Oxidation-responsive polymers are useful in the field of drug delivery as they allow the targeted release of drugs in areas where there is a high concentration of reactive oxygen species, i.e. from autoimmune diseases. In this thesis a variety of dual temperature- and oxidation-responsive materials were synthesised.
In chapter 2, a series of polymers of N-isopropylacrylamide (NIPAM) were synthesised by Reversible Addition−Fragmentation chain-Transfer (RAFT) polymerisation with varying degrees of polymerisation from 10 – 100 using different chain transfer agents (CTA). These polymers were oxidised to determine the applicability of the RAFT CTA end group as oxidative triggers for morphological change. Full polymer characterisation was performed before and after oxidation via 1H NMR, GPC, IR spectra and mass spectra and their cloud points were determined. A difference was seen in the colour of the polymer samples, from yellow to white on oxidation which indicated cleavage of the RAFT chain end group. This was confirmed by mass spectrometry showing cleavage of the Z group of the RAFT agent converting the trithiocarbonate to a sulfonic acid group as the major product. This changed the hydrophobicity of the polymer as can be seen with an increase in the cloud point. This effect was greater for shorter chain polymers as the ratio of polymer to RAFT chain transfer agent is lower. When the RAFT agent was changed to one which was dodecyl based and therefore more hydrophobic, there was no change in cloud point and the mass spectra were complex showing multiple species present after oxidation.
To determine whether the cleavage of the RAFT chain end group could cause disassembly of self-assembled polymers, a series of short pNIPAM block diblock copolymers were synthesised using three different RAFT agents and N-(2-Hydroxyethyl)acrylamide (HEA) as the water-soluble macro chain transfer agent (mCTA). These were analysed by DLS to assess any self-assembly above the LCST. DLS showed that self-assembly did not occur above the LCST.
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Using MPP and PPA as the RAFT agents, a series of pHEA-b-pNIPAMn copolymers were synthesised with varying lengths of the pNIPAM block from a targeted length of 100 – 300. Also, diblock copolymer were synthesised with 2 mol% of BAPE, an oxidation responsive monomer incorporated into the pNIPAM block, pHEA-b-p(NIPAM-stat-BAPE)n, with the targeted DP of these blocks the same as above. The samples with a temperature responsive block length of over 150 units showed self-assembly above the LCST and their effective diameter, as determined by DLS was less than 100 nm. On oxidation of the polymer with a pNIPAM based hydrophobic block, no change in size is seen, however when BAPE is incorporated into the hydrophobic block of the polymer there is an increase in size of the particles as seen by DLS. The change in the LCST on oxidation of BAPE does not cause the polymer chains to become soluble at 37 °C and self-assembly is still observed.
Complementary nanogels (NG) were synthesised of pHEA-b-pNIPAM and pHEA-b-p(NIPAM-stat-BAPE) with the core crosslinked with varying ratios of N,N′-Methylenebis(acrylamide) during synthesis of the second block. This was to ensure more stable assembly of the particles than the micelles formed above. The targeted lengths of the second block were the same as with the above diblock copolymers. The conversion was determined via moisture analysis and the particle size was determine by DLS and TEM. There was a difference seen in the trend of size and morphology between the NG with and without oxidation responsive groups, yet both shrunk in size on heating to temperatures above the LCST.
The oxidation responsive behaviour of the nanogels with H2O2 was determined in detail using DLS to analyse the particle size and by TEM to assess the morphological difference between the native and oxidised particles. The particles swell on oxidation when the nanogels contain BAPE with the amount of swelling being dependant on the crosslinker concentration in the core. The higher the concentration of H2O2, the faster the response, however the nanogels are sensitive to concentrations as low as 0.1 mM. The sensitivity to reactive oxygen species other than H2O2; SIN-1
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and hydroxy radicals, and a common reducing agent, glutathione, was also determined using in situ DLS studies, which showed that these nanogels are specifically sensitive to H2O2.
In-depth analysis of the morphology of numerous samples was performed. The combination of DLS, SAXS, TEM and AF4 combined with DLS and MALS allowed for successful determination of the size and shape of some of the NGs. This showed that these nanogels were composed of two different particle morphologies, and therefore particle sizes, usually with one more elongated, although the length of these varied. The NGs with BAPE in the core had more well-defined morphologies than those with pNIPAM alone. Overall, a more accurate view of these particles was achieved by minimising the limitations of individual techniques.
