Price: 10.00
Vinyl ethers are well established as radiation curable materials exhibiting a wide diversity of chemistry 1]. A good selection of reactive monomers and prepolymers is commercially available. These materials possess several attractive features such as low toxicity and high reactivity. The cationic cure (both uv and eb initiated) of these materials is particularly rapid often far outstripping the cure of epoxides. Since the cationic cure is a vinyl polymerisation process it is not surprising that shrinkage occurs on cure but this is no more than that exhibited by (meth)acrylates having the same percentage polymerisable groups per kilogramme. The use of epoxide-vinyl ether combinations alleviates this problem and the presence of the vinyl ether increases the cure speed compared to a formulation in which the vinyl ether is absent 21. Many of the vinyl ethers exhibit good solvating properties and have been used as reactive diluents fo this purpose. An example of such a use is that of vinyl ethers being used as diluents with epoxidised polybutadiene 3]. Another feature of vinyl ethers is the electron richness of the double bond and makes them susceptible to attack by electrophilic radicals such as thiyl radicals. The radical produced by attack of a thiyl radical on the terminal methylene of the vinyl ether is stabilised by the oxygen atom of the ether group and therefore it is not surprising to find that vinyl ethers are very reactive ene cOmponents of the thiol-ene reaction 4] The ether substituent on the double bond reduces the ionisation potential of the alkene and it is well established that vinyl ethers form charge transfer complexes with electron deficient species such as alkenes carrying electron withdrawing groups and carbonyl compounds such as quinines 5]. The electron donor properties become magnified when one of the components is in an excited state e.g.vinyl ether quinone systems. Of particular relevance to radiation curing is the reaction of vinyl ethers with maleate and fumarate esters which leads to the formation of 1:1 alternating copolymers. This reaction radical mediated and is initiated by the use of Type I photoinitiators. This reaction has found use in powder coating formulations which have been commercialised. The more powerful electron
accepting alkenes, maleimides form charge transfer complexes with vinyl ethers which manifest
themselves by giving rise to new absorption bands. Excitation into the charge transfer band has been shown to lead to polymerisation with the formation of a 1:1 alternating copolymer. Such systems have been coined as initiator-free systems 6]. An attractive feature of these systems is that the maleimide is consumed in the reaction and as a consequence the cured coating does not contain photoactive species. Vinyl ethers have been used as diluents for acrylate oligomers where they not only act as good solvents but they also increase the percentage of acrylate groups utilised in the curing process 7]. If such a hybrid system is cured solely using a radical initiator, then the percentage of vinyl ether used must be kept to below 10% if unreacted vinyl ether is not to be present in the cured coating. If higher percentages of vinyl ether are to be used then use of a mixture of cationic and radical initiators is to be recommended. Propenyl ethers have been used far less in radiation curing than vinyl ethers and this probably reflects the fact that very few are commercially available. The synthesis of propenyl ethers from allyl ethers is reasonably straightforward and may be carried out using a base catalyst (giving the trans isomer as the sole product) or with a ruthenium catalyst (tris(triphenylphosphine)ruthefliUm (II) dichloride) which leads to a mixture of cis and trans isomers being formed 8]. Not surprisingly the cis and trans
isomers cure cationically and at similar rates 9]. The radical mediated cure of a propenyl ether
admixed with a maleate ester has been shown to be a little slower (based on results from Photo DSC runs) than the reaction in which a comparable vinyl ether is used.
The present work was undertaken to establish the relative reactivity, under uv curing conditions of
vinyl and propenyl ethers 9] in both cationic and radical mediated reactions.
