Synthesis, Analysis, and Thermolysis of Copolymers of Methyl Methacrylate With Alkali Metal Methacrylate

This work deals with random copolymers of methyl methacrylate with the lithium, sodium and potassium salts of methacrylic acid. The three series of copolymers together with the related homopolymers, have been prepared in a homogeneous solvent medium under vacuum. The characterization and analysis of the copolymers have been carried out and ultimately, the reactivity ratios for the three systems were obtained. These showed that, during copolymerization, the reactivity of the alkali metal methacrylate radical towards MMA addition, increased with the decrease of the size of the metal ion. This has been explained in terms of variation of the electrostatic forces along the chain (due to cation binding character) with the size of the metal ion. In a second major part, the work deals with the thermal degradation in vacuo of these copolymers. The basic experimental technique used was thermal volatilization analysis with differential condensation of the products, coupled with thermogravimetric analysis. Differential thermal analysis was also used to some extent. The analysis of the degradation products was carried out using mainly infra red spectroscopy and gas liquid chromatography. In general, the copolymers show degradation products similar to those obtained from the corresponding homopolymers, but additional products arise by interaction between dissimilar monomer units along the chains, and this effect also leads to an increase in the overall thermal stability compared with PMMA depolymerization. Two main breakdown stages are discerned during the programmed thermal decomposition of the copolymers: (a) Firstly, depolymerization of MMA segments along the chains occurs in competition with production of methanol presumably originating from the decomposition of pendant ester groups. Methanol production is favoured by the formation of anhydride rings between unlike monomer units after elimination of alkali metal methoxide. At this stage of the reaction, the anhydride structures are also decomposing to yield carbon dioxide, carbon monoxide and isolated methacrylic acid units in the chain which will escape, at a later stage, to the cold ring together with other isolated MMA units. (b) At higher temperatures, the remaining alkali metal methacrylate units, which are more stable, decompose independently in a more or less similar manner to that of their parent homopolymers (production of alkali metal methacrylate and metal isobutyrate by depolymerization, carbon dioxide, carbon monoxide, methane, alkenes, carbonyl containing compounds and a residue consisting mainly of charcoal and metal carbonate). The major degradation products (i.e., methyl methacrylate, methanol, carbon dioxide, metal carbonate, alkali metal methacrylate and alkali metal isobutyrate) were assayed and the results are summarised below: (i) At any particular copolymer composition, the production of methanol and carbon dioxide increases as the size of the metal ion increases, whereas, in this same order, the evolution of MMA monomer decreases significantly. (ii) The depolymerization of the alkali metal methacrylate parts of the copolymers were found to: increase as the size of the metal ion decreases in the composition region ranging between 0 and 50% MMA in the copolymers; increase as the size of the metal ion increases in the rest of the composition region of the copolymers. These results are discussed in detail in chapter V and VI. To determine the origin of methanol, additional experiments were needed: (1) Thermal degradation of PMMA/alkali metal methoxide blends. (2) Considerations on the sequence distribution of the two types of monomer units along the chains. (3) Assay of methanol and methyl methacrylate production under isothermal conditions. In short, copolymerization of methyl methacrylate with alkali metal methacrylates, stabilizes PMMA decomposition and concurrently reduces the MMA monomer yield. This inhibition of depolymerization is believed to result from a direct blockage of the "unzipping" process by the presence along the chains of anhydride rings, formed by interaction between dissimilar monomer units. Because of this blockage phenomenon, chain scission preceding monomer release needs to be re-initiated more often along the chains and this results in an increase in the activation energy for MMA production, an effect which will allow scission to occur, not only along the chains, but also on pendant ester groups, thus allowing methanol formation in significant amounts