Mechanistic Aspe~.ts of -Free-Radical Catalytic Chain Transfer Polymerization Darren Forster School of Chemical Engineering and Industrial Chemistry University of New South Wales Thesis submitted in fulfilment of the requirements for the degree of Doctor of Philosophy November 1999 ii Summary This thesis contains the results of an investigation into several aspects relating to the kinetics of free-radical catalytic chain transfer polymerization. This study is an endeavour to obtain a greater mechanistic understanding of the transfer process in bulk and solution polymerizations. The specific areas of investigation include the rate of transfer to methacrylates, the kinetics of catalytic chain transfer polymerization in a supercritical medium, the role of monomer on the transfer process, the effect of solvents on the kinetics of the transfer reaction and the kinetics of catalytic chain transfer polymerization in styrene. The catalytic chain transfer polymerizations of methyl, ethyl, and butyl methacrylates are studied over the temperature range 40-70°C with cobaloxime boron fluoride (COBF) and its tetraphenyl derivative (COPhBF). It is found that for both catalytic chain transfer agents, the chain transfer constant decreases in going from methyl to butyl methacrylate, and that there is no significant temperature effect on the observed chain transfer constants. The re~ults are consistent with a diffusion-controlled rate coefficient for the chain transfer reaction. The mechanism of catalytic chain transfer polymerization with methyl methacrylate is studied in a range of media. The chain transfer reaction in supercritical C02 is found to be significantly enhanced compared with similar experiments in toluene or in bulk methyl methacrylate. The results provide further evidence for a diffusion-controlled rate-determining step in the transfer process with cobaloxime catalysts. The gas like viscosities in the supercritical medium result in an approximate chain transfer coefficient (i<u-) of 108 L.mol-1.s- 1: an order of magnitude higher than values obtained in conventional organic solvents. The role of monomer in catalytic chain transfer polymerization is studied by determination of the chain transfer constant of COPhBF in methyl methacrylate at 60°C, varying the monomer concentration instead of the COPhBF concentration, as is common practice. Toluene and tert-butyl acetate were used as diluents in these studies iii and it was found that the chain transfer constants obtained in the present studies were not significantly different from those observed in conventional experiments. These results suggest the absence of a direct participation of monomer molecules in the hydrogen abstraction step in catalytic chain transfer. The catalytic chain transfer constants of COBF and COPhBF, of methyl methacrylate is studied in bulk and the presents of toluene and t-butyl acetate at 60°C, and are found to be similar in all cases. The results indicate that there is no solvent effect on catalytic chain transfer polymerization from weakly co-ordinating solvents. The effect of a strong base ligand, pyridine, was tested with COBF and COPhBF in MMA and styrene. Pyridine was found to a detrimental effect, with both catalysts, on the transfer reaction and rate of polymerization in MMA. In styrene, however, the transfer reaction was enhanced at low concentrations of pyridine, but on further additions of pyridine the transfer constant dropped again. This behaviour is thought to be caused by, at first, a single pyridine as the axial ligand catalyst, changing the catalytic process. Secondly it is thought that an inactive species is formed on further additions of pyridine, which maybe a hi-ligand species. The effect of pyridine was shown to have a significant effect on the kp of MMA, but had no effect on the kp of styrene. In the transfer polymerization of styrene, in the presence of COBF and COPhBF, the transfer constant is found to decrease rapidly in the initial stages of the polymerization, due to the formation of cobalt-carbon bonds between the styryl radicals and the catalyst. The transfer constant is found to increase dramatically after 80°C, due to the cobalt­ carbon bonds formed between the styryl radicals and the cobalt species becoming more labile changing the equilibrium between the active cobalt species and the inactive Co(III). There is little effect of the catalyst in p-methoxy styrene possible due to the catalyst binding irreversibly to the radical. Overall this thesis provides insight into many aspects of the mechanism of catalytic chain transfer polymerization. iv Declaration I hereby declare that this submission is my own work and that, to the best of my knowledge and belief, it contains no material previously published or written by another peFson nor material which to a substantial extent has been accepted for the award of any other degree or diploma of the university or other institute of higher learning, except where due acknowledgement is made in the text. I also declare that the intellectual content of this thesis 1s the product of my own work, even though I may have received assistance from others on the style, presentation and language expression. Darren Forster v Acknowledgments I would like to sincerely thank everybody who in some way is connected with the work in this thesis, especially: My supervisor Tom Davis, for giving me the opportunity to do a Ph.D., and being there with good advise, ideas, and inspiration, for the project, and creating a friendly, relaxed work environment. I would like give special mention of Dr. Johan P. A. Heuts who was an inspiration throughout the project, and who's insight led to some very fruitful discussions. I would like to acknowledge the financial support by ICI and ORICA. I would also like to thank all my colleagues, from the University of New South Wales for not only help when it was needed but also for all the good times that we shared, especially Lucy Baker, Michelle Coote, Heidi Kapfenstein, Dax Kukulj, David Morrison, Leesa Morris, Lisa Muratore, Evan Rodrigues, Lachlan Yee and Michal Zammit. Finally, I would like to thank my wife Chattragom and my parents for there encouragement, love and support. vi Contents 1 'INTRODUCTION ............................................................................................................................... 1 1.1 GENERAL INTRODUCTION ................................................................................................................ 1 1.2 AIMS OF INVESTIGATION ................................................................................................................. 1 1.3 OUTLINE OF THE THESIS .................................................................................................................. 2 1.4 PUBLICATIONS ...................................................................... : .......................................................... 4 2 BACKGROUND .................................................................................................................................. 5 2.1 FREERADICALPOLYMERIZATION ................................................................................................... 5 2. I. I Introduction .......................................................................................................................... 5 2.I.2 Mechanism ............................................................................................................................ 5 2. I .3 Diffusion Controlled Reactions ........................................................................................... I I 2.I.4 Molecular Weight Distributions .......................................................................................... I I 2. I. 5 Measuring Rate Coefficients ............................................................................................... I 3 2.2 CATALYTIC CHAIN TRANSFER POLYMERIZATION .......................................................................... 19 2.2.I Introduction ........................................................................................................................ I9 2.2.2 Mechanism .......................................................................................................................... I9 2.2.3 Cobalt-Carbon Bond Formation ......................................................................................... 22 2.2.4 Cobalt catalysts ................................................................................................................... 24 2.2.5 Temperature effects ............................................................................................................. 28 2.2.6 Solvent effects ..................................................................................................................... 28 2.3 EXPERIMENTAL TECHNIQUES ........................................................................................................ 32 2.3. I Molecular weight analysis .................................................................................................. 32 2.3.2 Rate Analysis....................................................................................................................... 36 2.4 REFERENCES ................................................................................................................................. 37 3 THE EFFECTS OF ESTER CHAIN LENGTH AND TEMPERATURE ON THE CATALYTIC CHAIN TRANSFER POLYMERIZATION OF METHACRYLATES .....................