New butene-1/1,5-hexadiene copolymers with 0-2.15 mol % methylene-1,3-cyclopentane (MCP) co-units were synthesized by dimethylpyridylamidohafniurn/organoboron catalyst, which for the first time introduces structural unit of five-carbon ring type into the polybutene-1 main chain. The effects of these novel co-units on the crystallization polymorphism, and the featured memory effect that often appears above the equilibrium melting point in copolymers, were investigated with differential scanning calorimetry, wide-angle X-ray diffraction, and polarized optical microscopy. First of all, it was found that the hexagonal form I' can directly crystallize from the melt in the copolymers possessing the two highest co-unit concentrations of 0.65 and 2.15 mol %. With just 2.15 mol % co-unit, pure form I' crystallites were obtained by the isothermal crystallization at 65 degrees C, whereas pure form II appeared at 25 degrees C and a crystallite mixture of form I' and form II was generated in the intermediate temperature region. Such dependence in butene-1/1,5-hexadiene copolymer that high temperature favors form I' formation is opposite to the case of butene-1/propylene copolymer, where low temperature induces more form I'. Second, the crystallization kinetics depends on the temperature of initial melt, referred to a memory effect due to the local segregation of long crystallizable segments in the heterogeneous melt. The critical temperature for observing:this memory effect decreases with increasing the incorporation. Interestingly, this memory effect has no influence on the modification of, the formed crystallites, though the crystallization kinetics is significantly accelerated.