Quaternary boron salts are used as initiators of polymerisation in adhesive compositions for bonding low surface energy substrates such as polyolefins. The quaternary boron salts are of the formula I ##STR00001## wherein R.sup.1 is C.sub.1 C.sub.10 alkyl, R.sup.2, R.sup.3 and R.sup.4, which may be the same or different, are, C.sub.1 C.sub.10 alkyl or C.sub.3 C.sub.10 cycloalkyl, phenyl, or phenyl-substituted C.sub.1 C.sub.10 alkyl or C.sub.3 C.sub.10 cycloalkyl, provided that any two of R.sup.1 R.sup.4 may optionally be part of a carbocyclic ring, and M.sup.+ is a metal ion or a quaternary ammonium ion. Particular examples of initiator compounds include sodium tetraethyl borate and lithium tetraethyl borate, lithium phenyl triethyl borate and tetraethylammonium phenyl triethyl borate.