Methide polymerization reactions

The results presented above indicate that the previously unknown head-to-tail polymerization is the major reaction product of the iminium methide species. To investigate the generality of this reaction, we next studied a neutral ene-imine species shown in Scheme 7.9.48 As illustrated in this scheme, the generation of this reactive species requires quinone reduction followed by elimination of acetic acid. The ene-imine is structurally related to the methyleneindolenine reactive species that is a metabolic oxidation product of 3-methylindole (Scheme 7.9).57 59... 

Fig. 7-25. Main reactions of the phenolic /8-aryl ether structures during alkali (soda) and kraft pulping (Gierer, 1970). R = H, alkyl, or aryl group. The first step involves formation of a quinone methide intermediate (2). In alkali pulping intermediate (2) undergoes proton or formaldehyde elimination and is converted to styryl aryl ether structure (3a). During kraft pulping intermediate (2) is instead attacked by the nucleophilic hydrosulfide ions with formation of a thiirane structure (4) and simultaneous cleavage of the /3-aryl ether bond. Intermediate (5) reacts further either via a 1,4-dithiane dimer or directly to compounds of styrene type (6) and to complicated polymeric products (P). During these reactions most of the organically bound sulfur is eliminated as elemental sulfur.

Similarly, isomerized carbenium ions are formed in polymerizations of a-methylstyrene derivatives by either a 1,3-intramolecular or bimolecular methide anion shift, or by reaction of carbenium ion with an exo-unsatu-rated oligomer [cf., Eq. (10)] [13]. 

The formation of stable carbenium ions can be observed visually and/ or spectroscopically. For example, styrene and a-methylstyrene polymerizations are generally colorless because the growing carbenium ions absorb at approximately 340 nm (cf., Sections II.B and IV.B.l). However, these systems may turn brown or dark red at longer reaction times due to formation of indanyl carbenium ions (A 440 nm) [14,26,325] and other delocalized carbocations similar to those in Eq. (121). The stable cyclic diaryl carbenium ions are generated by hydride transfer from the initially formed indanyl end groups [Eq. (124)] in styrene polymerizations, and by methide transfer in a-methylstyrene polymerizations. The prerequisite for this termination is therefore intramolecular transfer by Friedel-Crafts alkylation protons liberated in the first stage can then reinitiate polymerization. 

The red color observed at later stages in polymerization of p-me-thoxystyrenes is probably due to formation of a charge transfer complex with two adjacent aromatic rings as shown in Eq. (9). The internal tertiary carbocation is sterically not accessible and is unreactive. It may be formed by a 1,3-methide shift or by reaction of unsaturated oligomers with growing species [13]. In the latter case, transfer by elimination occurs before termination. 

Attachment of B ansformation Products of Stabilizers. Up-to-date knowledge dealing with the chemistry of transformation products of phenolic [6, 15, 17, 20] and aromatic aminic [16, 43, 230] antioxidants and photoantioxidants based on hindered piperidines [10] indicates the possibility of attaching compounds having structures of quinone imine or quinone methide, or of radical species like cyclohexadienonyl, phenoxyl, aminyl or nitroxide to polymeric backbones. These reactions proceed mostly via reactivity of macroalkyl radicals derived fi-om stabilized polymers. Various compounds modelling this reactivity have been isolated [19, 230]. These results are of importance mainly for the explanation of mechanisms of antioxidant activity [6, 22, 24]. 

In some cases, the propagation reaction is accompanied by intramolecular rearrangements due to hydride ion (H ) or methide ion (CH3 ) shifts. Such polymerizations are referred to as isomerization polymerizations. Consider, for example, the polymerization of 3-methyl-1,2-butene in which the carbocation (XV), formed initially, isomeiizes by a 1,2-hydride shift. The resulting ion (XVI), being a tertiary carbocation, is more stable than (XV) which is a secondary carbocation. 

The factors that control the feasibility and the stereochemical course of the coupling process, as well as the methods to establish the configuration at C(4) of the condensation products and the mode of interflavanyl linkage were sufficiently reviewed (4, 27-29). Acid-catalyzed reactions to produce flavan-4-carbocations or A-ring quinone methides that may react with the A-rings of flavan-3-ols to produce oligo- and polymeric proanthocyanidins have been so successfully employed that they were called biomimetic syntheses (39, 40). The most recent variations of this theme are now briefly discussed. The nomenclature delineated in ref. (1) will be consistently employed. 

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