1 Bio-based elastomer: highly stereoregular poly--myrcene from a lanthanide-based coordination catalyst S. Loughmari 1,2,3,4,5 , A. Hafid 6 , A. Bouazza 5 , A. El Bouadili 5 , P. Zinck 1,2,3,4 , and M. Visseaux* 1,2,3,4 1 Université Lille Nord de France, F-5900 Lille, France 2 ENSCL, UCCS, CCM, F-59652 Villeneuve d’Ascq, France 3 USTL, UCCS, CCM, F-59655 Villeneuve d’Ascq, France 4 CNRS, UMR8181, F-59652 Villeneuve d’Ascq, France 5 Laboratoire de Chimie Appliquée et Science de l’Environnement, USMS - FST-BM - B.P 523 - 23 000 Béni-Mellal – Maroc 6 USMS - FST-BM - B.P 523 - 23 000 Béni-Mellal – Maroc Abstract. Polymerisation of -myrcene with neodymium borohydride based coordination catalysts affords poly--myrcene (polymyrcene) in high yield. With stoichiometric amounts of n- butylethylmagnesium (BEM) as co-catalyst, good control over macromolecular data along with a cis-stereoselectivity up to 98.5 % are obtained. Transfer reactions between neodymium and magnesium are observed in the presence of excess BEM whereas the selectivity switches to 3,4-. With aluminum triisobutyl associated to a borate activator, poorly soluble material is isolated, likely due to the occurrence of cross-linking. Graphical abstract. 1. Introduction