Polymer particles, growth

Polymer parameters, catalysts and, 26 534 Polymer particle growth... 

Figure 5 Polymerization mechanism in AOT globular microemulsions. (I) Before polymerization AOT micelles (c/ 6 nm), (II) Polymer particle growth (a) by collisions between particles (b) by monomer diffusion through the toluene phase. (Ill) End of polymerization. Polymer particles ( / 40 nm) plus small micelles (r/ 3 nm). (From Ref 23.)...

Modelling of the polymer particle growth process [82] has resulted in the conclusion that diffusion limitations are the single reason for the wide polydispersity of synthesised polymers. The model has demonstrated that the main transport limitations localise on the level of macroparticles. Modelling results are confirmed by data obtained in gas and liquid polymerisation experiments on titanium-magnesium catalysts. Authors also consider that the wide polydispersity of polymers can be explained by the existence of more than one type of active centre. Each specific type is responsible for a certain portion of polymer with a different MWD. However, the authors did not succeed in characterising the active centre [82] because it required the optimisation of many kinetic parameters. 

Talamini and Peggion [145] visualize the process as a modified heterogeneous solution polymerization. The monomer has an appreciable solubility in the aqueous phase. These authors estimate the solubility to be on the order of 0.5 moles per liter. (Presumably this is under the pressure conditions of a typical reactor. Our Table I gives the solubility as 0.1 or approximately 0.02 moles per liter at standard temperature and pressure.) Polymerization starts in the aqueous solution. The polymer that forms separates. The emulsifier in the solution protects the particle from coagulation. By imbibing monomer on the surface of the polymer particle, growth takes place until latex-sized particles form. When the surfactant is consumed by adsorption on these particles, radicals precipitate from solution onto existing particles. Then the number of particles remains constant, very much as in a conventional emulsion polymerization. The total surface area of the polymer particle appears to be involved in the polymer process. 

Population balances and the method of moments can also be combined with the multigrain model and other polymer particle growth models. In this case, the population balances are defined for each position in the particle to obtain the radial profiles of chain length averages [36, 51-60]. 

Reaction kinetics and polymer particle growth are determined by cocatalyst, comonomer, chain transfer agent, temperature, pressure, process, etc. (Fig. 12). Complete success in the catalyst design is reached only when a broad set of requirements is met (Table 2). 

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