Molecular Weight Control in Emulsion Polymerization by Catalytic Chain Transfer : Aspects of Process Development

Abstract

The molecular weight distribution (MWD), amongst others, governs the end use properties of polymeric materials, e.g., coatings. Robust molecular mass control is therefore a key issue in polymer production. Catalytic chain transfer (CCT) has proven to be a robust technique for the control of the MWD. In CCT the radical activity of a propagating polymer chain is transferred via the active cobalt complex to a monomer molecule. The catalytic nature of catalytic chain transfer agents (CCTA), combined with the high activity towards chain transfer allows for the use of very low amounts to achieve proper molecular weight control. This study aims at obtaining a thorough and fundamental understanding of the consequences of the heterogeneity of the emulsion polymerization reaction mixture for the application of CCT in a technical scale. The average molecular weight of the polymer formed can be predicted fairly accurately by the Mayo equation in bulk and solution polymerization, which relates the catalyst activity and the amount of catalytic chain transfer agent to the instantaneous numberaverage degree of polymerization. For emulsion polymerization an extended Mayo equation was derived which incorporates the effects of catalytic chain transfer agent partitioning. The lower apparent activity of these cobalt complexes observed in emulsion polymerization, when compared to bulk and solution polymerization, can be explained by the effects of partitioning. CCTA partitioning is a crucial parameter governing the performance of CCT in emulsion polymerization. The emulsion polymerization reaction system has some important consequences for the application of CCT. The absolute number of polymer particles in an emulsion polymerization very often exceeds the number of CCTA molecules, which implies that fast CCTA transport is required for proper molecular weight control. Partitioning of the CCTA in emulsion polymerization allows for fast transport via the aqueous phase. However, this is not the only transport mechanism in emulsion polymerization. This transport even occurs for a very sparingly water soluble CCTA, which also shows proper molecular weight control, suggesting that a CCTA (or other very hydrophobic species) can be transported by a shuttle mechanism. CCTA transport can be limited by the increasing viscosity of the polymer particles as the weight fraction of polymer is increasing. The high viscosity of the polymer particles can affect the rate of entry and exit of the CCTA. This results in compartmentalization behavior and a discrete distribution of CCTA molecules over the polymer particles, which is represented by a multimodal molecular weight distribution. The efficiency of chain transfer also severely changes throughout the course of an emulsion polymerization, which is governed by the polymer volume fraction in the polymer particles. The application of catalytic chain transfer also affects the course of the emulsion polymerization. Aqueous phase chain transfer, as a consequence of partitioning, affects the entry rate of radicals as well as the chemical nature of those radicals. This results in an extended nucleation period and as a consequence a broader particle size distribution, lower rates of polymerization throughout the entire course of the polymerization and possibly a loss of colloidal stability. Monomeric radicals, originating from the CCT process, can readily desorb from the polymer particles to the aqueous phase. This monomeric radical desoption, i.e. exit, results in a decrease in the rate of polymerization, relatively small polymer particles and a narrow particle size distribution. The reduced rate of entry in combination with the increased rate of exit results in a decrease of the average number of radicals per particle and consequently a decrease in the rate of polymerization. CCT mediated emulsion polymerizations obey Smith-Ewart Case 1 kinetics. Application of CCT in continuous emulsion polymerization was demonstrated in a pulsed sieve plate column (PSPC), which combines low net flow rates with limited axial mixing. For a very sparingly water soluble CCTA batch performance was approximately observed in the PSPC. For more water soluble CCTAs deviation from batch performance were observed. The observed differences could originate from CCTA backmixing. The results presented in this thesis illustrate the potential of CCT as a powerful technique for molecular weight control in emulsion polymerization. The obtained enhanced fundamental understanding allows for application of CCT on a technical scale.