Microgel functionalisation using click chemistry

• Main Author: Farley, Robert
• Published: University of Manchester 2016
• Subjects: 541
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.680005

The research described in this thesis aimed to assess the suitability of the copper(I) catalysed azide-alkyne cycloaddition (CuAAC), a click chemistry reaction, in preparing microgel particles with specific functionalities. A series of acetylene-functionalised microgels were synthesised by the co-polymerisation of propargyl acrylate (PA) with ethyl acrylate (EA), N-isopropylacrylamide (NIPAm) and 2-vinylpyridine (VP). The CuAAC reaction was employed to introduce primary amine functionality to the microgels via their reaction with 2-azido-1-ethylamine (AEA). The reaction proved to be high yielding and the degree of primary amine functionalisation was only limited by the concentration of available acetylene groups in each microgel. However, this was not sufficient to produce the desired pH-responsive microgels and the incorporation of PA into the VP based system severely restricted the swelling capacity of the poly(VP-PA) microgel. An alternative route to high primary amine content microgels was presented by the synthesis and emulsion polymerisation of the azide-bearing monomer 3-azidopropyl methacrylate (AZPMa). A CuAAC reaction between the poly(AZPMa) microgel and propargylamine (PAm) resulted in a pH-responsive microgel with a very high primary amine content. This poly(AZPMa-PAm) microgel demonstrated pH-triggered swelling from 225 nm at pH 9 and above to over 370 nm at pH 7 and below. The use of the CuAAC in precisely controlling the compositions of microgels with specific functionalities was examined via the incorporation of incremental amounts of PA into a poly(VP-co-AZPMa) microgel. This concept was further examined via the CuAAC reaction between a poly(VP-co-PA) microgel and AZPMa. The high efficiency of the CuAAC reaction enabled highly precise control over the extent of functionalisation. The pH-responsive ‘clicked’ microgels demonstrated strong pH-triggered swelling and formed physical gel networks below pH 4. Double-crosslinked (DX) microgels were manufactured by polymerising the pendant alkene groups of neighbouring particles, creating covalent inter-particle linkages. The DX microgels had significantly greater mechanical properties than singly crosslinked (SX) physical gel precursors. DX gel storage modulus (G’) increased linearly with the degree of alkene group functionality. The CuAAC reaction proved to be highly efficiently in introducing desired functionalities to microgel systems. Considerations for the use of the reaction were that high concentrations of ionisable primary amine groups were necessary to produce a pH-responsive microgel and the incorporation of either PA or AZPMa into pH and temperature responsive systems resulted in microgels with reduced swelling capacities. The reaction may therefore be of most use at low levels of functionalisation, or in microgel or hydrogel systems in which a swelling response is not of paramount importance.