Addition — Fragmentation type initiators for cationic polymerization

Abstract

Addition — fragmentation reactions are a versatile and exceptionally interesting tool in preparative polymer chemistry. With the aid of specially designed allyl‐onium salts (addition — fragmentation agent, AFA) in conjunction with conventional radical initiators it is possible to very efficiently initiate cationic polymerizations. The mechanism involves (a) the formation of radicals by heating or irradiating the radical initiator, (b) the addition of these radicals to an AFA molecule and (c) the fragmentation of the AFA. In step (c) radical cations are produced, which with high rates initiate the polymerization of cationically polymerizable monomers (e.g., cyclohexene oxide, CHO). In order to show good performance in initiation reactions, AFAs have to be equipped with functional groups or substituents which either promote radical addition or facilitate the fragmentation of reaction intermediates.

The Effect of the Heteroatom Moiety of Allylic Salts on the Addition Fragmentation Initiation of Cationic Polymerization

“…Previously experimental support for this mechanism [19] was obtained by the detection of the low-molecular weight fragmentation products. Considerable lowering of the initiation temperature by introduction of the allylic structure seems to be attributable to the participation of the radicals in the addition fragmentation scheme.…”

Control of reactions and network structures of epoxy thermosets

“…Similarly to photolatent onium initiators that may be activated through the use of free radical coinitiator upon visible light, they suggested to activate thermolatent cationic initiators by using free radical sources that are released at lower temperature than the normal onium salt's temperature threshold [349][350][351][352]. To this end, they developed a series of specially designed allyl onium salts.…”

Photoinitiated polymerization of ethyl cyanoacrylate by phosphonium salts

“…During the past two decades considerable attention has also focused on technically applied cationic photopolymerizations involving vinyl ethers and epoxides as monomers 2) . The major classes of photoinitiators which can successfully be applied to initiate cationic polymerization include diaryl iodonium 3) , triaryl sulfonium 4) and Nalkoxy pyridinium salts 5) , and iron arene complexes 6,7) .…”