4-Methyl-1,3-pentadiene was polymerized with the MAO/CpTiCl(3) (MAO: methylaluminoxane, Cp: cyclopentadienyl) system to yield a highly stereoregular syndiotactic 1,2-polymer. Fiber diffraction patterns indicate two crystalline modifications: a major one with a repeat of 11.73 Angstrom and minor amounts of a zig-zag planar polymorph (axial repeat 5.05 Angstrom). Energy calculations account for three nearly isoenergetic conformations: the more stable one, with repeat of 11.65 Angstrom, is (T(6)G(2)T(2)G(2)) analogous to the one recently reported for syndiotactic poly(propylene) (Chatani et al., 1991).