Cationic polymerisation Subject

Some explanation is required why my title involves the adjective cationoid instead of the traditional cationic . As most of those familiar with the subject have known for some years, I use this term to include both the cationic polymerisations by carbenium ions and also those polymerisations in which an alkene is inserted into the strongly polarised covalent bond of an ester, the cationoid insertions I have seen no convincing reason for changing my well known opinion that these are different types of reactions, and that a clear distinction between them is heuristically useful. 

The next chapter is a contribution to a collective volume containing detailed treatments of various themes in polymer science. The author took the opportunity to provide answers to some of the well-aimed questions about cationic polymerisations that had been put by two experienced polymer chemists (A) and from this the piece evolved into a conspectus of most parts of the subject. As such it is a useful snap-shot of this author s thinking at that time. However, because of his familiarity with the subject, his piece turned out to be a rather daunting assembly of worst case scenarios , but for this very reason the article is also useful in showing just how complicated the subject can become. 

This paper may be regarded as a sequel to my second book on Cationic Polymerisation [1]. I have aimed here at providing a fairly detailed discussion of some theoretical aspects of the subject which is still (or perhaps now more than ever before) in Dainton s words rudis indigestaque moles (a crude and ill-digested, i.e., confused, mass) [2], I also intend to discuss specifically some of the problems raised by Mayo and Morton in their article Ionic Polymerization in the book Unsolved Problems in Polymer Science [3]. 

Reaction with solvent - The solvent influences the course of cationic reactions not only through its dielectric constant, but also because many substances used as solvents are far from inert in these reactions [22, 23]. Although much more experimental material is required before a full treatment of the subject becomes possible, at least one example, the cationic polymerisation of styrene in toluene, is amenable to quantitative discussion. Experiment shows that polymerisation is rapid and complete, the molecular weight is low and the polymer contains para-substituted rings which are almost certainly tolyl endgroups [22]. Theoretically, a polystyryl carbonium ion can react with toluene in six different ways, only two of which (a.l and b. 1 below) can lead to tolyl endgroups in the first case the tolyl group is at the end of the terminated chain, in the second it is the start of a new chain. The alternative reactions can be represented as follows... 

No doubt some will say What - again , since I first reviewed this subject of propagation rate-constants in cationic polymerisations in 1971 [1] and discussed it briefly in 1973 [2]. Of the other reviews which have dealt with this subject thereafter, that by D. J. Dunn... 

It is evident from the foregoing exposition that almost all the discussions in the literature about the propagation rate-constants of cationic polymerisations are defective, and that most attempts at their measurement are flawed in some more or less important respect. The present author notes with regret that his own earlier writings on the subject are no more than preliminary approaches, and he quite expects that even the present, much more profound, discussion may yet turn out to be defective in some essential respect. 

The subject of this paper is the group of polymerisations which have living character in the sense defined by Penczek et al., [1]. The term cationoid comprises both the cationic and the pseudo-cationic ( /-cat) polymerisations however, it is a principal contention of this paper that none of the living polymerisations which have been adequately characterised actually show any of the characteristics of cationic polymerisations, but that they all show important features of the j/-cat polymerisations [2, 3]. The converse of this proposition is not true not all /-cat polymerisations are living. 

We feel that several important considerations justify this venture. First, the specific topic we chose within the general subject of cationic polymerisation, i.e., initiation, is perhaps the most critical and we believe that the comments we have to offer might contribute to the clarification of some issues, or at least to the promotion of helpful debates. 

The subject has been subdivided into specific sections according to the chanical type of initiator(s) or the physical technique used to induce initiation. These include both classical methods as old as cationic polymerisation itself, and more modern ones, such as photo-, electro- and nuclear initiation, which offer ajme interesting new ways of exploring the birth of chain carriers. We have devoted our main effort to the discussion of initiation patterns with Br nsted and Lewis acids, which are the most documented and, to our taste, the most attractive ones. The remaining sections are perhaps less thorough, but nonetheless we have attempted to touch upon all the fundamental problems and achievements. 

Photo-initiated cationic polymerisation of epoxidised oils has been the subject of intense scrutiny by the research team led by Crivello, starting with the groundbreaking study in 1992 [1-3] applied to several epoxidised triglycerides, with the linseed homologue shown in Scheme 4.1. Since that contribution, this [1-3] and other research teams have carried out further research, including use of thermally activated... 

In the 1960s, after Kennedy and Thomas [25] had established the isomerisation polymerisation of 3-methylbutene-l, this became a popular subject. From Krentsel s group in the USSR and Aso s in Japan there came several claims to have obtained polymers of unconventional structure from various substituted styrenes by CP. They all had in common that an alleged hydride ion shift in the carbenium ion produced a propagating ion different from that which would result from the cationation of the C C of the monomer and therefore a polymer of unconventional structure the full references are in our papers. The monomers concerned are the 2-methyl-, 2-isopropyl-, 4-methyl-, 4-isopropyl-styrenes. The alleged evidence consisted of IR and proton magnetic resonance (PMR) spectra, and the hypothetical reaction scheme which the spectra were claimed to support can be exemplified thus ... 

Polymerisations of undiluted, bulk monomer are rare except for those initiated by ionising radiations and they require a special treatment which will be given later. The most common situation is to have the propagating ions in a mixture of monomer and solvent, and as the solvation by the solvent is ubiquitous and may dominate over that by other components of the reaction mixture, mainly because of the mass-action effect, it will not be noted by any special symbol, except in a few instances. This means that we adopt the convention that the symbol Pn+ denotes a growing cation solvated mainly by the solvent correspondingly kp+ denotes the propagation constant of this species, subject to the proviso at the end of Section 2.3. Its relative abundance depends upon the abundance of the various other species in which the role of the solvent as the primary solvator has been taken over by any or all of the anion or the monomer or the polymer. The extent to which this happens depends on the ionic strength (essentially the concentration of the ions), and the polarity of the solvent, the monomer and the polymer, and their concentrations. 

Organolanthanides have been investigated as anionic polymerisation initiators for methacrylates and acrylates and a review on this subject has appeared [88]. In one example THF was polymerised cationically using a tri- or tetra-functional initiator (e.g. l,2,4,5-tetrakis(bromomethylbenzene)) with AgOTf as coinitiator and terminated by NaOOCCMc2Br. The bromo- terminated chain was treated with Sml2 and MMA polymerised in THF at —78 C (7.2 < M /(kg/mol) < 16 1.04 < Mw/M < 1.21) [89]. 

---