- Chain-length dependence in living polymerization: evidence from kinetic experiments and Monte Carlo simulation of RAFT agents in emulsion polymerization. SW Prescott, MJ Ballard, E Rizzardo and RG Gilbert. 26th Australasian Polymer Symposium, Noosa QLD, July 2003.
Chain-length dependent termination is found to play an important role in emulsion polymerization systems containing RAFT agents, and provides a qualitative explanation of observed features of RAFT/emulsion systems such as retardation in the rate of polymerisation and the acceleration in the rate of polymerisation throughout the course of the reaction.
Living free-radical polymerisation techniques have recently provided unprecedented control over polymerisation reactions. Techniques such as RAFT allow control of molecular weight distribution and molecular architecture . Emulsion polymerization offers significant benefits to living polymerization systems, offering faster rates of polymerization while maintaining good temperature control, negligible change in viscosity with conversion and would have the various environmental advantages of emulsion polymerization compared to solution systems .
While initial RAFT/emulsion studies illustrated the problems in tying these two techniques , subsequent work demonstrated the successful use of RAFT in both seeded  and ab initio  emulsion polymerisation. Importantly, the seeded emulsion polymerization experiments showed :
- at low conversion, nbar is less than that of the non-RAFT system,
- in the presence of RAFT, an increase in nbar is evident throughout the experiment (noting that the experiments described were Interval III experiments so the concentration of monomer is decreasing throughout),
- an inhibition period that decreases with increasing initiator concentration, but is not observed in the equivalent bulk system.
Subsequent kinetic experiments using gamma-radiolysis on the seeded emulsion polymerisation of styrene, mediated by RAFT agents present similarly interesting results. In these experiments, the sample is removed from the initiation source and the rate of polymerisation tracked. The relaxation from in-source to out-of-source shows :
- the in-source nbar is reduced in the presence of the RAFT agent,
- the out-of-source nbar is similarly reduced,
- the relaxation occurs much faster (when treated as a first order loss or a second order loss).
The origins of effects such as these have recently received considerable attention; theories including the slow fragmentation of the intermediate bipolymeric radical [7-9], reversible  and irreversible [11,12] termination of the intermediate radical have been espoused. Their applicability to emulsion polymerisation particles containing only one radical is problematic, however [13 ].
Simulations of emulsion polymerisation systems offer an interesting insight into the systems; however, in the case of the RAFT/emulsion systems, traditional kinetic simulations (numerically solving the differential equations) become prohibitively complex once both the termination and transfer to dormant species reactions are allowed to be chain-length dependent. A Monte Carlo approach is shown to be applicable to these systems and provides interesting insights into the relative importance of the different reaction pathways that are possible . Moreover, the applicability of various kinetic models for emulsion polymerisation are critically evaluated. The consequences for experimental design are as follows:
- Long-chain dormant species lead to an increased lifetime for the radicals compared to short-chain dormant species. Use of an oligomeric RTA-adduct is thus an advantage.
- [RAFT] has little direct effect on the lifetime of the radicals in the particles.
- [RAFT] may have a secondary effect on the kinetics of particle growth with high-activity RAFT agents. An increased [RAFT] will mean that it takes longer to progress from short dormant chains to longer dormant chains, so the retarding effects of short chains lasting for longer into the polymerization.
- Zero-one kinetics may be appropriate for the initial stages of the experiment but are unlikely to be applicable thereafter; pseudo-bulk kinetics may be appropriate (but with a reduced pseudo-first-order termination rate coefficient) at later stages in the experiment. However, there are cases when neither zero-one nor pseudo-bulk kinetics are applicable.
- RAFT-mediated polymerization is never in a true steady state due to CLD kinetics, with the rate coefficients for various processes changing by orders of magnitude over the course of the reaction.
In summary, these effects are due to the change in the chain length of the polymeric radical due to transfer to dormant species, such that significant amounts of short-short termination are seen at low conversions and long-long termination is required at high conversions.
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- S.W. Prescott, (submitted).
Last edited: Friday September 10, 2010
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