Abstract: In one of several methods of producing polybutadiene or a butadiene-styrene copolymer with an organolithium initiator, an organotin compound such as tetraallyltin, tetrabenzyltin and tetraphenyltin is added to the polymerization system in a period of growth of the polymer chain between a time immediately after a start of the polymerization to a time before an end of the polymerization. In another of the methods, tin tetrachloride or tin dichloride is added at an earlier time after a start of the polymerization. A polymer comprising a considerable amount of high molecular weight polymer containing the chain having a tin-carbon linkage is formed. A rubber composition for tires comprises specified amounts of the polymer described above, natural rubber, carbon black and vulcanizing agents. The rubber composition has a small loss factor and excellent fracture properties.

Abstract: A method for preparing a polymer having low hysteresis loss value and good breaking properties using a novel lithium initiator compound for polymerization of a conjugated diene and/or an aromatic vinyl hydrocarbon. Polymers produced in accordance with this method contain tin in an amount more than usual, and can be manufactured with good productivity.

Abstract: A method for preparing a polymer having low hysteresis loss value, good breaking properties and good processability with good productivity is provided by adding a tin compound during the propagation step of polymerization of a conjugated diene and/or an aromatic vinyl hydrocarbon. Polymers produced in accordance with this method have high molecular weight and star-shaped structures with branches having the same or different molecular weight.