Work finally moved on to see if it was possible to improve the polymerisation of MMA by attempting copolymerisation reactions. Styrene was chosen as the co- monomer. The method used for the copolymerisation was taken from the literature31, in which MMA was copolymerised with butyl acrylate and used 0.05 equivalents of free radical for every equivalent of initiator. Polymerisations were run at 125˚C.
Three sets of experiments were run initially using13(R = Ph) varying the amount of MMA and styrene from 1:1 to 3:1 and 9:1. Linear kinetic plots were observed and the rates of polymerisation were similar at the start of the three reactions but started to vary a bit over time (Figure 1.4-24). The polymerisation using 3:1 MMA to styrene proceeded the fastest (▲) followed by 1:1 (▲) and then 9:1 (▲). An increase in molecular weight with conversion was observed but the increase was small over time and much lower than the theoretical molecular weight (Figure 1.4- 25). The observed polydispersity was higher than would be expected for an ideal living polymerisation, approximately 1.4 after 25 hours, but was lower than those observed for the homopolymerisation of MMA at 25 hours where the polydispersity was > 2.5 (prior to precipitation).
Figure 1.4-24. First order kinetic plot for the verdazyl mediated copolymerisation of MMA and styrene by 13 (R = Ph) at ratios of 1:1 (▲), 3:1 (▲) and 9:1 (▲) using 0.05 equivalents of free
verdazyl radical (8) for every equivalent of initiator
Figure 1.4-25. Evolution of molecular weight (Mn) and PDi versus conversion for the verdazyl
mediated copolymerisation of MMA and styrene by 13 (R = Ph) at ratios of 1:1 (▲), 3:1 (▲) and 9:1 (▲) using 0.05 equivalents of free verdazyl radical (8) for every equivalent of initiator.
The same three copolymerisations were repeated, however, this time no excess free radical was added to the polymerisation to observe any changes as a result. Similar results were observed with reasonably linear first order kinetics with respect to monomer, with the 3:1 polymerisation the fastest and the 9:1 the slowest (Figure 1.4-26,▲and▲respectively).
Figure 1.4-26. First order kinetic plot for the verdazyl mediated copolymerisation of MMA and styrene by 13 (R = Ph) at ratios of 1:1 (▲), 3:1 (▲) and 9:1 (▲)
Figure 1.4-27. Evolution of molecular weight (Mn) and PDi versus conversion for the verdazyl
mediated copolymerisation of MMA and styrene by 13 (R = Ph) at ratios of 1:1 (▲), 3:1 (▲) and 9:1 (▲), DP = 200, target Mn= 20,000 g/mol (- - -)
After an initial jump in molecular weight, only a small increase in molecular weight was observed (Figure 1.4-27). Polydispersity values were again higher than expected for a living polymerisation. The conversion, molecular weight and polydispersity data for all six aforementioned copolymerisation reactions are presented in Table 1.4-2. The overall conversion was calculated based on the conversions of the individual monomers adjusted for their ratio in the polymerisation.
Table 1.4-2. Molecular weights and polydispersities for the verdazyl mediated copolymerisation of MMA and styrene before and after precipitation
I V. MMA:Sty Overall conversion Conversion (%) Mn (g/mol)a PDia Mn (g/mol)b PDib (%) MMA Sty 1 0.5 100:100 58.3 80.8 35.9 2700 1.41 3600 1.28 1 - 100:100 63.8 87.0 40.5 3300 1.43 3800 1.36 1 0.5 150:50 66.2 68.6 29.5 2500 1.41 3100 1.42 1 - 150:50 73.6 72.3 38.7 3900 1.53 4144 1.55 1 0.5 180:20 50.2 52.0 22.4 2500 1.38 3100 1.33 1 - 180:20 60.2 63.2 33.2 3500 1.60 6300 1.51
abefore precipitation,bafter precipitation
Conversions were higher when free radical was not added at the start of the polymerisation. In each polymerisation, the conversion of MMA was higher than for styrene at the end of the polymerisation. As seen with the homopolymerisation of MMA, after precipitation the copolymers had an increased molecular weight, although to a much lesser extent. Molecular weights rarely exceeded 4,000 g/mol for the copolymerisations, again lower than molecular weights observed for the homopolymerisation of MMA. The changes in polydispersity were small after precipitation. Based on the results displayed in Table 1.4-2, higher molecular weights and polydispersity values were observed in the polymerisations that did not include the addition of free verdazyl at the start of the polymerisation.
The evolution of molecular weight typically expected for a controlled radical polymerisation was observed when free verdazyl was added at the start of the polymerisation (Figure 1.4-28, left hand side). As conversion increased the low molecular weight tail increased and a shoulder in the trace became more prominent (Figure 1.4-28, top left), similar to the homopolymerisation of MMA (Figure 1.4- 23), but to a lesser extent. As the amount of MMA was increased this effect increased. As already seen in Figure 1.4-27, without the addition of free radical at the start of the polymerisation, the evolution of molecular weight was much smaller and decreased further on increasing amounts of MMA (Figure 1.4-28, right hand side). The traces also displayed a low molecular weight tail but were more mono- modal. After precipitation all the molecular weight distributions were mono-modal but they retained the low molecular weight tail.
.
The1H NMR of the precipitated polymer was different to both those for polystyrene and poly methyl methacrylate suggesting that a random copolymer has been formed over individual homopolymers or a block copolymer (Figure 1.4-29). Further work is required to optimise the copolymerisation reaction to find the correct conditions to achieve a completely controlled system. All polymerisations were brightly orange coloured after 25 hours indicating the build up of verdazyl radical 8 (R = Ph) from dissociation. The amount of free radical was not quantified for these polymerisations; however, doing so in the future may improve understanding of the polymerisation system and help to further optimise the reaction conditions.
Figure 1.4-28. Overlaid differential molecular weight distribution curves for the verdazyl mediated copolymerisation of MMA and styrene with 0.5 equivalents of free verdazyl (left) and
Figure 1.4-29.1H NMR of polystyrene homopolymer (PS,─), poly methyl methacrylate homopolymer (PMMA,─) and poly methyl methacrylate/polystyrene copolymer (─)