The effect of side methyl and dimethyl groups of the soft segment component on the microphase-separated structure and mechanical properties of polyurethane elastomers (PUEs) was investigated. Poly(oxytetramethylene) glycol (PTMG), and PTMG incorporating dimethyl groups (PTG-X) and methyl side groups (PTG-L) were used as a polymer glycol, which forms a soft segment in the PUEs. The PUEs were synthesized with 4,4'-dipheylmethane diisocyanate [1,1'-methylenebis(4-isocyanatobenzene)], 1,4-butane diol, and 1,1,1-trimethylol propane by a prepolymer method. The degree of microphase separation of the PUEs became weaker with increasing side group content in polymer glycols. Dynamic viscoelastic properties measurement showed reorganized-crystallization and melting of the soft segment for the PUEs based on PTMG, PTG-L, and PTG-X with a lower content of the side groups, but not for a PTG-L and PTG-X with higher content of the side groups. Tensile testing revealed that increasing methyl group concentration made the PUEs soften and weaken. The PTMG-based PUEs obviously exhibited strain-induced crystallization of the soft segment chains during elongation process. In contrast, for the PTG-L and PTG-X-based PUEs, crystallinity decreased with increasing side group content, and the PUEs with PTG-L and PTG-X with highest methyl group content did not crystallize even at a large strain.(C) 2008 Wiley Periodicals, Inc.