A high-trans 1,4-butadiene/isoprene copolymer (TBIP) was synthesized in a 5-L autoclave with hydrogen as an effective molecular weight modifier. The effects of hydrogen on the catalyst efficiency and molecular weight of the copolymers were investigated. The processability and physicomechanical properties of TBIP and their relationship to the composition, composition distribution, and molecular weight of TBIP were examined in detail. Increasing the H, pressure effectively reduced the molecular weight of TBIP. The optimum Mooney viscosity of TBIP and the 1,4-butadiene molar content in the feed were 30-50 and 5-25%, respectively. No cis-trans isomerization was observed during the roll processing procedure for TBIP. The vulcanization characteristics of TBIP were similar to those of general rubbers, and no reverse vulcanization was observed for TBIP. A high green strength was the typical characteristic of TBIP. Vulcanized TBIP (TBIR) with an optimum composition and molecular weight presented outstanding antifatigue properties and low heat buildup in comparison with general rubbers. Compared with general sidewall stock [natural rubber (NR)/butadiene rubber (BR) = 50/50], TBIR exhibited a greater than 15-fold increase in its crack-initiation resistance. The other mechanical properties of TBIR were similar to those of 50/50 NR/BR. The heat-aging mechanism of TBIR was crosslinking aging. (C) 2004 Wiley Periodicals, Inc.