Anionic-cationic polymerization stages

Transformation of Anionic Polymerization into Cationic Polymerization. Richards et al. (26. 27, 73-75) proposed several methods for the transformation of a living anionic polymeric chain end into a cationic one. Such a process requires three distinct stages polymerization of a monomer I by an anionic mechanism, and capping of the propagating end with a suitable but potentially reactive functional group isolation of polymer I, dissolution in a solvent suitable for mechanism (2), and addition of monomer II and reaction, or change of conditions, to transform the functionalized end into propagating species II that will polymerize monomer II by a cationic mechanism (73). 

Up till now, the predominant and, it should be mentioned, successfully solved problems have been related to the determination of the nature (cationic, free-radical or anionic) and the structure of the active center of the growing polymer chain represented by an asterisk in Scheme 1. However, the investigation of the process of the direct insertion of the monomer in the polymer chain, i.e. everything represented in Scheme 1 by an arrow - was considered to be of secondary importance, with the exception of anionic coordination polymerization. It is usually a priori assumed that this is an elementary single-stage activation transition in the literal sense without any peculiar features, and if these features even exist, they are completely predetermined by (Fig. 1). 

As in the case of cationic polymerization, the presence of a metal atom can drastically change the electronic parameters of ring opening. In other words, the counterion may play the role of a symmetry switch , i.e. it can induce the reversion of stereospecificity of the active center. The possibility of anionic chain propagation on contact ion pairs via on intermediate stage of the formation of monomer-separated ion pairs was considered by Erusalimsky as far back as 1970 [61]. However, even 20 years later the author of the present paper does not attempt to discuss this problem in detail. 

Only a few of the radical cations formed in this way dimerize, however, to the corresponding dications. Consequently, in contrast to anionic polymerizations, electron transfers are poor methods of synthesis for living cationic polymerizations to triblock polymers. However, dications can be produced in a single-stage process from trifluoromethane sulfonic anhydride with, for example, tetrahydrofuran ... 

The anionic pol5mierization, similarly as the cationic polymerization, takes place in three stages ... 

Many researchers have investigated the use of amines and alcohols as initiators for the ROP of lactones. As a rule, amines and alcohols are not nucleophilic enough to be efficient initiators, and it is then mandatory to use catalysts to perform the polymerization successfully. Nevertheless, highly reactive p-lactones exhibit a particular behavior because their polymerization can be initiated by nucleophilic amines in the absence of any catalyst. As far as tertiary amines are concerned, the initiation step implies the formation of a zwitterion made up of an ammonium cation and a carboxylate anion, as shown in Fig. 20. Authors coined the name zwitterionic polymerization for this process [80]. Nevertheless, this polymerization is not really new because the mechanism is mainly anionic. Interestingly, Rticheldorf and coworkers did not exclude the possibility that, at least at some stage of the polymerization, chain extension takes place by step-growth polycondensation [81]. 

The key feature distinguishing anionic (and cationic) from free-radical polymerization is the absence of spontaneous binary termination and has already been mentioned. Unless chain transfer occurs, polymer chains keep growing until all monomer is used up. At that stage, the polymer still carries reactive centers [65] —it is said to be a "living polymer" [66,67]—, and polymerization can be started anew by addition of further monomer. Block copolymers can be synthesized from a living polymer by addition of a different monomer [68,69]. 

Bawn, Ledwith, and Matthies59 have put forward a polymerization scheme which is consistent with all the known facts. They suggest that, depending on the stage of polymerization, fluoride or —(CH2) F anions migrate to and neutralize the cation. 

In chain polymerization initiated by free radicals, as in the previous example, the reactive center, located at the growing end of the molecule, is a free radical. As mentioned previously, chain polymerizations may also be initiated by ionic systems. In such cases, the reactive center is ionic, i.e., a carbonium ion (in cationic initiation) or a carbanion (in anionic initiation). Regardless of the chain initiation mechanism—free radical, cationic, or anionic—once a reactive center is produced it adds many more molecules in a chain reaction and grows quite large extremely rapidly, usually within a few seconds or less. (However, the relative slowness of the initiation stage causes the overall rate of reaction to be slow and the conver-... 

The electrochemical polymerization of PVK in LiClOVacetonitrile solution on an SWNT electrode was studied by cyclic voltammetry [194,195], The mechanism of the electropolymerization reaction of VK on the SWNT film was characterized by three stages, chemical-electrochemical-chemical [194]. The main difference between the mechanism of electropolymerization of VK on a Pt electrode only, and an electrode covered with a SWNT film consists in the fact that during the first stage, the formation of a charge-transfer complex results in the formation of VK radical cations and the SWNT radical anions. 

In this process, the only chain-extension reaction is that of attachment of a monomer to a growing active chain. The active end group may be a free radical, an anion, or a cation. In contrast to step-growth polymerization, some high-molecular-weight polymer is formed in the early stage of the addition polymerization. 

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