AbstractThe presence of the sterically hindered Lewis acid, trimesityl boron, during the anionic polymerisation of isoprene is shown to affect the structure of the resulting polymer only marginally. This behaviour contrasts with its effect when the monomer is butadiene. The principal change is an increase in 1,4‐content at the expense of 1,2‐structure, with the proportion of 3,4‐structure remaining unaffected. These results are explained in terms of the isomeric forms of the propagating