A solution polymerization process for obtaining isotactic, crystalline or crystallizable 1-butene/propylene polymers comprising contacting under polymerization conditions at a temperature ranging from 50° C. to 90° C. 1-butene and propylene in the presence of a catalyst system obtainable by contacting: a) at least one metallocene compound; b) an alumoxane or a compound capable of forming an alkyl metallocene cation; and optionally c) an organo aluminum compound; wherein the polymerization medium consists of a mixture of 1-butene and propylene wherein the content of propylene in the liquid phase ranges from 1% to 60% by weight.