Development of a new synthetic polymerisation technique for the fabrication of speciality block copolymers

Abstract

Block copolymers have many applications in a wide range of industries. In medicinal research, the production of amphiphilic block copolymers has gained a lot of interest, for drug delivery and tissue engineering. The current methods used for the block copolymer synthesis are controlled radical polymerisation such as RAFT, ATRP and NMP. These methods have many drawbacks such as a limited selection of monomers for ATRP, specific reaction conditions for NMP and expensive purchasing costs for RAFT agents. Recently, there has been a surge in research to identify alternatives to these techniques. The addition of alkyl halides to radical polymerisation is one alternative which has gained some attention. In the literature, brominated haloalkanes have been used as transfer agents, but the use of bromoform has not been studied extensively. Articles suggested polymers produced using bromoform lead to labile end groups which can undergo further polymerisation. This project focuses on understanding the role of bromoform in radical polymerisation and producing block copolymers using this method. The results from bromoform-mediated polymerisation showed polymers produced using this technique offered some control for the molar mass for the polymerisation of styrene and MMA. Generally, low disparity polymers were produced when compared to those produced by the conventional radical polymerisation. Using high concentrations of bromoform resulted in low molar mass polymers being obtained (Ð < 1.5) . The results from thermal polymerisation of MMA showed bromoform can behave as a thermal initiator at temperatures above 60 °C. Temperatures below 60 °C resulted in low conversions, and high molar mass polymer being achieved. The activation energy for the thermal polymerisation of MMA using 1 molar equivalents bromoform as a thermal initiator in THF was calculated using an Arrhenius plot, Ea = 95.1 kJ/mol. Following this, attempts were made to produce PMMA-PS block copolymers using the homopolymers produced from the bromoform-mediated polymerisation. It was discovered the order of homopolymer used as macro-CTA/macro-initiator plays an important role in the formation of the block copolymer. The results showed copolymers can be produced using PMMA (Mn = 24,000 g/mol, Ð = 1.56) as a macroinitiator for the polymerisation of styrene monomer in THF. The target ratio of MMA to styrene units for the copolymer was 1:1 in the initial study. The kinetic studies showed a linear progression in molar mass and monomer conversion. GPC data showed the final polymer had an Mn = 54,000 g/mol, and a broad, but unimodal distribution dispersity (Ð = 2.41). The actual ratio from 1H NMR was found to be equal to 1:3.4. Following this, further experiments were conducted with different degree of polymerisations. The research conducted on the polymerisation of styrene and MMA under non-aqueous system has been largely successful. The addition of bromoform to a radical polymerisation has a few advantages: The purchasing price of bromoform is relatively cheap compared to other CTAs and its solubility in water, which could allow reactions to be conducted under aqueous media. For a better understanding of the role bromoform plays in polymerisation reactions, characterisation techniques such as MALDI-TOF should be used to identify the functionality of the polymer end groups.