RESEARCH
UV irradiation alters 1,4-polyisoprene Cyclization accompanies cis-trans isomerization, loss of 1,4 unsaturation, and vinyl group formation When 1,4-polyisoprene film is irradiated with ultraviolet light under vacuum it undergoes a number of structural changes: • Cis-trans isomerization occurs. • The degree of 1,4 unsaturation drops. • Vinylidene and vinyl double bonds appear. • Gyclopropyl groups are formed along the polymer backbone. Discovery of cyclopropyl rings in the UV-irradiated polymer came as a complete surprise, admit Dr. Morton A. Golub and Curtis L. Stephens, who did the study at Stanford Research Institute in Menlo Park, Calif. Photochemical production of such a ring system in an unsaturated macromolecule is unprecedented, they contend [/. Pol· ymer Set., A-1,6, 763 ( 1 9 6 8 ) ] . Until now there hasn't been any detailed evaluation of the effect of UV irradiation under vacuum on polyisoprene's microstructure, Dr. Golub points out. This is surprising in view of the considerable interest in the effects of light aging and weathering on rubber and the many studies on radiolysis of polyisoprene in vacuo, he adds.
The only prior work that dealt with the photolysis of 1,4-polyisoprene under vacuum dates back to the mid1940's when Dr. L. Bateman at the Natural Rubber Producers Research Association in Welwyn Garden City near London, England, showed that UV irradiation of a purified, degassed, film of natural rubber leads to evolution of gas which is chiefly hydrogen [/. Polymer Set., 2, 1 (1947)]. "We set out to extend that work to a detailed examination of the irradiated polyisoprene using analytical tools not available when Dr. Bateman did his study, and at the same time follow up our prior investigation of the photolysis of 1,4-polybutadiene," explains Dr. Golub, who is currently a National Research Council senior postdoctoral research associate at NASA's Ames Research Center, Moffett Field, Calif. This work is the first IR and NMR study of photoinduced microstructural changes in 1,4-polyisoprene, he claims. Dr. Golub and Mr. Stephens cast thin films of the purified polymer onto sodium chloride infrared disks. The disks were then sealed in quartz tubes at about 10 - 5 torr. The films
CHARTING CHANGES. Dr. M. Golub (standing) and C. L. Stephens study NMR results. NMR and IR spectroscopy were used to determine photochemically induced changes in 1,4-polyisoprene.
were irradiated at room temperature with UV light from a Hanovia 450watt quartz medium-pressure lamp. Most irradiations were carried out on cis- 1,4-polyisoprene (hevea). Supporting data were derived by irradiating trans- 1,4-polyisoprene (balata) and cis-l,4-polyisoprene-3
