Visible light induced conventional step-growth and chain-growth condensation polymerizations by electrophilic aromatic substitution

Abstract

The importance of photochemical processes has been on the rise over the last two decades, this being both due to their numerous self-evident advantages over conventional processes, and the ever growing need for making existing processes more environmentally friendly, which is among the superior aspects of photochemical processes in contrast to conventional ones. Some of the most prominent features of photochemical processes are: spatial and temporal control, cost-effectiveness, the ability to work at low temperatures, and as formerly mentioned, being eco-friendly. Photochemical processes, particularly photoinduced polymerizations, have various applications, including dental fillings, 3D printing, inks, adhesives, coatings, and photoresists. The implementation of photomediation into step growth polymerization, which is widely used and is of the two general modes of polymerization, has thus far received less investigation and development compared to its alternative. PPM, also known as "polybenzyl", is one of the earliest compounds synthesized belonging to the class of poly(phenylene alkylene). Its usual methods of synthesis involve the use of strong Lewis or Bronsted acids in order to induce the step growth condensation polymerization of either benzyl halides or benzyl alcohol. PPM polymers are known to have a very interesting property, that is photoluminescence activity in the absence of conventional mechanisms that may cause a compound to fluoresce, such as pi conjugation along the polymer chain or excimer formation as seen in compounds like anthracene and pyrene. In this thesis, a PPM derivative has been synthesized by the visible light induced step-growth cationic polymerization of the monomers dihalomethane and dimethoxybenzene in the presence of Mn2(CO)10 and an onium salt. The pathway that was chosen for this polymerization reaction was expected to lead to a situation where growing chains would have higher reactivities than monomers, leading to polymerization kinetics that would differ from being purely step growth. Such polymerization reactions, called chain condensation polymerizations (CCP), in which the reactivities of dimers, trimers and so on towards monomers, are higher than the reactivities of the monomers towards themselves, have been established by others in the literature, and are known to respond positively to deviations from the 1:1 stoichiometric ratio, contradictive to the trends seen in step growth polymerizations. Polymerizations using different monomer to monomer ratios were carried out and the polymer molecular weights were indeed found to increase with larger stoichiometric imbalances rather than decline, confirming that the polymerization follows a CCP mechanism at imbalanced monomer ratios. Moreover, the telechelic nature of the polymer was evaluated with a grafting reaction that was carried out using the same cationic photoinitiating system and PPM was successfully grafted onto a poly(MMA-co-GMA) prepolymer as confirmed by GPC and 1H-NMR measurements, confirming the presence of bromine terminating ends on the polymer.