The photochemically induced emulsion polymerization of styrene, methyl methacrylate, or acrylic acid is photosensitized by oil-soluble ketone initiators (e.g., dibenzyl ketone) to conversion and average molecular weights that are comparable to those achievable by employing conventional aqueous-soluble thermal initiators such as persulfates. The efficiency of polymerization and the average molecular weight of the polymers formed are significantly increased by the application of laboratory magnetic fields when photoinduced initiation is sensitized by oil-soluble ketones but not when aqueous-soluble thermal initiators are employed. With 1 ,l'-diphenyl- 1,l'-azoethane as initiator, triplet-sensitized initiation, but not thermal or direct photoinduced initiation, shows magnetic field effects on the average molecular weight and the yield of the products. No magnetic field effects are observed for photoinitiated polymerization of styrene or methyl methacrylate in toluene solution. With dibenzyl ketone that is enriched with I3C at both benzylic carbons as photoinitiator, the molecular weight distribution is bimodal. Varying the ratio of IT-enriched and natural-abundance DBK changes the relative yield of the lower and higher molecular weight fractions. It is concluded that external magnetic fields decrease the efficiency of triplet to singlet radical-pair intersystem crossing within micelles and accordingly increase the fraction of radical pairs that escape without terminating polymerization chains. An important characteristic of emulsion polymerization (EP) is the production of high molecular weight polymers at a relatively rapid rate.) The simultaneous achievements of high-molecu- lar-weight polymer and high polymerization rate result from the occurrence of initiation by single radicals at a large number of isolated microscopic reaction vessels (the micelle aggregates in- itially present), so that chain termination rates are decreased. Extensive work has been carried out on the mechanism of such polymerizations, and it has been found that, in general, water- soluble initiators are best suited for thermally initiating EP.' Oil-soluble initiators are commonly ineffective in EP either as thermal initiators or photoinitiators, presumably because such initiators produce pairs of radicals in the initial-polymerization loci and because these pairs undergo termination before substantial polymer growth can be effected.Id This result is generally expected if singlet radical pairs are generated in the initial bond-cleaving process. However, photochemically generated triplet radical pairs in micelles have been shown to survive long enough to compete, so that escape from micellar aggregates can compete with radical recombination in the mi~elle.~-~ Furthermore, application of relatively weak magnetic fields (GOO G) has been shown to substantially enhance the efficiency of escape of radicals from micelle aggregates. Since enhanced escape of a radical from a radical pair should increase polymerization rates and molecular weights in EP, we have explored (1) the influence of the application of weak magnetic fields on the rate of EP and molecular weight distribution (MWD) of polymers produced in polymerizations initiated by dibenzyl ketone (DBK) and other ketones that are known to produce triplet benzyl radical pairs in direct photolysis and by triplet sensitization of 1,l'-diphenyl- 1,l'-azoethane (DPAE) and (2) the I3C isotope effects on polymer molecular weight distribution produced by DBK photoinitiation. Results Comparison of Molecular Weights and Yields as a Function of Initiation Method. Since photoinitiation has not been commonly employed for initiation of EP, we have compared averaged polymer ~~~~
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Jan 1, 1993
N I Avdievich + 3
Open Access