Neodymium trisborohydride, associated with MgR2 alkylating agents, affords an efficient initiator for the stereospecific polymerization of isoprene. This system presents good activity (150 kg of polyisoprene/mol of Nd/h; 95% yield in 2 h) and is trans‐stereospecific (up to 97.7%), and its polymerization shows a quasi‐living character. The molecular weights are controlled by the [Isoprene]/[Nd] ratio, with a polydispersity index lower than 1.6. A full study is presented here, including the influence of the nature of the alkylating reagent (MgR2), the solvent, the reaction temperature, the [MgR2]/[Nd] ratio, and the replacement of neodymium by other lanthanides. This work also includes a kinetic study of the system and a detailed NMR study of the microstructure of the trans‐polyisoprene obtained, providing mechanistic insights about the polymerization reaction. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 2400–2409, 2007

We have studied the dynamics of amorphous 1,4-trans-poly(isoprene) (trans-PI) with different molecular weights using broadband dielectric spectroscopy over a wide range of temperature ∼T g -70 K to T g + 130 K, and frequencies (10 -1 to 10 6 Hz). Avoiding crystallization by quenching, three dielectric processes (global chain, segmental and local relaxations) were observed in order of decreasing temperature. Both the segmental and local relaxations of the here studied trans-PI, were found to be markedly different compared with the corresponding processes in cis-PI. In contrast, the temperature dependence of the relaxation time related to the global chain mode was found to be independent of the chain configuration. However, its relaxation strength was 1 order of magnitude lower in trans-than in cis configuration indicating a small parallel component of the dipole moment in trans configuration. In addition we also studied the dielectric spectra of trans-PI upon crystallization. The kinetics during crystallization as observed by dielectric spectroscopy results independent of molecular weight. During isothermal crystallization the dielectric spectra can be analyzed by a combination of three processes: the R m -relaxation related with the modified amorphous phase, the so-called R CAP -relaxation induced by crystallization and the β-relaxation, the two later only varying in relaxation strength. At the end of isothermal crystallization a percentage of crystallinity about 30% was estimated.