Anionic processes

Anionic Processes.— The methods of block polymer synthesis from living polyanions are now well understood and the recently reported syntheses are summarized in Tablet. 

A-B-C-B-A poly(ethylene oxide-b-isoprene-b-styrene-b-isoprene-b-ethylene oxide) 

Morton and L. Fetters, Polymerisation Processes , ed. C. E. Schildknecht and I. Skeist, Wiley-Interscience, New York, 1977. 

Many of the successful syntheses have involved the conversion of a living polyanion chain end to a reactive functional group. One route to a polyolefin-b-polyamide is by co-reaction of two bifunctional prepolymers. Stehlidek and Sebenda - have used the alternative approach of building up the polyamide 

Associated with this well-known propagating species equilibrium, P-lactones behave differently to other larger lactones, due to their high pwlarity and high internal strain of the four-membered ring (Table 9.1). Since 1948, when details of the P-propiolactone (PL) ionic polymerization were first pubhshed [20], much more informahon regarding the polymerization not only of P-lactones but also of substituted P-lactones, has become available [21-27]. 

Such deprotonation/elimination side reactions yield either acrylate (from P-propiolactone) or crotonate species (P-butyrolactone) that are also capable of 

When weak bases or ammonium carboxylates are used as initiators, a similar mechanism has been observed which involves the carboxylate anions responsible for propagation. Subsequently, it was confirmed that, in the case of a,a -dialkyl-P-propiolactone, no chain transfer could occur as a-proton abstraction could no longer take place [29]. Interestingly, a similar result was recently highlighted by Guerin et al. who showed that, compared to poly(malolactonate) (as obtained from a,a, P-trisubstituted P-lactones), polymers prepared by the ROP of P-lactones without substitution in the a-position were characterized by major discrepancies between the experimental molecular weights and the theoretical values expected for a controUed/ Uving polymerization (see below) [30]. 

The Uving character of the anionic polymerization of PL has enabled determination of the absolute rate constants of propagation on free macroions (fe ) and macroion pairs ), since the overaU propagation rate coefficient was shown to be the result of various forms of active species. For the polymerization of PL in CH2CI2 or in dimethylformamide (DMF), the reactivity of the macroion pairs was found to be almost independent of the initial monomer concentration, and weakly dependent on the temperature [33a,b] (cf. AHJ in Table 9.2). 

Haggiage et al. showed the macroion pairs to be more reactive than macroions in 

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One of the key benefits of anionic PS is that it contains much lower levels of residual styrene monomer than free-radical PS (167). This is because free-radical polymerization processes only operate at 60—80% styrene conversion, whereas anionic processes operate at >99% styrene conversion. Removal of unreacted styrene monomer from free-radical PS is accompHshed using continuous devolatilization at high temperature (220—260°C) and vacuum. This process leaves about 200—800 ppm of styrene monomer in the product. Taking the styrene to a lower level requires special devolatilization procedures such as steam stripping (168). 

The standard cation—anion process has been modified in many systems to reduce the use of cosdy regenerants and the production of waste. Modifications include the use of decarbonators, weak acid and weak base resins. Several different approaches to demineralization using these processes are shown in Figure 1. 

A valence bond isomer of pentakis-(trifluoromethyl)-l, 3-diazepine (44) was prepared from (43) (81TL1113) (44) can be transformed thermally or photochemically to a 2,4-diazabicyclo(3.2.0)hepta-2,6 diene (45), which was subsequently photolysed to an imidazole in an anionic process. Compound (45) is highly acidic arising out of the bishomoaromaticity of the anion and forms a salt with Et3N (81TL1369). 

These TMS-carbamate-mediated NCA polymerizations resemble to some extent the group-transfer polymerization (GTP) of acrylic monomers initiated by organo-silicon compounds [40]. Unlike GTPs that typically require Lewis acid activators or nucelophilic catalysts to facilitate the polymerization [41], TMS-carbamate-mediated NCA polymerizations do not appear to require any additional catalysts or activators. However, it is still unclear whether the TMS transfer proceeds through an anionic process as in GTP [41] or through a concerted process as illustrated in Scheme 14. 

There are, however, also many examples of mixed domino processes , such as the synthesis of daphnilactone (see Scheme 0.6), where two anionic processes are followed by two pericydic reactions. As can be seen from the information in Table 0.1, by counting only two steps we have 64 categories, yet by including a further step the number increases to 512. However, many of these categories are not - or only scarcely - occupied. Therefore, only the first number of the different chapter correlates with our mechanistic classification. The second number only corresponds to a consecutive numbering to avoid empty chapters. Thus, for example in Chapters 4 and 6, which describe pericydic and transition metal-catalyzed reactions, respectively, the second number corresponds to the frequency of the different processes. 

Besides the numerous examples of anionic/anionic processes, anionic/pericydic domino reactions have become increasingly important and present the second largest group of anionically induced sequences. In contrast, there are only a few examples of anionic/radical, anionic/transition metal-mediated, as well as anionic/re-ductive or anionic/oxidative domino reactions. Anionic/photochemically induced and anionic/enzyme-mediated domino sequences have not been found in the literature during the past few decades. It should be noted that, as a consequence of our definition, anionic/cationic domino processes are not listed, as already stated for cationic/anionic domino processes. Thus, these reactions would require an oxidative and reductive step, respectively, which would be discussed under oxidative or reductive processes. 

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