Effect of side‑group structure and temperature on chain transfer to polymer and branching in acrylate homopolymerizations

The paper reports studies of the effects of ester side-group size and structure on chain transfer to polymer (CTP) and mol% branches (%br) in the final polymers from monomer-starved semi-batch emulsion homopolymerizations of a series of alkyl acrylates with different numbers of ester OR side-group carbon atoms (N _sgC) in the range 1–10. Although the high instantaneous conversions and high gel contents in the final polymers show that intermolecular CTP is significant, the kinetics have been analysed principally in terms of intramolecular CTP (intraCTP) because this is expected to be numerically the dominant CTP process. For linear OR side groups, there is a small, continuous increase in %br (from ~ 3.4 to ~ 5.0 mol%) as N _sgC increases from 1 to 8, which arises predominantly from the acrylate molar mass (M _acrylate) reducing the molar monomer concentration and from the small increase in rate coefficient for propagation (k _p) as N _sgC increases; no discernible effects of N _sgC on the rate coefficient for intraCTP (ktrPintra) are evident. For isomeric butyl acrylates, there is no measurable effect of isomer structure on %br (4.3–4.4 mol%), indicating that butyl groups are too small for changes in side-group bulkiness to influence ktrPintra and k _p. A similar observation was made for n-hexyl acrylate (nHA) and 2-ethylbutyl acrylate (%br 4.7–4.8 mol%). However, cyclohexyl acrylate (cHA) gives a much higher %br (~ 7.2 mol%), which has been assigned to steric effects increasing ktrPintra because k _p values for nHA and cHA are very similar. For OR side-groups with N _sgC of 8 and10, there is a strong effect of side-group non-linearity (2-ethylhexyl, iso-octyl and iso-decyl) increasing %br, with polymers from n-alkyl acrylates having much lower %br (~ 4.5–5.0 mol%) than those from the equivalent non-linear acrylates (~ 6.7–7.1 mol%); since the k _p values are expected to be similar for these acrylates, it is evident that the much bulkier non-linear OR groups cause a significant increase in ktrPintra compared to the linear equivalents. Tentative hypotheses for these steric effects have been postulated. Studies of the effect of temperature (0–80 °C) on CTP and %br for methyl acrylate solution homopolymerization (in which intraCTP should be completely dominant) show the expected reduction in %br as temperature reduces, such that CTP is negligible at 0 °C; Arrhenius analysis gives an activation energy of 40.7 kJ mol ⁻¹ for intraCTP, which is within the range of values reported for n-butyl acrylate.