~et--The photoinitiated polymerization of methyl methacrylate by triethyl-aluminium is shown to occur by a free radical mechanism, initiated by a photoexcited state of a 1 : I Wittig '-ate' complex between MMA and Et3AJ.
A survey of related compounds shows that only monomers having an unsaturated group, containing an atom possessing filled, non-bonding orbitals, conjugate with the vinyl group, polymerize in the presence of Et3Al at ambient and below ambient temperatures. The formation of a complex between monomer and Et3AI is a prerequisite for polymerization. However, these complexes, e.g. MMA:ALEt3, can initiate polymerization of monomer inert to EtjAI, e.g. styrene.
No anionic potymerization occurs in the MMA:EtaAI system. However a 1:2 MMA:Et3AJ complex, which prevails when Et,Al is in excess, undergoes rapid reaction in the dark to form nucleophilic addition products involving the carbonyl group of the monomer. WHILE the alkyl metal compounds of Group Ia and I[a initiate anionic polymerization of methyl methacrylate, the unstable compounds of Group Ib and certain transition metals initiate free radical processes. (I~ In Part II1 of this series, (2) we reported preliminary results on initiation by triethyl-aluminium. The evidence concerning the activity of trialkyl-aluminiums has been conflicting. Even if certain reports could be disregarded on the grounds that traces of oxygen can lead to a spurious radical mechanism by way of peroxide intermediates, (3' *~ a serious contradiction remained even where the techniques seemed unexceptionable. Minsker reported that, while tri-isobutyl-aluminium polymerized methyl methacrylate at 60 ~, triethyl-aluminium did not. (5~ This is contrary to earlier reports cited in patents, c2) In our previous paper, C2~ we showed that initiation by triethyl-aluminium was photochemical, involving visible or u.v. light. This is one possible explanation, though the crucial factor, viz. which of the conflicting experiments were carried out under ambient illumination, cannot be ascertained from the published reports. As we reported previously, ~2~ the true initiator is a yellow Et3AI:MMA complex. At that time, the evidence was balanced in favour of a non-radical polymerization. This can no longer be sustained. The evidence reported below leaves little doubt that radical polymerization occurs. Anionic polymerization does not develop; the nucleophilic chains are stopped short at the first adduct. i. EXPERIMENTAL General purification and manipulation procedures have been described previously. (2~ Dioxan and tetrahydrofuran were purified by passage through a column of" freshly dried alumina and were