Synthesis and Characterisation of Polymeric Materials via RAFT Polymerisation

Abstract

Well-defined polymeric materials incorporating N-vinylpyrrolidone (NVP), vinyl acetate (VAc) and / or N-vinylcaprolactam (NVCL) were synthesised using reversible addition fragmentation chain transfer (RAFT) polymerisation. Chapter 1 is a general introduction on controlled / living radical polymerisation methods, in addition to a brief background on poly(N-vinylpyrrolidone) (PNVP), poly(vinyl acetate) (PVAc) and poly(N-vinylcaprolactam) (PNVCL). Chapter 2 describes the synthesis of RAFT agents (RAFT agents 1-7) used within this study comprising either a dithiocarbamate (RAFT agent 1) or xanthate (RAFT agents 2-7) structure. Several novel RAFT agents with pyrrolidone functionality and based on xanthates (RAFT agents 4-7) were synthesised to improve the RAFT polymerisation of “less activated” monomers (LAMs). Furthermore, multi armed RAFT agents (RAFT agents 9-11) based on xanthates were also synthesised with the aim of generating star-like polymeric structures incorporating LAMs. 1H and 13C nuclear magnetic resonance (NMR) spectroscopy methods were used to characterise the RAFT agents synthesised. Chapter 3 involves the use of RAFT agents 1-8, to mediate the polymerisation of NVP, VAc and NVCL in order to synthesise linear homopolymers with controlled molecular weight and narrow PDI. The kinetics of NVP RAFT mediated polymerisations using novel RAFT agents 5-7 were also investigated and showed that the polymerisations had controlled / living characteristics. Furthermore, the effect of having either primary, secondary or tertiary R groups was explored, for the controlled polymerisation of NVP. RAFT agent 4 which incorporates a primary R group was found to be ineffective in controlling the polymerisation of NVP, whereas RAFT agents with a secondary or tertiary R groups were found to be effective. The resulting polymers were characterised by 1H NMR spectroscopy and size exclusion chromatography (SEC). Chapter 4 focuses on the synthesis of linear block and novel random copolymers incorporating various combinations of PNVP, PVAc and PNVCL. PNVP macroCTA’s (12-14) were used to synthesise PNVP-block-PVAc and PNVP-block-PNVCL, whereas PVAc macroCTA’s (15-17) were used to synthesise PVAc-block-PNVP and PVAc block-PNVCL. Bimodal molecular weight distributions were observed in all the block copolymers synthesised. Novel linear PNVP ran PVAc, PNVCL ran PVAc and PNVP-ran-PNVCL were also synthesised in the presence of RAFT agent 5, with monomodal molecular weight distributions and narrow PDI’s. Chapter 5 describes the synthesis of more complex polymeric structures using multi-armed RAFT agents prepared in Chapter 2 (RAFT agents 9-11). A “core first” R group approach was implemented instead of a “core first” Z group approach to synthesise the polymeric stars, in order to maintain the integrity of the star structure. NVP, VAc and NVCL were polymerised via RAFT in the presence of RAFT agents 9 11, to synthesise Star 1-6. PNVP and PVAc three and four armed stars (Star 1-4) were found to exhibit monomodal molecular weight distributions and low PDI. However, PNVCL three and four armed stars (Star 5 and 6) were found to show bimodal molecular weight distributions. Star 3 (4 arm star of PNVP) and Star 4 (4 arm star of PVAc) were then subsequently used as star macroCTA’s and chain extended with NVP, VAc and NVCL to synthesise novel Star-block 1-4. Star block copolymers were found to either have broad or bimodal molecular weight distributions. In addition, novel three and four armed star random copolymers (Star-random 1-6) were also synthesised via RAFT using RAFT agents 9 and 11, respectively. All Star-random copolymers were observed to have monomodal molecular weight distributions and narrow PDI. Chapter 6 investigates the temperature responsive behaviour of polymeric materials containing NVCL using UV-Visible spectroscopy and optical microscopy. PNVCL synthesised via conventional free radical polymerisation, with a Mn of 9.97 x 104 gmol-1, was found to exhibit an LCST at 33°C. In comparison, linear PNVCL samples prepared via RAFT polymerisation, with Mn ranging from 1.02 x 104 to 2.62 x 104 gmol-1 were observed to exhibit higher LCST’s in the region of 38-40°C. This suggests that the LCST of PNVCL is dependent on the polymer chain length; i.e. “classical” (Type 1) Flory Huggins behaviour. Furthermore, PNVCL synthesised using RAFT agents 2-5 exhibited LCST’s in the region of 39-40°C, which is known as fever temperature. Novel linear PNVCL-ran-PNVP, PNVCL-ran-PVAc and Star-random 2, 3, 5-6 were also analysed to determine their temperature responsive behaviour. The introduction of a hydrophobic (PVAc) and hydrophilic (PNVP) entities into PNVCL is shown to significantly decrease and increase the LCST, respectively. Comparison of the LCST transition range for PNVCL-ran-PVAc synthesised via RAFT and conventional FRP, indicated that the former showed a much narrower transition. Novel Star-random 5 and 6 (four armed random copolymers) were found to have a lower LCST compared to Star random 2 and 3 (three armed random copolymers) despite having similar monomer compositions. A thermal hysteresis was found to be present in all polymer samples, which was attributed to the possibility of weak cross-linking interactions between water molecules and PNVCL carbonyl groups. Chapter 7 is a general conclusion of the work discussed in Chapters 1-6 and future work.