Polymerization of beta-myrcene with neodymium borohydride-based coordination catalysts is very efficient, affording poly-beta-myrcene (polymyrcene, PMy) with high selectivity. With stoichiometric amounts of n-butylethyl magnesium (BEM) as co-catalyst, good control over macromolecular data along with cis-stereoselectivity up to 98.5%, are obtained. In the presence of excess BEM, high level of transfer reactions efficiency between neodymium and magnesium is clearly evidenced whereas the selectivity switches to 3,4-rich. Combining the neodymium pre-catalyst with triisobutyl aluminum in the presence of a boron activator affords PMy in good yield, but the polymer material displays low solubility, likely due to the occurrence of crosslinking. (C) 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012