VAc polymerization

The classic method for establishing the proportion of head addition occurring in VAc polymerization involves a two step process.5 The PVAc is converted to PVA by exhaustive hydrolysis and the number of 1,2-glycol units is determined by periodate cleavage. 

The process is favored by low monomer concentrations as occurs at high conversions and in starved feed polymerizations.307 Theoretical calculations suggest that the incidence of backbiting should be strongly dependent on the tacticity of the penultimate dyad. 08 Double backbiting in VC or VAc polymerization will lead to 2-chloroethyl or 2-acetoxy ethyl branches respectively (as for E in Scheme 4.41 ).302... 

Early reports37 157 167 suggested that termination during VAc polymerization involved predominantly disproportionation. However, these investigations did not adequately allow for the occurrence of transfer to monomer and/or polymer, which are extremely important during VAc polymerization (Sections 6.2.6.2 and 6.2.7.4 respectively). These problems were addressed by Bamford et who used the gelation technique (Section 5.2.2,2) to show that the predominant radical-radical termination mechanism is combination (25 °C). 

Certain monomers may act as inhibitors in some circumstances. Reactivity ratios for VAc-S copolymerization (r< 0.02, rVu -2.3) and rates of cross propagation are such that small amounts of S are an effective inhibitor of VAc polymerization. The propagating chain with a terminal VAc is very active towards S and adds even when S is present in small amounts. The propagating radical with S adds to VAc only slowly. Other vinyl aromatics also inhibit VAc polymerization.174... 

Aliphatic disulfides 4 are not particularly reactive in chain transfer towards MMA and S (Table 6.3). However, they appear to be ideal transfer agents (Cu 1.0) for VAc polymerizations. 

The catalysts 153-155 shown in Table 9.7 have been used for polymerizations of acrylates and methacrylates and S. The catalyst 155 used in conjunction with an iodo compound initiator has also been employed for VAc polymerization.3"0 Catalytic chain transfer (Section 6.2.5) occurs in competition with halogen atom transfer with some catalysts. 

VAc polymerization 294 retarders 264 7 definition 264 reverse transfer constant... 

Figure L Solution of kinetic equations for molecjdar weight and LCB development in bulk VAc polymerization. Ms, Mw, and Bs are plotted as a function of conversion. Model predictions Cm = 2.0 X Cp = 3.0 X i0 K == 1.0.

Rate constants change in the course of MMA and VAC polymerizations [39, 40] (Tables 3 and 4). The considerable decrease in k, at high conversion is caused by a switch of the rate-determining step from the kinetic domain to a diffusion-controlled process [41] (see Chap. 6, Sect. 3.1)... 

Figure 2. Molecular weight-conversion dependence for VAc polymerization initiated by Al(iBu)3/Bpy/TEMPO (1 1 2). Initiator concentration [I]o. 0.05 M, 60 °C (n) 0.05 M, 20 °C (o) 0.3 M, 60 °C (6). (Reproduced with permission from American Chemical Society)...

Scheme 6. VAc polymerization mediated by [Fe(Cp)(CO)2)]2 (Reproduced with permission from American Chemical Society).

The terminal group of the prepared PVAc macroinitiator was analyzed by NMR (Figure 12). Besides the strong peaks (peak f, d and e) from the repeat unit of VAc, small peaks (a, b, c) from initiator EBiB were also observed. The co-terminal methine proton (g) with a C-Br bond appeared at 5.8 ppm, indicating the preservation of end group in VAc polymerization. The primary C-... 

Br bond originated from the head-to-head chain propagation (i) was also found in the polymer. Based on the integration ratio of peak g and peak b, 89% of terminal bromide was found to be preserved. The number average molecular weight of PVAc was estimated to be 4230 from the integration of peak f and peak b, which is very close to the value (4500) measured by GPC. The presence of the terminal carbon-bromide group further proved that the VAc polymerization by the catalyst of CuBr/tPy was an ATRP process. 

Analysis of C NMR spectra of PVA provides a direct estimate of the extent of head addition occurring in VAc polymerizations,Another advantage of the NMR method over chemical methods is that both head-to-head and tail-to-tail linkages can be observed. The polymers examined in these studies w ere of relatively high molecular weight and prepared by emulsion polymerization. They... 

---