Boron-catalyzed polymerization offers unique C3-polymer structures because, in contrast to 1,4-polydienes, each double bond is separated by only one methylene group. However, the geometrical regularity of such unique C3-structures was less discussed, and their properties have not been reported. In this work, well-defined poly(2-methyl-propenylene)s with different molecular weights are prepared in the gram scale by boron-catalyzed polymerization of 2-methylallyl arsonium ylide. 1H NMR, 13H NMR, and two-dimensional 1H–13C heteronuclear single quantum coherence NMR spectra confirm the high selectivity toward trans-configuration (>99%) and C3 monomeric insertion (>98%). Density functional theory (DFT) calculations at the wb97xd/tzvp level (solvent = tetrahydrofuran) explain the high trans/C3. Furthermore, the thermal parameters, Tc, Tm, Tg, ΔHc, ΔHm, and crystallinity degree, of poly(2-methyl-propenylene)s are determined by differential scanning calorimetry (DSC), fast scanning chip calorimetry (Flash DSC), and Wide Angle X-ray Scattering (WAXS) for the first time and are compared with those of trans-polyisoprene.