Bromoform-assisted Free Radical Polymerisation: A Simple, Inexpensive Route For the Preparation of Block Copolymers

Abstract

The last three decades have marked unprecedented advances in polymer chemistry enabling the production of a wide range of well-defined block copolymers. Such macromolecules are crucial for structure-property relationship studies, bulk block copolymer self-assembly and in the pursuit of sequence-controlled macromolecules for biomimicry. However, in most cases the conventional RDRP (reversible-deactivation radical polymerisation) techniques, used to synthesise such materials, rely on toxic transition metals, sulfur or unstable compounds to provide control and often produce inherently coloured polymers [e.g. in the case of reversible addition-fragmentation chain transfer (RAFT)]. This highlights one of the key challenges in polymer chemistry; the need to produce block copolymers without the use of sulfur or transition metals. In the quest for commercially relevant block copolymer materials, for which overall average molecular composition is key but molar mass distribution is of little importance, a straightforward, sulfur- and metal-free aqueous route to block copolymers using commercially available starting materials is described. Based on synthetic techniques first described in the 1950s for hydrophobic monomers in organic solvents, the alkyl halide bromoform (CHBr3) has been used to synthesise block copolymers. Unlike common bromine-containing chain transfer agents such as carbon tetrabromide (CBr4), bromoform is partially water-miscible and relatively inexpensive. In addition, bromoform is readily available, stable (easily stored) and can be used directly at low and ambient temperatures. Interestingly, bromoform has been reported to photodissociate under UV light and as a result of this the reactions described in this thesis are conducted under UV conditions. Herein, this new aqueous-based technology has been studied using N,N-dimethylacrylamide (DMA) and N-isopropylacrylamide (NIPAM) as exemplar monomers to synthesise poly(N,N-dimethylacrylamide)-block-poly(N-isopropylacrylamide) [PDMA-b-PNIPAM] block copolymers of varying composition directly in water. Detailed kinetic studies, using this bromoform-assisted polymerisation technique were conducted to identify the optimal conditions for synthesising potentially bromine-terminated PDMA and PNIPAM macro-initiators for subsequent chain extension. Following these kinetic studies, PDMA (made using 2 mol % bromoform, relative to monomer) was used as a macro-initiator for subsequent PDMA-b-PNIPAM copolymer synthesis. Both one-pot and two-step studies were conducted to identify potential routes to block copolymer synthesis. The one-pot study was completed as the simplest, cheapest route to forming the PDMA-b-PNIPAM copolymers. However, due to unwanted impurities formed during the one-step synthesis, alongside the need to understand the process in unprecedented detail, a two-step synthetic route was explored. The two-step synthetic route was completed using PDMA macro-initiators (using PDMA synthesised to both 91 and 70 % conversion) in order to further optimise the methodology. Finally, a series of control reactions were conducted to provide further evidence that bromoform was required to impart the reversibly-cleavable chain end functionality under UV-irradiation, for block copolymers to be formed. Additionally, control reactions were undertaken to further indicate that block copolymers were formed in this study; demonstrating the potential of this technique as a simple, inexpensive route for the creation of functional block copolymers.