Price: 10.00
INTRODUCTION
Cationic polymerization of varios functionálized alkenylethers and oxiranes have often
been induced photochemically by a direct photolysis of an óniurn salt. The use of high
energy irradiation (yrays) and EB (Electron Beam ) are technically not well developed
for the use in cationic polymerization, despite the very efficient utilization of the energy
input compared to the acrylate free radical chemistry used today.The cationic induced
polymerization involves a cleavage of onium salts in order to produce the active
Brönsted acid. As alternatives to direct photocleavage of the onium salt, little
attention have been given to latent thermally activated initiators and indirect routes.
However diaryliodonium, dialkyl-4-hydroxyphenylsulfoniuml, phenacylsulfoniuml,
2,5-dihydroxy-phenylsulfonium and benzylsulfonium sals can all be triggered
thermally and in some cases also by a photoredox process to induce a cationic
polymerization.
The positive charge at the sulfoniumgroup makes these salts susceptible to
nucleophilic attacks, usually a SN2 process. Attack at the positively charged sulfur
atom is not as common as an attack at a carbon atom adjacent to the sulfur.
Polarization by the sulfonium group and its ability as a leaving group directs the
nucleophilic attack to a carbon attached to the sulfur atom. Unimolecular mechanisms
are also possible in sulfoniumsalts containing a group that after cleavage can form a
stable carbocation, e.g., benzylsulfoniüm and tert.-butyldimethylsulfoniumsalts. For
benzylsulfonium and 2,5dihydroxyphenylsulfonium salts the structure determines
whether the SN1 or SN2 mechanism predominates. By kinetic parameter
considerations based on the Arrhenius equation, Pappas9 has shown that the first
order mechanism is preferable. The ratio of SN1 to SN2 is strongly dependent on the
bulkiness of the substituents adjacent to the sulfur Increased bulkiness will be in
favour for the SN1 mechanism. In common for these types of latent thermally:or
photoredox activated initiators, are that the activation leads to a fragmentation,
dissociation of the initiator molecule into decomposition productsofIower molecular
'weight along with the initiation of a cationic polymerization.
