Technology of industrial production of liquid rubber under trademark SKOP is based on the cationic polymerization of 1,3-penadiene (piperylene) in the presence of TiCl4 or AlCl3-based catalytic systems. The disadvantage of these catalytic systems is the high probability of formation of branched and insoluble fractions due to the chain transfer to polymer. This deteriorates the useful qualities of SKOP. Here we propose the new initiating systems for the cationic polymerization of 1,3-pentadiene based on the homogeneous (dissolved in a minimal amount of diethyl ether) zinc halides (ZnCl2 and ZnBr2) as coinitiators and hydrochloric acid, tert-butyl chloride or trichloroacetic acid as initiators. These initiating systems allow to synthesize fully soluble low molecular weight (Mn = 10003000 gmol1) poly(1,3-pentadiene)s with relatively narrow molecular weight distribution (Mw/Mn < 2.0), which do not contain any high molecular weight and insoluble fractions in the whole range of monomer conversion. The polymers synthesized in the presence of zinc halides possess the same microstructure that those prepared with TiCl4 as coinitiator. (c) 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013