The polymerisation of styrene was attempted using a similar procedure to that described by Georges.53 The monomer was stabilised with free verdazyl at a concentration of 1 mg of verdazyl per 40 mL of styrene. The verdazyl radical used to stabilise the monomer was the same as that in the initiator used in the polymerisation, in this case verdazyl initiator16(R = H) and verdazyl radical7(R = H). The polymerisation was performed in bulk at 125 ˚C using mesitylene as an internal standard to determine conversion by 1H NMR. After 50 hours, 71% conversion had been reached.
The pseudo first order kinetic plot shown in Figure 1.4-1 (●) is linear indicating that the concentration of propagating radicals remains relatively constant throughout the reaction. However, it should be noted that the rate began to slow down over time, seen as the kinetic plot starting to curve (after approximately 40 hours). The polymerisation mixture was yellow at the end of the reaction indicating a build up of free radical, due to termination reactions, which can inhibit the polymerisation reaction. The molecular weight versus conversion plot (Figure 1.4-2, ●) shows that the experimental values for the molecular weight deviated from the calculated theoretical molecular weights, in particular at conversions above 30% where the molecular weight plateaus at a lower molecular weight than targeted.
Figure 1.4-1. First order kinetic plot for the verdazyl mediated polymerisation of styrene by 16 (R = H) using verdazyl stabilised styrene (●), styrene (●) and results from the literature ()53
Figure 1.4-2. Evolution of molecular weight (Mn) and PDi versus conversion for the verdazyl
mediated polymerisation of styrene by 16 (R = H) using verdazyl stabilised styrene (●), styrene (●) and results from the literature ()53, DP = 380, target Mn= 39,500 g/mol (- - -)
The kinetic plot shows the concentration of radical to be constant for the majority of the reaction indicating that the monomer is being consumed at a steady rate, however, the levelling out of the molecular weight indicates that chain termination occurs. The number average molecular weight (Mn) is plotted against conversion in
Figure 1.4-2 and even though some monomer chains are getting longer, the number of terminated lower molecular weight chains is also increasing which overall averages out for no change in the value. This is confirmed by an increase in polydispersity as the molecular weight plateaus and can be visualised in the differential molecular weight distributions104 as a low molecular weight tail (Figure 1.4-3).
Figure 1.4-3. Overlaid differential molecular weight distribution curves for the verdazyl mediated polymerisation of styrene (●) by 16 (R = H)
The reaction was then repeated, this time without stabilising the monomer with free verdazyl prior to performing the reaction. The polymerisation produced identical results (Figures 1.4-1 and 1.4-2, ●), also reaching 70% conversion in 50 hours. As no differences were observed, styrene was not stabilised in further reactions.
Verdazyl initiator 13 (R = Ph) was investigated and the same trends were seen as with 16 (R = H). A linear kinetic plot was observed (Figure 1.4-4, ▲) and the molecular weight plateaued along with an increase in polydispersity (Figure 1.4-5, ▲) towards the end of the reaction.
Figure 1.4-4. First order kinetic plot for the verdazyl mediated polymerisation of styrene with 0% (▲), 25% (▲), 50% (▲) and literature values for 0% ()53toluene with respect to
Figure 1.4-5. Evolution of molecular weight (Mn) and PDi versus conversion for the verdazyl
mediated polymerisation of styrene with 0% (▲), 25% (▲), 50% (▲) and literature values for 0% ()53toluene with respect to monomer by 13 (R = Ph), DP = 485, target Mn= 50,500 g/mol
(- - -)
As the polymerisations were carried out in bulk it was speculated that increasing viscosity in the polymerisation mixture may affect growth of the polymer. Therefore, two further reactions were performed (Figure 1.4-4) using 13 (R = Ph) with either 25% (▲) or 50% (▲) toluene with respect to styrene. As expected, the rate of polymerisation slowed with increasing amount of solvent, but the molecular weight and polydispersity showed the same trends and almost identical values (Figure 1.4-5).
The polymerisation of styrene in bulk at 125 ˚C was then repeated for each of the four verdazyl initiators and the TEMPO initiator in order to observe if any differences between radicals. Figure 1.4-6 shows the kinetic data for the five polymerisations.
Figure 1.4-6. First order kinetic plot for the verdazyl mediated polymerisation of styrene by 12 (■), 16 (●), 13 (▲), 17 (), 18 (►) and literature values for 16 ()53
Slight differences were observed between initiators. Verdazyl initiator16(R = H,●) displayed the same rate as the TEMPO initiator (12, ■). Adding the phenyl substituents to the verdazyl results in a reduction of the rate in the order of PhNO2
(►) < Ph (▲) < PhOMe (). The rate was seen to slow down over time in a more pronounced fashion for17(R = PhOMe,) then for any of the other initiators. The
molecular weight plot (Figure 1.4-7) shows that the same trends as already seen with 13(R = Ph) and16(R = H).
Figure 1.4-7. Evolution of molecular weight (Mn) and PDi versus conversion for the verdazyl
mediated polymerisation of styrene by 12 (■), 16 (●), 13 (▲), 17 (), 18 (►) and literature values for 16 ()53, DP = 485, target Mn= 50,500 g/mol (- - -)
The slower rate resulted in low conversion for 18 (PhNO2, ►) and the lowest
molecular weight of all five initiators was observed. The trend in rate for the phenyl substituted verdazyls ran through with the molecular weight so that molecular weights were observed in the order PhNO2(►) < Ph (▲) < PhOMe (). Initiators
16 (R = H, ●) and 12 (TEMPO, ■) reached the same conversion and molecular weights were similar, although were slightly higher for the verdazyl initiator. The polydispersities were substantially higher for the phenyl substituted verdazyls than
for 12 (TEMPO) and 16 (R = H). Figure 1.4-8 compares the differential molecular weight distributions for the polymerisation of styrene by 16 (R = H) and 18 (R = PhNO2). Low molecular weight tails were visible in both polymerisations after 8
hours; however, the distribution was much broader in the latter.
Figure 1.4-8. Overlaid differential molecular weight distribution curves for the verdazyl mediated polymerisation of styrene by 16 (R = H,●, top) and 18 (R = PhNO2,►, bottom)
In the literature53, results are reported up to between 5-7 hours for the polymerisation of styrene at which conversions were around 40%. This suggests that above this conversion the same low molecular weight tail was observed and were therefore not included. This was confirmed during email correspondence with Georges.105 These results indicate that it is possible to polymerise styrene using verdazyls; however, an improvement over TEMPO mediated polymerisation has not been established.