POLYMERISATION INITIATOR Subject

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]. 

An alternative to the preparation of polymer stars by the multifunctional coupling route is a multifunctional initiation route. In practice this is not a viable possibility under apolar conditions for anionic polymerisation. However, multifunctional initiation under different conditions and for different mechanisms has been explored. A couple of reviews on the subject have appeared recently [107,108]. A trifunctional analogue of MDDPE, TDPE was investigated for its potential as a precursor for a hydrocarbon soluble trifunctional anionic polymerisation initiator and coupling agent. TDPE reacts with sBuLi in... 

The styrene-diene triblocks, the main subject of this section, are made by sequential anionic polymerisation (see Chapter 2). In a typical system cc-butyl-lithium is used to initiate styrene polymerisation in a solvent such as cyclohexane. This is a specific reaction of the type... 

This individualism also implies that different potential co-catalysts may react differently with the different MtXn. Here is not the place for a treatise on this complicated subject, but it is the occasion to describe how we established the manner in which the A1X3 initiate polymerisations in the most rigorously purified systems. 

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. 

The generality of this title is misleading, since a worthwhile amount of information is only available for perchloric acid. The mechanism of the initiation and propagation in the polymerisation of DCA, especially 1,3-dioxolan (DXL), has been discussed many times [1-7], and the origin and nature of the differences of opinion on these subjects need not be explained here at length. The most important facts can be summarised as follows. 

By far the most studied PolyHIPE system is the styrene/divinylbenzene (DVB) material. This was the main subject of Barby and Haq s patent to Unilever in 1982 [128], HIPEs of an aqueous phase in a mixture of styrene, DVB and nonionic surfactant were prepared. Both water-soluble (e.g. potassium persulphate) and oil-soluble (2,2 -azo-bis-isobutyronitrile, AIBN) initiators were employed, and polymerisation was carried out by heating the emulsion in a sealed plastic container, typically for 24 hours at 50°C. This yielded a solid, crosslinked, monolithic polymer material, with the aqueous dispersed phase retained inside the porous microstructure. On exhaustive extraction of the material in a Soxhlet with a lower alcohol, followed by drying in vacuo, a low-density polystyrene foam was produced, with a permanent, macroporous, open-cellular structure of very high porosity (Fig. 11). 

A high-molecular-weight, insoluble polymer is obtained when perfluoro-2-butyne is subjected to various initiators for free-radical polymerisation (Figure 7.87). The off-white colour of this material is remarkable for a polyacetylene [307, 308]. Indeed, it is largely ignored in discussions on polyacetylenes because, of course, the fact that it is not coloured also means that the system is not conjugated the trifluoromethyl groups keep the TT-systems out of plane relative to each other. 

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. 

Topics which have formed the subjects of reviews this year include excited state chemistry within zeolites, photoredox reactions in organic synthesis, selectivity control in one-electron reduction, the photochemistry of fullerenes, photochemical P-450 oxygenation of cyclohexene with water sensitized by dihydroxy-coordinated (tetraphenylporphyrinato)antimony(V) hexafluorophosphate, bio-mimetic radical polycyclisations of isoprenoid polyalkenes initiated by photo-induced electron transfer, photoinduced electron transfer involving C o/CjoJ comparisons between the photoinduced electron transfer reactions of 50 and aromatic carbonyl compounds, recent advances in the chemistry of pyrrolidino-fullerenes, ° photoinduced electron transfer in donor-linked fullerenes," supra-molecular model systems,and within dendrimer architecture,photoinduced electron transfer reactions of homoquinones, amines, and azo compounds, photoinduced reactions of five-membered monoheterocyclic compounds of the indigo group, photochemical and polymerisation reactions in solid Qo, photo- and redox-active [2]rotaxanes and [2]catenanes, ° reactions of sulfides and sulfenic acid derivatives with 02( Ag), photoprocesses of sulfoxides and related compounds, semiconductor photocatalysts,chemical fixation and photoreduction of carbon dioxide by metal phthalocyanines, and multiporphyrins as photosynthetic models. 

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... 

Kolb and Meier [43] prepared a malonate derivative of methyl 10-undecenoate, which was polymerised further with 1,6-hexanediol using titanium (IV) isopropoxide as a catalyst. This polymalonate, bearing a C9 aliphatic side chain with terminal double bonds, was then subjected to grafting by ruthenium-catalysed cross-metathesis reactions with acrylates or thiol-ene addition reactions. This functionalisation enabled a subsequent Passerini multi-component reaction [44] using the pendant carboxylic-acid moiety of the modified polymers that resulted from the thiol-ene addition of 3-mercaptopropionic acid into the initial double bonds of the polymer. 

The main question here is whether or not polymerisation preserves, alters or destroys initial micelle structure. The answer is still a subject of debate. 

Readers interested in polymer science should be familiar with the photochemistry of polymers. Photochemistry plays a role in polymerisation reactions, in the degradation of the backbone chain, or in cross-Hnking of different chains, all of which can be initiated by light Further, photochemistry has been used in a variety of different ways to change and probe polymer structure and intermolecular interactions. However, there are a number of primary physical processes, which take place between the photon impinging on the polymer and the commencement of chemical reaction. It is these primary physical processes that are the subject of this chapter. 

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