Polymerization of Methyl Methacrylate by Organometallic Compounds. V. Dark Stoichiometric and Polymerization Reactions between Trialkylaluminums and Methyl Methacrylate

Abstract

The stoichiometric reaction products of the reaction of triethylaluminum and methyl methacrylate (MMA) derive from a different complex to that responsible for the photosensitized, radical polymerization. The stoichiometric products are the result of nucleophilic attack on the carbonyl group of MMA (1,2 addition). No 3,4 nucleophilic adducts are found and it is questioned whether the one product usually believed to be the result of conjugate (1,4) attack does so arise. The reactions have been followed, in situ, using 60 MHz NMR and mechanisms are discussed. The nucleophilic addition reactions do not develop into an anionic polymerization chain. The equilibrium constants governing MMA-triethylaluminium complex formation are such that the 1:1 complex, the precursor of radical polymerization, prevails overwhelmingly in MMA-rich mixtures and the precursor of stoichiometric, nucleophilic, addition reactions prevails in A1-rich mixtures. The validity of diagnostic tests for polymerization mechanism based on the nature of concomitant, low molecular weight products is discussed. It is confirmed that triethylaluminium does not initiate MMA polymerization in the dark over the temperatures 233-333°K. We find diisobutylalumin-ium hydride inert, in dark or light, at 298 and 333°K. Triisobutylaluminium only initiates MMA when illuminated at 298°K, but at 333°K there is also a significant dark rate. Preliminary copolymerization experiments, devised to elucidate the mechanism of this dark polymerization, suggest that, as in the case of the photo-sensitized, triethylaluminium-initiated, radical polymerization, it proceeds from a 1:1 MMA: trialkylaluminium complex.