Polymerisation rearrangement

With appropriately substituted oxetanes, aluminum-based initiators (321) impose a degree of microstmctural control on the substituted polyoxetane stmcture that is not obtainable with a pure cationic system. A polymer having largely the stmcture of poly(3-hydroxyoxetane) has been obtained from an anionic rearrangement polymerisation of glycidol or its trimethylsilyl ether, both oxirane monomers (322). Polymerisation-induced epitaxy can produce ultrathin films of highly oriented POX molecules on, for instance, graphite (323). Theoretical studies on the cationic polymerisation mechanism of oxetanes have been made (324—326). 

Polyether Polyols. Polyether polyols are addition products derived from cyclic ethers (Table 4). The alkylene oxide polymerisation is usually initiated by alkah hydroxides, especially potassium hydroxide. In the base-catalysed polymerisation of propylene oxide, some rearrangement occurs to give aHyl alcohol. Further reaction of aHyl alcohol with propylene oxide produces a monofunctional alcohol. Therefore, polyether polyols derived from propylene oxide are not truly diftmctional. By using sine hexacyano cobaltate as catalyst, a more diftmctional polyol is obtained (20). Olin has introduced the diftmctional polyether polyols under the trade name POLY-L. Trichlorobutylene oxide-derived polyether polyols are useful as reactive fire retardants. Poly(tetramethylene glycol) (PTMG) is produced in the acid-catalysed homopolymerisation of tetrahydrofuran. Copolymers derived from tetrahydrofuran and ethylene oxide are also produced. 

The third approaeh to synthetic polymers is of somewhat less commereial importance. There is in fact no universally accepted deseription for the route but the terms rearrangement polymerisation and polyaddition are commonly used. In many respects this process is intermediate between addition and condensation polymerisations. As with the former teehnique there is no moleeule split out but the kinetics are akin to the latter. A typical example is the preparation of polyurethanes by interaction of diols (di-alcohols, glycols) with di-isocyanates Figure 2.7). 

One variation of rearrangement polymerisation is ring-opening polymerisation. Important examples include the polymerisation of trioxane, ethylene oxide and e-caprolactam Figure 2.8 (a) to (c) respectively). It is to be noted that... 

In turn the oxazoline-containing polymer may then react very rapidly (e.g. at 240°C) with such groups as carboxyls, amines, phenols, anhydrides or epoxides, which may be present in other polymers. This reaction will link the two polymers by a rearrangement reaction similar to that involved in a rearrangement polymerisation without the evolution of water or any gaseous condensation products (Figure 7.14). 

Such reactions allow chain extension and/or cross-linking to occur without the elimination of small molecules such as water, i.e. they react by a rearrangement polymerisation type of reaction. In consequence these materials exhibit a lower curing shrinkage than many other types of thermosetting plastics. 

This particular polymer is a fibre-forming material (Perlon U). Although in many respects this reaction resembles the formation of polyesters and polyamides it is not a condensation reaction but involves a transfer of hydrogen atoms and thus may be considered as an example of rearrangement polymerisation. 

Rearrangement polymerisation Here the mechanism resembles condensation polymerisation hut no stnall mbleculfr is split out. In the first example l 4-butane di6l reacts with hexaniethyiehe di-isocyanate to give 6,4-poly-... 

It explodes on heating or in contact with concentrated acids (the latter possibly involving Beckmann rearrangement and/or polymerisation of the oxime form, which is effectively a 1,3-diene). 

It can decompose violently, for unknown reasons, during vacuum distillation (probably exothermic Beckmann rearrangement and subsequent polymerisation — Editor)... 

The rearrangement of platinacyclobutanes to alkene complexes or ylide complexes is shown to involve an initial 1,3-hydride shift (a-elimina-tion), which may be preceded by skeletal isomerization. This isomerization can be used as a model for the bond shift mechanism of isomerization of alkanes by platinum metal, while the a-elimination also suggests a possible new mechanism for alkene polymerisation. New platinacyclobutanes with -CH2 0SC>2Me substituents undergo solvolysis with ring expansion to platinacyclopentane derivatives, the first examples of metallacyclobutane to metallacyclopentane ring expansion. The mechanism, which may also involve preliminary skeletal isomerization, has been elucidated by use of isotopic labelling and kinetic studies. 

The ether-catalyst complex (II) splits into a complex anion (III) and a carbonium ion (IV), which rearranges to the configuration of maximum stability (V). This carbonium ion (V) could itself initiate polymerisation, but it is more likely that it attacks the double bond of the closely associated anion (III), giving the double ion (VI) in equilibrium with the aldehyde (VII). Rearrangements of the type (I)-(VII) have been observed for vinyl ethers [7], and a closely parallel isomerisation is that of isobutyl phenyl ether into para-tertiary butyl phenol under the influence of A1C13 [8]. It is unlikely that the steps from (II) to (VI) take place in a well defined succession. The process probably proceeds by a single intramolecular transformation. 

The well-known addition of tert-butyl chloride to ethylene by means of A1C13 without skeletal rearrangement is simply another example of the initiation step of a /-cat polymerisation, involving the insertion of the alkene into an ester, namely the C-Cl bond, which is activated by the A1C13, via a six-centred transition state (I), as shown in equation (ii) ... 

Further examples of zwitterionic products are formed by B(C6F5)3 attack on tuck-in complexes to give 20 [57-59] and 21 (Scheme 8.13) the latter is a single-component polymerisation catalyst which deactivates under rearrangement to an allyl complex [60]. Attack by B(C6F5)3 on the coordinated ethene in zirconium(II) complexes gives zwitterions of type 22 which act as single-component polymerisation catalysts [58]. 

The determination of the microstructure of vinyl polymers is not merely a characterisation tool. Each polymer molecule is unique, and each polymer chain is a record of the history of its formation, including mis-insertions, rearrangements, the incorporation of co-monomers, and the mode of its termination. NMR analysis of polymers can therefore be used to provide detailed mechanistic and kinetic information. This approach has been applied particularly successfully to the microstructure, i. e. the sequence distribution of monomer insertions, of polypropylene, giving rise to a wealth of studies far too numerous to cover here. Progress in this area has recently been summarised in two excellent and very comprehensive review articles [122, 123[. Here we will cover only the most fundamental aspects of stereoselective polymerisations. 

Conjugated dienes, 11 Conjugation, 11, 368 Conrotation, 345 Coordination polymerisation, 322 Copolymerisation, 322 Cope reaction, 268 Cope rearrangement, 354 Cracking, 112, 305 Cram s rule, 235 Cross-linking, 323... 

Dienes, 11 addition to, 194-198 cisoid conformation, 197, 350 conjugated, 11 Cope rearrangement, 354 cycUsation, 346 cycloaddition to, 348 Diels-Alder reaction, 197, 349 excited state, 13 heat of hydrogenation, 16,194 isolated, 11 m.o.s of, 12 polymerisation, 323 Dienone intermediates, 356 Dienone/phenol rearrangement, 115 Dienophiles, 198, 350 Digonal hybridisation, 5 Dimedone, 202 Dimroth s Et parameter, 391 solvatochromic shifts, 391 solvent polarity, 391 Y and,392 Dinitrofluorobenzene proteins and, 172... 

Radical addition, 312-323 carbon tetrachloride, 320 halogens, 313 hydrogen bromide, 316 sulphenyl halides, 320 vinyl polymerisation, 320 Radical anions, 218 Radical rearrangements, 335 Radicals, 20, 30,299-339 acyl, 306, 330, 335 addition to 0==C, 313-323 alkoxyl, 303... 

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