Activated monomer mechanism reactions

A variety of protonic and Lewis acids initiate the cationic polymerization of lactams [Bertalan et al., 1988a,b Kubisa, 1996 Kubisa and Penczek, 1999 Puffr and Sebenda, 1986 Sebenda, 1988]. The reaction follows the mechanism of acid-catalyzed nucleophilic substitution reactions of amides. More specibcally, polymerization follows an activated monomer mechanism. Initiation occurs by nucleophilic attack of monomer on protonated (activated) monomer (XXIV) to form an ammonium salt (XXV) that subsequently undergoes proton exchange with monomer to yield XXVI and protonated monomer. The conversion of XXIV to XXV involves several steps—attachment of nitrogen to C+, proton transfer from... 

Another misconception arises from the statement that a strong base, e. g., C(Ph)3-, Na+ will be converted into an extremely weak acid, i. e. Ph3CH, and will not provide a proton for the decarboxylation of carbamate ions. This again is true, but the proton need not be provided by the conjugated acid — it is given by the non-activated NCA which is converted in this process into an activated NCA. This reaction cannot take place with the N-substituted NCA s and therefore the activated monomer mechanism is not operative for these monomers. 

As discussed already for cationic polymerization of oxiranes, cycliza-tion can be eliminated if polymerization is performed under the conditions at which the activated monomer mechanism operates. This approach was used for cationic polymerization of e-caprolactone and other higher lactones [191]. Thus, in the polymerization of e-caprolactone in the presence of ethylene glycol (EG) and (C2Hs)30 +, PF6- catalyst, linear increase of molecular weight with conversion was observed up to M 3000 and polymers with DP = [M]o/[EG]0 and relatively narrow molecular weight distribution (MJM 1.3) were obtained. No cyclic oligomers were detected in reaction products. Similar results were obtained for polymerization of 5-valerolactone and j8-butyrolactone. Kinetic studies of the AM polymerization of lactones have been reported [192]. 

The activated monomer mechanism was based on the assumption that the major part of propagation reaction was carried out by NCA species rather than NHCOOH or -NH Cwith CO elimination) group at the growing chain end. 

The mechanism, developed by Penczek, in the presence of an excess of hydroxyl groups, is very similar to a solvolysis reaction [55] and is characterised by the presence of the active cationic centre to the monomer in the form of a secondary oxonium cation. The polymer chain extension takes place by the SN2 reaction of hydroxyl groups with the activated epoxide. This mechanism is called activated monomer mechanism (AM mechanism), characterised by the relationship [OH] [PO] (reaction 7.21). 

The random copolymerisation of THF with alkylene oxides by activated monomer mechanism is characterised by reactions 7.27-7.30 [54]. 

A very interesting variant of cationic polymerisation of CPL is based on the polymerisation initiated by hydroxyl compounds, at room temperature [42,43,44]. The mechanism called hydroxo-mechanism is very similar to the activated monomer mechanism developed for cyclic ethers. This kind of polymerisation is practically a living cationic polymerisation and in the particular case of CPL, using various polyols as starters, it is possible to obtain hydroxy-telechelic poly CPL) polyols, with various MW, depending on the molar ratio of CPL per polyol (reactions 8.28). 

Anionic polymerization of 2-hydroxymethyloxetane is imsuccessfiil (34). The failure of such a reaction is most likely due to the fact that oxetanes are not known to ring-open imder basic conditions. The successful cationic ring-opening polymerization of 3-ethyl-3-(hydroxymethyl)oxetane gave hydroxy-functional hyperbranched polyethers (45,46) (Fig. 7). The cationic polymerization can proceed according to two different mechanisms, activated chain end (ACE) or activated monomer mechanism (AMM) (45) (Fig. 8). 

Looking at these reactions, a specularity between the anionic and the cationic mechanism is observable. Different from conventional ionic polymerizations, the growth center in cationic and anionic lactam polymerizations is always neutral, and it is the lactam monomer that carries the ionic charge. Such polymerizations follow what is sometimes referred to as activated monomer mechanism, since the reaaive species are lactam cations/anions, thus activated monomers. However, in order to avoid misunderstandings, it must be well dear that this definition has nothing to do with the term activated... 

The reaction mechanism for the polymerization of a hydroxyalkanoic acid (Eqs. 2-243 through 2-246) is a chain polymerization, often called an activated monomer polymerization. The active site of lipase is its serine a-amino acid unit, which contains a hydroxyl group. The acyl carbon of the hydroxyalkanoic acid undergoes nucleophilic attack by the hydroxyl group of serine to form lipase-activated monomer (Eq. 2-243). Initiation consists of reaction... 

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