Radical polymerisation polymer particle formation

The Mechanism of Polymer Particle Formation in Polymer Polyols Synthesis by Radical Polymerisation [19]... 

The practical technique to obtain polymer polyols by radical polymerisation is to add an homogeneous mixture of vinylic monomer, initiator, chain transfer agent and a part of polyether polyol, to the rest of polyether polyol containing the NAD (macromer or nonreactive NAD), at 115-125 °C. The mechanism of solid polymer particle formation during radical polymerisation of vinylic monomers in liquid polyethers, in the presence of a nonreactive NAD, in the form of very stable dispersions, is described next. 

The particles are heterogeneous by definition. As with singlepolymer particles, heterogeneities in density can arise as a result of the mechanism of particle formation. Usually a polymer has a different density to the liquid monomer from which it is derived, and in most emulsion polymerisation processes to produce reasonably concentrated dispersions, propagation is dominated by arrival at the particle surface of oligomeric radicals which can lead to non-homogeneous shrinking within the particle. 

For polymerisation in a normal emulsion, the hydrophobic monomer is dispersed in the aqueous phase, in which it is almost completely insoluble, with the help of a surfactant. On the whole it is then located in the form of reservoir droplets of diameter d 1-10 gm, and in surfactant micelles of diameter d 5-10 nm. A small fraction may be solubilised in the continuous aqueous phase. The initiator is generally present in the aqueous phase (see Fig. 6.5). Polymer particles are generated by two simultaneous processes. The first is free radical capture by micelles, called micellar nucleation. There are far fewer droplets than micelles and the total surface area of the micelles is extremely large. Consequently, radicals tend to penetrate micelles rather than monomer droplets. The latter act mainly as monomer reservoirs. The second of the processes mentioned above is the formation of oligomer radicals in the continuous phase, and is referred to as homogeneous nucleation. When radicals reach a certain size, they become insoluble and group together to form polymer particles similar to those formed by micellar nucleation. 

Ab initio emulsion polymerisations of styrene were also conducted at 90°C, using the stable acyclic phosphonylated nitroxide radical SGI (NIO) as a mediator together with a water-soluble redox initiator (Lansalot et al, 2000). A long induction period was observed, assigned to the formation of water-soluble alkoxyamines before nucleation. In this system, molar mass of the polymer increased with conversion following the theoretical line, but the MMD was rather broad (PDI between 2.0 and 2.5). Rather small particles were obtained (average diameter was 120 nm) with a broad particle size distribution. It was also found that a few per cent of coagulum formed usually. 

---