Molecular weight control in emulsion polymerization by catalytic chain transfer : aspects of process development

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

A bulky phosphite modified rhodium catalyst for efficient hydroformylation of disubstituted alkenes and macromonomers in supercritical carbon dioxide

The hydroformylation of disubstituted alkenes and related macromonomers in supercritical CO2 is demonstrated. Higher turnover frequencies were observed for the 1,2-disubstituted alkenes than for the 1,1-disubstituted alkenes. The turnover frequency for poly(styrene) macromonomer hydroformylation compares well with that observed for cyclohexene. The turnover frequency observed for poly(methyl methacrylate) macromonomer hydroformylation is considerably lower than that observed for methyl methacrylate. Unprecedented turnover frequencies are observed in comparison with previous studies where CO2 has been applied as a solvent. This is achieved using rhodium modified with a readily available bulky phosphite ligand without the need of ligand modification to improve solubility in supercritical CO2

Catalytic chain transfer and its derived macromonomers

An overview is given of cobalt-catalyzed chain transfer in free-radical polymerization and the chemistry and applications of its derived macromonomers. Catalytic chain transfer polymerization is a very efficient and versatile technique for the synthesis of functional macromonomers. Firstly the mechanism and kinetic aspects of the process are briefly discussed in solution/bulk and in emulsion polymerization, followed by a description of its application to produce functional macromonomers. The second part of this review briefly describes the behavior of the macromonomers as chain transfer agents and/or comonomers in second-stage radical polymerizations yielding polymers of more complex architectures. The review ends with a brief overview of post-polymerization modifications of the vinyl endfunctionality of the macromonomers yielding functional polymers with applications ranging from initiators in anionic polymerization to end-functional lectin-binding glycopolymers

Catalytic chain transfer in a miniemulsion copolymerization of methyl methacrylate and n-butyl acrylate

This paper reports on the application of catalytic chain transfer (CCT) in miniemulsion copolymn. It has been found that Co(Et)4BF has a similar chain transfer activity in bulk as CoBF. CCT copolymns. of MMA and BA can be performed in miniemulsion; large mol. wt. redns. are achieved, and at high conversions significant macromer incorporation occurs. [on SciFinder (R)

Facile and selective synthesis of aldehyde end-functionalized polymers using a combination of catalytic chain transfer and rhodium catalyzed hydroformylation

A novel synthetic pathway towards aldehyde end-functionalized polymers is presented from a combination of catalytic chain transfer polymerization (CCTP) and rhodium catalyzed hydroformylation in supercritical carbon dioxide. CCTP allows for the synthesis of well-defined macromonomers in terms of the average molecular weight and the terminal unit carrying the unsaturated bond. The rhodium catalyzed hydroformylation allows for a high selectivity towards aldehyde end-group functionalized polymers. The introduction of the synthetically versatile aldehyde end-group opens up a broad range of possible applications

Effect of catalyst partitioning in Co(II) mediated catalytic chain transfer miniemulsion polymerization of methyl methacrylate

The effect of catalyst partitioning over the organic and water phases in the catalytic chain transfer mediated miniemulsion polymerization was investigated and a mathematical model developed to describe the instantaneous degree of polymerization of the formed polymer. Experimental and predicted instantaneous degrees of polymerization prove to be in excellent agreement. © 2008 Wiley Periodical

Catalytic chain transfer mediated emulsion polymerization : compartmentalization and its effects on the molecular weight distribution

We present the first population balance calculations which encompass the complete molecular weight distribution (MWD) to discuss the implications of both radical and catalytic chain transfer agent (CCTA) compartmentalization in a catalytic chain transfer (CCT) mediated emulsion polymerization system. Compartmentalization effects are attributed to reduced frequencies of entry and exit of the CCTA (bis[(difluoroboryl)dimethylglyoximato]cobalt(II) or COBF). Two limiting scenarios were identified. In instances of fast CCTA entry and exit, monomodal MWDs are obtained governed by a global CCTA concentration. In instances of slow entry and exit, bimodal MWDs are obtained; one peak can be attributed to the generation of a bimolecular termination product produced in polymer particles devoid of CCTA, while a transfer-derived peak can be attributed to polymer particles containing one or more CCTA molecules. We present theoretical evidence that experimentally observed multimodal MWDs ( Macromolecules 2009, 42, 7332-7341) originate from a reduced mobility of the CCTA and that when viscosity is high in the polymer particles, compartmentalization of the CCTA becomes important

The effect of Co(II)-mediated catalytic chain transfer on the emulsion polymerization kinetics of methyl methacrylate

The effect the catalytic chain transfer agent, bis[(difluoroboryl) dimethylglyoximato] cobalt(II) (COBF), on the course of the ab initio emulsion polymerization of methyl methacrylate, and the product properties in terms of the molecular weight distribution were investigated. The emulsion polymerization kinetics have been studied with varying surfactant, initiator, and COBF concentrations. The experimentally determined average number of radicals per particle strongly depends on the concentration of COBF and proves to be in good agreement with the results of model calculations. The apparent chain transfer constant, determined up to high conversion, is in excellent agreement with the predicted value based on a mathematical model based on COBF partitioning and the Mayo equation. The results of this work enhance the fundamental understanding of the influence a catalytic chain transfer agent has on the course of the emulsion polymerization and the control of the molecular weight distribution

Evidence of compartmentalization in catalytic chain transfer mediated emulsion polymerization of methyl methacrylate

Evidence of compartmentalization of the catalytic chain transfer agent in seeded emulsion polymerization of methyl methacrylate (MMA) is shown experimentally. The addition of bis[(difluoroboryl)dimethylglyoximato]cobalt(II) (COBF) to seed particles swollen below their maximum saturation concentration exhibited multimodal molecular weight distributions (MWD) which are attributed to a statistical distribution of COBF molecules over the polymer particles. The experimental observations suggest that there are two limits for catalytic chain transfer in emulsion polymerization: (i) at the earlier stages of the polymerization where a global COBF concentration governs the MWD and (ii) at the latter stages of the polymerization where a statistical distribution of COBF molecules governs the MWD. To the best of our knowledge, these results are the first to suggest evidence of compartmentalization in catalytic chain transfer mediated emulsion polymerization