Controlled radical polymerisation

Ideally, bimolecular termination should be eliminated from the reaction. The only way to do this is to have a very low concentration of radicals, which in turn would mean a very low rate of polymerisation. In many circumstances this would be too slow for use in industrial processes, and a compromise between molar mass control and the rate of the reaction should be made. 

The active radical is turned into a dormant species. This dormant species is reactivated every once in a while. In total the time that the radical is in an active form is the same as for a free radical polymerisation (seconds), but because the active/dormant ratio is low the overall reaction times are large. Because the active/dormant ratio is low, the concentration of fi ee radicals is low. Recalling Equation 2.5 

The number average molar mass, Mn, and molar mass distribution (MMD) are controlled by interplay of kinetic parameters, which will be described in more detail in Chapter 5. In principle there are two ways of terminating a growing chain bimolecular termination and transfer. These two modes of termination have led to the two main categories of living radical polymerization (a) reversible termination and (b) reversible chain transfer. 

Reversible termination requires the deactivation of active polymeric radicals through termination reactions to form dormant polymer chains, and activation of dormant polymer chains to form active chains. On the other hand, reversible chain transfer requires active chains to undergo transfer reactions with the dormant chains, and thus the reversible chain transfer end-group is transferred from dormant to active species. A narrow MMD is observed when this exchange reaction is fast. In aU these techniques, the MMD can be controlled such that the polydispersity index (D) is below 1.1. 

A further discussion of the living radical polymerisation techniques and its application in heterogeneous polymerisation techniques can be found in Chapter 5. 

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Controlled Radical Polymerisation ed. K. Matyjaszewski, ACS Symposium Series 685, Amer. Chem. Soc., Washington DC, USA, 1998. 

Nitroxide-mediated living/controlled radical polymerisations (NMP)... 

Scheme 10.12 Nitroxide (T ) as the living reagent in nitroxide-mediated living/controlled radical polymerisation (NMP).

The nickel-containing systems are also active in controlled radical polymerisation (ATRP). See a) T. E. Patten, J. Xia,... 

However, despite considerable progress, the truly living character is far from being attained and it seems preferable to use the term controlled process rather than living process. Recently, various methods to synthesise block copolymers by radical polymerisation or telomerisation were reviewed [349]. But, to our knowledge, the literature does not mention any investigation of controlled radical polymerisation of fluorinated polymers. 

Such above chemical equations show the essential difference between the traditional radical polymerisation and the controlled radical polymerisation in the presence of counter radicals or metallic complexes. 

Figure 1.12 shows the timeline of discovery of various styrenic polymers and copolymers. It would be naive to suggest that the rate of invention and innovation will level off in this century. Rather, the pace of discovery of new styrenic polymers will probably increase. Advances in new catalyst technology and controlled radical polymerisation technology will undoubtedly yield new styrenic polymers with well-defined architecture, as we have recently seen with the introduction of syndiotactic PS and ethylene-styrene interpolymers. 

P. Tordo, presentation at EPF Workshop on Controlled Radical Polymerisation , 1999. 

The field of controlled radical polymerisation (CRP) has seen rapid growth during the past decade, growth that in more recent years has begun to embrace work on CRP in dispersed media. The two-phase nature of heterogeneous polymerisations, however, imposes severe constraints on what can be achieved and restricts the scope for extending the established methods of CRP to heterogeneous systems. 

A corresponding principle applies to controlled radical polymerisation performed in quite a number of modes such as nitroxide-mediated polymerisation (NMP), atom transfer radical polymerisation (ATRP), reversible addition fragmentation chain transfer (RAFT) or catalytic chain transfer (CCT) reactions. All of these variants of controlled radical polymerisation lead to well-defined architectures with the particular advantage that a much larger number of monomers are suitable and the reaction conditions are much less demanding than those of living ionic polymerisation reactions. 

Although in controlled radical polymerisation, termination reactions cannot be excluded completely, they are limited in their extent and consequently the molecular weight is controlled, the polydispersity index is small and functionalities can be attached to the macromolecules. These features are indicative of the realisation of well-defined polymer architectures such as block copolymers, starshaped and comb-shaped copolymers. 

The present volume is particularly concerned with the use of the different modes of controlled radical polymerisation for the preparation of copolymers such as random copolymers, linear block copolymers, as well as graft copolymers and star-shaped copolymers. It also presents the combination of controlled radical polymerisation with non-controlled radical copolymerisation, cationic and anionic polymerisation,both of vinyl monomers and cyclic monomers, and ringopening metathesis polymerisation. 

The power of controlled radical polymerisation is demonstrated convincingly and the limitations of the synthetic approaches clearly indicated. 

Last but not least the volume presents some potential applications for copolymers obtained by controlled radical polymerisation. It is expected that the first commercial products will appear on the market this year, giving convincing evidence for the importance of controlled radical polymerisation methods. 

Blencowe, A., Tan, J.F., Goh, T.K., Qiao, G.G. Core cross-linked star polymers via controlled radical polymerisation. Polymer 50, 5-32 (2009)... 

Figure 3.4 Equilibrium between sleeping and active species in controlled radical polymerisation.

Transition metal catalysed ATRP is one of the most efficient methods to control radical polymerisation [13]. ATRP is based on the reversible formation of radicals from alkyl halides in the presence of transition metal complexes, and is a direct extension to polymers of the Kharasch reaction, ATRA, (Scheme 4). Among the plethora of catalysts (or precatalysts) described in the literature for ATRP, the copper systems developed by Matyjaszewski [3, 14] and the ruthenium complexes introduced by Sawamoto [15] play a most prominent role and set the standards in the field (Scheme 5). 

With the rapid expansion in controlled radical polymerisation chemistry, for example, atom-transfer radical-polymerisation (ATRP) [47], it is clear that combinations with the inorganic polyphosphazene backbone, and its many unique properties, can add extra dimensions and multiply the opportunity for new hybrid materials. As the inorganic component in such polymers is low, often below 5%, the resultant polymers often possess the solution, chemical and biological properties of the attached organic component and can, in effect, be viewed as highly branched versions on an inorganic (potentially degradable) backbone. 

Barbey, R., Lavanant, L., Paripovic, D., Schuwer, N., Sugnaux, C., Tugulu, S., Klok, H. A. 2009. Polymer brushes via surface-initiated controlled radical polymerisation Synthesis, characterisation, properties, and applications. Chem. Rev. 109 5437-5527. 

Functionalised PO as block and graft copolymers used as compatibilisers or to increase interactions with other materials are prepared by free radical grafting (the simplest method), metallocene-catalysed copolymerisation of olefins with functional monomers, or anionic polymerisation (silane-containing PO). They are also produced by controlled/living polymerisation techniques such as nitroxide-mediated controlled radical polymerisation, atom transfer radical polymerisation (ATRP), and reversible addition-fragmentation chain transfer (RAFT). 

RAFT polymerisation is controlled radical polymerisation technique that is highly versatile and can be used with a wide range of monomers [38]. In RAFT polymerisation, a chain-transfer agent, typically a thiocarbonyl compound such as a xanthate, dithioester or thiocarbamate, is used in conjunction with a free-radical initiator to control the polymerisation (Figure 2.10). RAFT polymerisation has been... 

The book is focusing on emulsion polymerisation in combination with both conventional and controlled radical polymerisation. Except for miniemulsion polymerisation, more exotic techniques, such as inverse emulsion polymerisation, microemulsion polymerisation and dispersion polymerisation are not covered. Chapter 1 gives a historic overview of the understanding of emulsion polymerisation, while also focusing on the solution of the... 

In this chapter, the basics of free radical polymerisation are described in a concise way with an emphasis on development of molecular mass and rate of polymerisation. For a more extensive discussion of all aspects of free radical polymerisation and controlled radical polymerisation, the reader can resort to two excellent books Moad and Solomon (1995) and Matyjaszewski and Davis (2002). 

Note that when a certain monomer homopolymerises first in some kind of controlled radical polymerisation and the second one is supplied after complete conversion of the first one and so on, block copolymers will be obtained. Controlled radical polymerisation and block copolymer production are discussed in detail in Chapters 2 and 5. 

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