Cyclo‐ and cyclized diene polymers. II. Infrared study of the cyclization of cis‐1,4, trans‐1,4, and 3,4‐polyisoprenes

Cyclization of cis‐1,4, trans‐1,4‐, and 3,4‐polyisoprene was carried out in toluenesolution with sulfuric acid. With 1% H2SO4, calculated on polymer, a stationary state of cyclization was reached in which 33% of the original unsaturation and about 50% of the total unsaturation remained in cyclized trans‐1,4‐ and 3,4‐polyisoprene. The rate of cyclization of cis‐1,4‐polyisoprene was very small, and the limiting value was not reached even after 100 hr. at 80°C. Infrared spectra of cyclized polymers show decreased absorption intensity of the bands characteristic of groups in the linear polymer. With 10% H2SO4 calculated on polymer (a heterogeneous mixture), all three polyisoprenes cyclize to give polymer with none of the original unsaturation in the chain. Infrared spectra of cyclized trans‐1,4‐and cis‐1,4‐polyisoprene are identical. The spectrum of cyclized 3,4‐polyisoprene differs only by two peaks, at 1385 and 1370 cm.−1. The spectra of all three cyclized isomers show new bands in infrared spectrum at 2870, 1265, 1200, 1040, and 810 cm.−1. A comparison has been made of the infrared spectra of cyclopolymers of isoprene, butadiene, and chloroprene prepared from monomers by use of Ziegler‐Natta catalysts with spectra of cyclized polyisoprene. The spectra are very similar, which indicates that both types of polymer have the same polycyclic structure.