1.2 Catalytic Chain Transfer Polymerisation (CCTP)
1.2.1 Initial development
Catalytic chain transfer was first discovered by Boris Smirnov and Alexander Marchenko in 1975 as a result of research into the use of transition metal compounds to catalyse the redox decomposition of peroxy initiators in free radical polymerisations.14-19 Co (II) porphyrins (Figure 1.1 - 1) promoted by Ponomarev
method was the very low quantities of catalyst required to achieve this relative to the conventional chain transfer agents. 14,15,17 The chain transfer constant (Cs) for (1) in the free radical polymerisation of methyl methacrylate is 2.4x103compared to ~40 in the case of a thiol chain transfer agent.21
Figure 1.1: (1) Cobalt tetramethoxy hematoporphyrin-IX (shown without neutral axial ligands) and (2) Vitamin B12 with corryn ring as opposed to porphyrin ring.15
Further work assisted by the Enikolopyan group led to an understanding of the fundamental mechanism of reaction in 1977, this is discussed below.
Initial research in the USSR dealt with; catalytic inhibition by aprotic solvents,22 mechanistic investigations,22 the development of cobaloxime compounds (3) as potent second generation chain transfer catalysts,23,24 investigation into the structural basis of activity,25,26 and kinetic studies into the formation of low molecular weight polymers and oligomers.27,28 Further commercial development
was stymied by the issuing of a USSR patent detailing ‘for office use only’ in 1980, restricting research to the institution of the researcher.23,29
Figure 1.2; Cobaloxime general structure (unspecified neutral axial ligands denoted by L) (3), CoBF general structure (4).
Take up of the technique by DuPont under Steven Ittel led to the development of patents for air stable CCTP catalysts based upon cobaloximes with BF2 bridging ligands (CoBF) (4), allowing for the ready commercialisation of CCTP for methacrylates and styrene.30-33
The initial stage of publications generated in the Russian literature went largely unnoticed outside of Russia and further patents from this period also led to this technique being ignored in the west for a number of years.15,16,18,19,34The next stage of the development of CCT was to occur through industrial development. Issues at this point were encountered with the perceived unreactivity of CCTP macromonomers and the narrow range of monomers accessible to the technique
(being limited to α-methyl substituted monomers and styrene).
Scheme 1.4; Alternative proposed pathways for reaction with CCTP ω-vinyl
terminated polymers, showing addition fragmentation chain transfer and propagation. Figure adapted from reference.14
The late 1990s through to the 2000s saw industrial interest increase and with this a corresponding increase in academic publication. This can be seen through a range of patents issued from companies such as the Glidden Paint company (which became part of ICI/Dulux and now Akzo), DuPont and ICI/Zeneca Specialities (later to become a part of DSM). Glidden’s patents demonstrate the use of cobaloximes (4) as catalysts, however, the patent was quite narrow in that it made very limited claims only to the parent cobaloxime (3).38 The narrowness of the claim was exploited by DuPont who expanded upon the range of cobaloxime functionalities available. In turn DuPont also made a similar mistake in not filing broad enough claims and an “error” in the wording of their patents, where they claimed “R” in
structure4 as phenyl but not aryl, led to its exploitation by ICI/Zeneca who further developed a range of compounds where R is a substituted phenyl (aryl).
The development of cobaloximes first explored analogues of 3which proved to be cheaper to produce and with a higher chain transfer constant (Cs 2x104).14 These were soon replaced by cobaloximes with a BF2bridge as opposed to a H bridge (4), this once again increased the chain transfer constant (Cs4x104) whilst at the same time increasing the stability of the complexes to acidic hydrolysis and oxidation to Co(III).27,31
The use of (4) increased the utility of the catalysts and CCTP has now been used in a wide range of applications both in fine chemicals (rheology modifiers,39 macromonomers as hair care additives40, paint and coatings, automotive refinish, ink-jet inks, contact lenses) and in industrial applications (thermoformed sheets of MMA for sinks, baths and shower trays).41
1.2.2
Mechanism
There are three proposed mechanisms for catalytic chain transfer polymerisation, a technique that has been found to be truly catalytic through the recovery of the regenerated cobalt complex.15,16,18,19Two of the three techniques are characterised by activation of a substrate by the cobalt complex prior to attack by the monomer; the third involves a sequential reaction of two species with the metal centre (Scheme 1.5). The matter of contention between the three mechanisms is the action of hydrogen transfer in the initiation of a new propagating radical chain.
Scheme 1.5; Proposed mechanisms for CCTP. Where Rn and R1are the polymeric
and monomeric radicals respectively, M is the monomer, LCo(II) is the cobalt chelate CTA and Pn= is a polymer with unsaturated chain end. Adapted from
reference.15
The first proposed mechanism (1) requires the formation of an intermediate Co complex through reaction with the propagating radical followed by hydrogen abstraction from the monomer by the complex to give a new propagating radical chain.28,29 This mechanism was tested, and although the initial step has been observed, the mechanism is improbable as the monomer is not directly involved in the hydrogen abstraction step.42 The second mechanism (2) involves using a Michaelis-Menton-type mechanism akin to enzymatic action.16 This mechanism shows a dependence of the rate of chain transfer upon the concentration of monomer, which was quickly disproved.34,43 The third proposed mechanism (3) involves the disproportionation of the Co catalyst with a polymeric radical, yielding a Co(III) hydride followed by the reinitiation of a monomer to form a new
propagating chain. Although the LCo(III)H has never been observed in context, kinetic studies by Smirnov and co-workers,18,19 and, later work carried out by O’Driscoll and Gridnev in combination with well documented work on cobalt hydrides,27,44-46have led to general agreement on this mechanism.15,42,47-50
As with conventional chain transfer, the effect of CCTP is to reduce the molecular weight without reducing the rate of polymerisation in the reaction. It differs from conventional chain transfer in that the CTA is recovered in the process and is capable of reinitiating polymerisation, as a result the mechanism can be displayed as a catalytic cycle (Figure 1.3).
Figure 1.3; Mechanism for catalytic chain transfer of methacrylates cobalt complex COBF.
There still remain a few unexplained anomalies in the proposed mechanism, caused by the difficulty of analysing the active species due to the paramagnetism of the catalyst, rendering NMR difficult. One such anomaly is that it is considered unlikely
occurs via a radical pathway, and, is supported by kinetic isotope experiments by Gridnev.51