Controlled/living polymerisation

In this somewhat distinctive volume it is possible to trace Peter s thought processes over the years as well as the development of cationoid polymerisations. While much current emphasis in polymer chemistry is on so-called controlled, living polymerisation processes , an emphasis I consider exaggerated, the contents of this volume are concerned with the broader aspects of cationoid polymerisations and not of selected modern developments. 

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). 

Controlled/living radical polymerisation (CRP) is currently a fast developing area in polymer synthesis and it allows preparation of many advanced polymeric materials, including thermoplastic elastomers, surfactants, gels, coatings, biomaterials, materials for electronics and many others. 

K. Matyjaszewski (Ed.), Controlled/Living Radical Polymerisation, American Chemical Society, Washington, DC, 2000. 

During the last two decades, chemists have become increasingly focused on how molecules interact, i.e. on supramolecular chemistry. Dynamic intermolecular processes provide opportunities for incorporation of control, adaptation and function in man-made materials, as observed in living systems. In biology, these processes are tightly controlled by the catalytic action of enzymes. In this chapter, we focus on enzymatically controlled supramolecular polymerisation, whereby self-recognising molecular building blocks assemble to form extended onedimensional (ID) structures, or supramolecular polymers, with unique adaptive features. 

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

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

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. 

The polymer displays narrow molecular weight distributions characteristic of living polymerisation systems. Among catalysts exhibiting an ability to perform such controlled polymerisations of dicyclopentadiene according to scheme (40), titanacycles and Mo or W alkylidenes are representative [142, 143] ... 

As in the case of the ring-opening metathesis polymerisation of cycloolefins, an important matter is the control of polymerisation to prepare acetylenic polymers having precise structures. A living polymerisation is of practical importance in the synthesis of monodisperse polymers, such as terminally functionalised polymers and block copolymers. The metathesis catalysts that promote the living polymerisation of acetylene [42] and acetylenic monomers include M0OCI4 SnBu EtOFkNbCls and Ta, Mo and W alkylidenes [84, 133, 152, 153]. 

Within the context of stable carbenium salts initiatron, we already examined a very interesting and successful study on the block copolymeriation of a-methylstyrene with cyclopentadiene performed by Vairon and Villesange (see Sect. V-A-4-b). The preparation of the product required three basic operations (i) the living anionic polymerisation of a-methylstyrene to give monodisperse macromolecules, (ii) transformation of their end groups into stable carbocationic moieties, and (iii) initiation of the polymerisation of cyclopentadiene from these active ends under conditions of minimal transfer and termination reactions. Thus, the macroinitiators in the second polymerisation were generated by a controlled anionic polymerisation and allowed tiie synthesis of a triblock near-isomolecular copolymer. 

Figure 5 Comparison of conventional free-radical polymerisation, controlled living free-radical polymerisation and the middle ground of statistical I pragmatic modification...

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. 

Thus, the polymerisation of methyl methaciylate in toluene was investigated using an initial eatalyst/initiator/monomer molar ratio of 1 2 800 with ethyl 2-bromo-2-methyl-propionate as the initiator. Under these experimental eonditions, all the criteria of living polymerisation were fulfilled. Indeed, the plot of hi([M o/[Af]t) versus time followed a linear relationship, which is typical for a controlled polymerisation. Furthermore, a linear evolution... 

Matyjaszewski, K. and M. Sawamoto, Controlled/Living Carbocationic Polymerization, Chap. 4 in Cationic Polymerisations Mechanisms, Synthesis, and Applications, K. Matyjaszerski, ed., Maicel Dekker, New Ymk, 1996. 

A development of particular importance for the controlled production of block copolymers is the perfection of various so-called living polymerisation techniques. In the classical addition polymerisations there was always a termination stage, leading to the production of chains with non-reactive groups at both ends of the polymer chain. Polymerisation could therefore stop before all monomer had been exhausted, although ideally the termination step was of much lower probability than the propagation step. In living polymerisations there is no termination step and the reaction proceeds in the ideal case until all monomer has been exhausted. The chains still have reactive ends and a second type of monomer can then be added to the reaction to produce a block of a different type of polymer. 

Living polymerisations are processes that are virtually free of chain transfers and termination reactions. They permit the synthesis of homopolymers with controlled molecular weights, narrow polydispersities and well-defined terminal functionalities and also the synthesis of well-defined block copolymers. Living polymerisations proceed until all of the monomer has been consumed and further additions of monomer result in continued polymerisation. 

ROMP now represents a well-understood technique and titanium-, molybdenum-, and ruthenium-based systems permit living polymerisations. Living in this context means that a controlled initiation takes place and that chain transfer as well as chain terminating reactions, due to possible side reactions (backbiting and secondary isomerisation) of the active metal carbene species with the inner double bonds of the polymer chains, are absent during propagation or more realistically are very limited. 

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