Planificación detallada de las actividades Taller de Aprendizaje N°

In addition to the configuration of the catalyst itself there are other factors that effect the activity of a catalyst within a given system. Simple addition o f solvent to a system may cause a decrease in the apparent Cs due to competition between monomer,

solvent and propagating chains for reaction at the cobalt centre. For example, CCTP carried out in methanol, a strong co-ordinating solvent, significantly reduces catalyst performance41. Haddleton el al. reported a Cs value for bulk polymerisation of MMA

in the presence o f CoBF as 40000, under identical conditions with methanol this was reduced to 10000. Solvents have been shown to have an effect upon the Cs of a

system beyond that predicted by simple dilution of the system. Haddleton el al.

reported that purification by distillation o f butanone, removing trace acids, has a dramatic effect on the Cj of the system. The Cs of CoBF with MMA in untreated butanone was 8020 whereas under identical conditions with distilled butanone a Cs of

Introduction Chapter I

26500 was recorded41. This suggests that trace acids present in solvent cause hydrolysis o f the catalyst.

Suddaby el al. noted a strange effect when CCTP o f MMA and methacrylamide were

attempted in dimethlyformamide (DMF) in that no observable polymerisation occurred46. DMF is known to co-ordinate to transition metals via the oxygen atom, therefore it is not unlikely that it forms a complex with CoBF47. Suddaby el al.

suggest that the co-ordination of the DMF changes the electronic configuration of the cobalt centre and that this causes the cobalt-hydride to be more stable towards monomer. The increase in stability would mean an increase in competition between hydrogen atom transfer to monomer or the propagating chain. When transfer to a propagating chain occurs it terminates the polymer reducing the concentration of radicals thus inhibiting the polymerisation. As with the CCT mechanism the complex is back to its original form and thus this inhibition is catalytic.

If the proposed mechanism involving co-ordination o f the cobalt to the polymer were correct then a change in Cs with temperature would be predicted. However this has

not been found to be the case in several studies4148'49. Through kinetic analysis Heuts el al. drew the conclusion that the rate determining step must be diffusion controlled.

The effect caused by an increase in the size o f the monomer provides further supporting evidence. Calculation of Cs for a series o f alkyl methacrylates in bulk has

been carried out independently by both Mironychev et al. and Heuts et al. 49’50. They

found that with an increase in ester chain length a decrease in the Cs was observed.

This can be partially explained by the increase in propagation rate coefficients as the chain length increases. Mironychev el al. provided further explanation and suggested

that as a result o f increasing steric hindrance with an increase in ester chain length there would be an increase in steric hindrance during the hydrogen abstraction and an

Introduction Chapter I

increase in stability of the Co-monomer complex. This would result in a reduction in the concentration o f the active species. Heuts el al. claim to have evidence to the

contrary, as yet unpublished, and ascribe the observation to be consistent with a diffusion-controlled hydrogen-abstraction rate coefficient49-51.

CCTP has been shown to be most effective for monomers containing an a-methyl group. This was originally explained by the ease of hydrogen abstraction from the a-m ethyl group compared to abstraction from the polymer backbone. However, studies have shown that the reason lower Cs values are observed is due to a reduction

in the Co(II) level as a result of the stability o f the Co-C bond formed. Heuts et al.

studied the levels o f Co(II) in a mixture of CoPhBF, initiator and monomer, either MMA or styrene16-52. After heating at 60 °C for 1 hour and then freezing in liquid nitrogen they obtained electron paramagnetic resonance (EPR) spectra. They noted that in the case o f MMA there was no significant difference in levels o f Co(Il) from those taken before heating. However, in the styrene polymerisation the Co(II) signal had virtually disappeared. They concluded that the reduction in the Co(II) signal was due to the formation of a bond between the styryl radical and the Co(II) complex to form an organocobalt(IIl) complex, see figure 1.19. Earlier studies by Gridnev et al.

reported similar findings and conclusions through examination o f electronic absorption spectra 38-53.

R' + Co(II) - ---— R— Co(III)

Figure 1.19, Formation of a stable organocobalt(III) complex in the CCTP of styrenics and acrylates

Introduction Chapter I

Further evidence is provided by the observation that cobaloximes interact with the propagating chain of acrylates forming a reversible Co-C bond that allows living polymerisation, section /.<5.254"56.

Co-C bond formation is more significant in the styrene and acrylate polymerisations than with MMA as secondary carbon centred radicals are formed in the case of styrene and acrylates and tertiary radicals in the case of MMA. Firstly tertiary radicals are more stable than secondary and secondly the formation o f a bond between an MMA radical and the Co(ll) complex is more stericaly hindered than in the case of styrene. Thus for the polymerisation of styrene and acrylates the concentrations of both the propagating radicals and the Co(II) complex would be reduced. A reduction in the level o f Co(II) catalyst would mean that the actual ratios of [Co(II)]/[M] are lower than those used to calculate the Cs. Heuts et al. conclude that the chain transfer

rate coefficients for MMA and styrene are probably not as different as the experimentally determined chain transfer constants suggest.