exposed to 102 generated in the gas phase by microwave discharge, are hydroperoxidized at the surface only and experience no degradation. However, the 102-treated films are unstable and display pronounced autoxidation when heated in air above lOO°C (22,30,32,33); these same films, in the absence of air and put into solution, undergo chain scission when irradiated with near UV light (6,7,16,45). These effects, which are also observed for 102_ reacted polymers obtained by dye photosensitization (7,16,37), are a consequence of the facile thermal or photochemical decomposition of the polymer-OOH group.…”

“…However, the 102-treated films are unstable and display pronounced autoxidation when heated in air above lOO°C (22,30,32,33); these same films, in the absence of air and put into solution, undergo chain scission when irradiated with near UV light (6,7,16,45). These effects, which are also observed for 102_ reacted polymers obtained by dye photosensitization (7,16,37), are a consequence of the facile thermal or photochemical decomposition of the polymer-OOH group.…”

“…Concurrently, work in this laboratory on the photosensitized oxidation of 1,4-polyisoprene and 1,4-polybutadiene was reported at two meetings in the USA (10,11). Since then, additional related work has been performed by those researchers (12)(13)(14)(15)(16) and by us (17). The present paper surveys this literature, with emphasis on the spectroscopic analysis of various unsaturated polymers before and after reaction with 102, and on the relative reactivities of such polymers towards 102 vis-à-vis their simple olefinic analogs.…”