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An initial test reaction was performed to assess whether the MBM C=C double bond was susceptible to homopolymerisation. A degassed solution of 2ii and 0.1 eq. of AIBN in dioxane was heated at 65 °C for 6 h. During this time there was no evidence of polymerisation by 1H NMR spectroscopy with the resonance of MBM proton (indicated in Scheme 2.8) being unchanged, or SEC analysis which saw no change to the molecular weight distribution.

Scheme 2.8. RAFT polymerisation with 2ii as CTA. The MBM proton of the CTA is indicated by ⋆

Polymerisations mediated by 2ii of tBA, MA, TEGA and NIPAM were conducted with [AIBN]:[2ii] = 0.1:1, at 65 °C, with varying monomer/dioxane ratios of 1/1 (v/v), 1/1 (v/v), 1/2 (v/v), and 2/3 (w/w) respectively to ensure complete monomer dissolution. Sty

Chapter 2 polymerisations were conducted in bulk at 110 °C, without the addition of AIBN (thermal initiation of Sty). Details of the reactions are given in Scheme 2.8 and Table 2.3.

Table 2.3. RAFT polymerizations conducted with CTA (2ii), performed at 65 °C in dioxane.

Monomer [2ii:M:AIBN] Time (h) Mn a (kDa) Mn,th b (kDa) ĐM a p b (%) 2xvi tBA 1:100:0.1 3 31.2 11.5 1.53 85 2xvii MA 1:100:0.1 3 25.6 7.4 1.46 79 2xviii TEGA 1:50:0.1 16 16.9 10.9 1.54 93 2xix NIPAM 1:50:0.1 16 0.5 0.7 1.64 3 2xx Sty c 1:90:0 16 7.2 2.4 2.42 19

a _{Molecular weight data were obtained by SEC. Samples were taken without fractionation} or precipitation. b Monomer conversion (p) monitored by 1H NMR spectroscopy. Mn,th calculated from monomer conversion. c Styrene polymerizations were conducted in bulk,

with thermal initiation at 110 °C.

Polymerisation of tBA (2xvi) proceeded rapidly, with 85% conversion reached after 3 h. However, conversion 1H NMR spectra revealed that the MBM proton of the RAFT agent

2ii was consumed at a faster rate than tBA (Figure 2.14). SEC analysis showed a significant deviation of Mn from theoretical values, with high dispersity (ĐM > 1.4) throughout. Molecular weight distribution was multimodal throughout the reaction, even at 30 min when tBA and MBM conversions were very low (1% and 10% respectively). Analysis of the purified polymer by SEC with a viscometry detector allowed the plotting of a Mark-Houwink curve (ln[η ] vs ln[MW] where η is the intrinsic viscosity measured by the viscometry detector, and MW is the molecular weight calculated by a Universal Calibration).57 Measuring the gradient gave α = 0.29 (Figure 2.15) which indicates that the polymer adopted a more compact structure in solution than a linear flexible polymer, which have 0.5 < α < 0.8 (for example PtBA 2xi had α = 0.62 under the same conditions).58

Chapter 2

Figure 2.14. tBA RAFT polymerisation with 2ii (2xvi in Table 2.3); a) First order kinetics of tBA and MBM consumption with linear fits; b) Mn and ĐM (as measured by SEC) as a function of tBA conversion with theoretic values (line); c) Evolution of molecular weight

distribution (as measured by SEC) as a function of time.

Figure 2.15. SEC molecular weight distribution and Mark-Houwink plot for 2xvi. This suggests that 2xvi has a branched structure, due to copolymerisation of tBA and the MBM C=C double bond. In fact, a RAFT agent containing a polymerisable double bond would be expected to give a hyperbranched polymer, as demonstrated for example by the

a)

b)

Chapter 2 groups of Zhao, Rimmer and Sumerlin.59-61 Attack of propagating radicals at the less hindered position of the MBM C=C double bond would lead to the formation of a radical stabilised by the Br (Scheme 2.9.). This radical would then propagate with monomer, leading to the creation of a branch point. The result is a species with one C=C double bond and two trithiocarbonate groups, which can be considered as an AB2 monomer.62 Further polymerisation of AB2 monomers leads to the formation of highly branched polymers without gelation.63

Scheme 2.9. Proposed mechanism for the formation of hyperbranched PtBA in RAFT polymerisation mediated by 2ii.

1_{H NMR spectroscopy indicated that the ratio of trithiocarbonate end-group to P}t_{BA was} 90:1 (using the CH(CO2) backbone resonances), in reasonable agreement with 85% conversion of 100 eq (Figure 2.16). tBA. There was no resonance corresponding to the MBM proton (6.95 ppm), instead there was a broad resonance attributed to the poly(bromosuccinimide) unit (H8), as well as a broad resonance attributed to PtBA units adjacent to these poly(bromosuccinimide) units (H9).

Chapter 2

Figure 2.16. 1H NMR spectrum (400 MHz, CDCl3) of 2xvi.

RAFT polymerisations of MA, TEGA and Sty (2xvii, 2xviii and 2xx respectively) showed similar results, with large deviations of Mn measured by SEC from theoretical Mn, high dispersities (1.4 < ĐM < 2.5), and complete loss of the MBM proton (6.95 ppm) in conversion 1H NMR spectra. Again this suggested polymerisation of the C=C double bond of the CTA, with formation of hyperbranched polymers. In the case of NIPAM (2xix) almost complete loss of the MBM proton (6.95 ppm) was observed by 1H NMR spectroscopy (> 89%), however very little NIPAM conversion was obtained (3%) after 16 h reaction. SEC showed the appearance of a high molecular weight shoulder due to branching, leading to a large dispersity (ĐM = 1.64). These results suggest that the MBM double bond was reacting during the polymerisation, but that radicals centred on the resultant succinimide carbon were not able to propagate. These findings are further discussed in Chapter 6, in relation to other data concerning polymerisations in the presence of bromo-and thiomaleimides.

In an attempt to prevent the formation of hyperbranched polymers when using CTA 2ii, a series of tBA polymerisations were conducted with changes to various reaction conditions, as detailed in Table 2.4. The aim was to find conditions which favoured tBA propagation, and disfavoured polymerisation of the MBM group of 2ii.

Chapter 2 Table 2.4. Polymerisation of tBA with 2ii in dioxane at tBA/dioxane = 1/1 (v/v)

[2ii]:[tBA]:[Initiator] Initiator T (°C) 2xvi 1:100:0.1 AIBN 65 2xxi 1:100:0.025 AIBN 65 2xxii 1:100:0.1 AIBN 80 2xxiii 1:100:0.1 V-65 50 2xxiv 1:1000:0.1 AIBN 65

The polymerisation was conducted with a reduced radical concentration, by using 0.025 eq. of AIBN (2xxi). A higher temperature (80 °C), which would result in a higher radical concentration, was also attempted (2xxii). By using the initiator 2,2'-azobis-2,4- dimethyl valeronitrile (V-65) a lower reaction temperature (50 °C) was investigated (2xxiii). A much lower radical and CTA concentration was also attempted, with 1000 eq.

of tBA used (2xxiv). However, in all cases consumption of the MBM group of 2ii was significantly faster than tBA conversion, leading to high ĐM and significant deviations of

Chapter 2

Figure 2.17. tBA RAFT polymerisations with 2ii. First order kinetics of tBA and MBM consumption with linear fits (left), Mn and ĐM (as measured by SEC; THF eluent, PSty standards) as a function of tBA conversion with theoretic values as a line (right), for a&b)

2xxi, c&d) 2xxii, e&f) 2xxiii, g&h) 2xxiv.

a)

b)

c)

e)

g)

d)

f)

h)

Chapter 2