Christopher J. Ferguson

Chain Transfer to Polymer and Branching in Controlled Radical Polymerizations of n-Butyl Acrylate

Chain transfer to polymer (CTP) in conventional free-radical polymerizations (FRPs) and controlled radical polymerizations (ATRP, RAFT and NMP) of n-butyl acrylate (BA) has been investigated using (13) C NMR measurements of branching in the poly(n-butyl acrylate) produced. The mol-% branches are reduced significantly in the controlled radical polymerizations as compared to conventional FRPs. Several possible explanations for this observation are discussed critically and all except one refuted. The observations are explained in terms of differences in the concentration of highly reactive short-chain radicals which can be expected to undergo both intra- and inter-molecular CTP at much higher rates than long-chain radicals. In conventional FRP, the distribution of radical concentrations is broad and there always is present a significant proportion of short-chain radicals, whereas in controlled radical polymerizations, the distribution is narrow with only a small proportion of short-chain radicals which diminishes as the living chains grow. Hence, irrespective of the type of control, controlled radical polymerizations give rise to lower levels of branching, when performed under otherwise similar conditions to conventional FRP. Similar observations are expected for other acrylates and monomers that undergo chain transfer to polymer during radical polymerization.

Synthesis of latices with polystyrene cores and poly(vinyl acetate) shells. 1. Use of polystyrene seeds

Strategies for avoiding secondary particle formation in seeded emulsion polymerisation, based on a simple model for particle nucleation [Macromol. Syrup. 92 (1995) 131, are discussed and exemplified in the context of growing latex particles with polystyrene cores and poly(vinyl acetate) shells. With a polystyrene seed of unswollen radius 44 run, core-shell polymerisation was easily achieved. However, when the same recipes were used with a polystyrene seed of unswollen radius 200 nm, excessive new particle formation occurred and no poly(vinyl acetate) shells could be detected. A wide selection of the suggested strategies for overcoming this were implemented, but always either extensive secondary nucleation occurred or the system became colloidally unstable. These results are in full accord with the predictions of the simplified nucleation model

Modelling secondary particle formation in emulsion polymerisation: application to making core–shell morphologies

A simple model for particle formation in surfactant-free emulsion polymerisation [Macromol. Symp. 92 (1995) 13; Emulsion polymerization: a mechanistic approach, 1995], with extension to allow for induced decomposition of initiator, is explored. The object is to find conditions for secondary particle formation, especially to find conditions under which it would be possible to grow core-shell particles of vinyl acetate in styrene, and vice versa; core -shell particle formation requires that secondary particle formation be avoided. The system is described by homogeneous nucleation: a radical generated in the aqueous phase will either enter a latex particle, undergo termination, or grow in the aqueous phase until it becomes the nucleus of a new particle. The simplified kinetic description contains only easily specified parameter values and requires minimal computational resources. The model implies that secondary particle formation is suppressed by decreasing seed radius, by increasing solids content, and by starved-feed conditions; seed radius is by far the most influential, while monomer-catalysed initiator decomposition has negligible effect. The model predicts that new particle formation will be rampant when vinyl acetate is polymerised in the presence of large polystyrene particles (implying that large core-shell polystyrene/poly(vinyl acetate) (PS/PVAc) particles cannot be obtained in this way), but that there should be relatively little secondary particle formation when styrene is polymerised in the presence of large PVAc particles (implying that large core-shell PS/PVAc particles can be created by inverse core-shell polymerisation). The model was also used to estimate the particle numbers expected in ab initio, surfactant-free styrene and vinyl acetate systems. The model explains why such styrene systems give large, monodisperse particles, whereas such vinyl acetate systems give much smaller particles. Comparison of predictions of the model with those of more sophisticated treatments suggests that model contains the kinetic events which are most essential in determining the rate of particle formation, and thus is sufficient for stating whether or not massive secondary nucleation will occur

Synthesis of latices with hydrophobic cores and poly(vinyl acetate) shells. 2. Use of poly(vinyl acetate) seeds

A strategy is explored for synthesizing latex particles with polystyrene cores and poly(vinyl acetate) shells. The seed particles are poly(vinyl acetate), which theory indicates should be immune to secondary particle formation when a second-stage seeded emulsion polymerization with styrene is carried out. The objective is to form a single hydrophobic core by inversion of the second and first stages. While this morphology is favoured thermodynamically, conditions need to be optimized so that it is kinetically achievable: many attempts to implement this using straightforward synthetic procedures result in either no core (acorn morphology) or multiple polystyrene cores. A series of experiments enables this goal to be implemented by ensuring sufficiently fast diffusion of the first-stage hydrophilic polymer (using chain-transfer agent to reduce the molecular weight and, more importantly, the degree of branching of the parent poly(vinyl acetate) seed polymer), an initiator which minimized grafting between the first- and second-stage polymers, and modifying the seed poly(vinyl acetate) to increase its hydrophilicity.