A Theoretical and Experimental Investigation of a Photochromic RAFT Agent and the Photoiniferter Effect

Author: Oskar Majewski

Majewski, Oskar, 2019 A Theoretical and Experimental Investigation of a Photochromic RAFT Agent and the Photoiniferter Effect, Flinders University, College of Science and Engineering

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Reversible Fragmentation Chain Transfer (RAFT) polymerisation is increasingly utilised in research and industrial applications due to its ability to polymerise monomers with diverse reactivity and functionality across a wide range of polymerisation conditions with both conventional and photoinitiator systems. Two key limitations exist; tailoring the class of RAFT agent to the reactivity of the monomer being polymerised and polymerising monomers of sequentially lower reactivity during block copolymer synthesis.

In this thesis, the feasibility of a model photochromic RAFT agent whose reactivity towards radical addition and fragmentation can be switched remotely through a photoswitchable Z group was explored.

Density Functional Theory level quantum chemical studies showed that spirooxazine based RAFT agents (xanthate and dithiocarbamate) displayed changes to both their LUMO energy levels and electron density within the RAFT moiety, indicating potential changes in reactivity. Thermodynamic parameters used to qualitatively predict RAFT agent reactivity based on ab initio theory confirmed that the closed and open states of both spiro-RAFT compounds will have different reactivity towards radical addition and fragmentation, enabling controlled polymerisation of Less Activated Monomers (LAMs) and More Activated Monomers (MAMs) respectively.

A novel spirooxazine based xanthate (spiro-XEP) and its non-photochromic analogue (PXEP) were synthesised and tested in the polymerisation of methyl acrylate (MA), a typical MAM and vinyl acetate (VAc) a typical LAM under typical RAFT conditions both with and without UV irradiation.

The rate of MA polymerisation in the dark initiated by AIBN was the same for both PXEP and spiro-XEP. Irrespective of the presence of AIBN, with UV irradiation the polymerisation rate of MA increased by 10x and 2.67x for PXEP and spiro-XEP, respectively. Good agreement between the expected and obtained molecular weights and narrow dispersities (D < 1.25) were obtained with both RAFT agents, indicating control was maintained. Chain extension kinetics with a spiro-XEP macroinitiator replicated these trends, demonstrating living characteristics and that the main RAFT equilibrium reaction dominated polymerisation behaviour.

For VAc polymerisation with PXEP under UV irradiation, an enhancement was observed but it was only 1.15x higher than the AIBN alone. With spiro-XEP severe rate retardation was seen, only being 2% that obtained with PXEP under equivalent dark conditions. Furthermore, the polymerisation rate did not change with UV irradiation. In both cases there was good agreement between expected and obtained molecular weights, with dispersities being narrower with spiro-XEP. Solutions containing spiro-XEP underwent a series of colour changes when in the presence of UV and/or thermally generated radicals species. For polymerisations, the intensity of the colour changes depended on the monomer used and conversion attained. The evidence suggests that these colours arise due to radical reactions with the spiro-XEP compound which are non-reversible in nature.

Modelling the polymerisations of MA and VAc with PXEP in Predici revealed that for MA the dominant photolysis pathway and thus primary source of initiating radicals was the reversible photolysis of RAFT capped species, whilst for VAc it was the photolysis of AIBN. Under UV irradiation the model revealed that for both polymerisation systems the RAFT mechanism was responsible for molecular weight and dispersity regulation, with no evidence of propagation from the thiyl radical generated through the photolysis of RAFT species.

Finally, the limitations on leveraging the photoiniferter effect to synthesise block copolymers with block orders that are “forbidden” by the RAFT process was investigated. This involved the synthesis of copolymers with blocks comprised of methyl methacrylate (MMA), styrene (Sty), MA, and VAc. A variety of RAFT agents with identical R groups including PXEP, spiro-XEP and a trithiocarbonate were used to investigate the effect of RAFT agent class. Initiation systems ranging from a combination of conventional thermal initiation and purely photoinitiated systems, including different monomer orders, were tested to find the limits of this approach. It was found that PXEP was the superior RAFT agent in all cases and that only moderate reversals against the conventional block order were possible. Furthermore, dilute reaction mixtures featuring lower concentrations of initiating species gave superior consumption of starting macroinitiators and narrower molecular weight distributions.

Keywords: Reversible Addition Fragmentation Chain Transfer, RAFT, photo-RAFT, iniferter, photoiniferter, xanthate, spirooxazine, photochromic, photochromism, free radical polymerization

Subject: Nanotechnology thesis

Thesis type: Doctor of Philosophy
Completed: 2019
School: College of Science and Engineering
Supervisor: David Lewis