Zwitterionic Polymerizations

Zwitterionic propagating species have been implicated in both homopolymerization and copolymerization reactions. An early example of the involvement of zwitterions in an alternating copolymerization was provided by the copolymerization of p-benzoquinone diazides with tetrahy-drofuran [Eq. (88)] [314,315]. This reaction could be thermally or photo-chemically initiated. A similar 1 1 alternating copolymer was obtained 

Several types of zwitterionic polymerization reactions have been identified. One type is spontaneously initiated upon mixing a nucleophilic monomer with an electrophilic monomer. Another type involves the thermal generation of a reactive zwitterion. Finally, zwitterions that are sufficiently stable to be isolated have been polymerized. 

Spontaneous Initiated Alternating Copolymerization of Nucleophilic-Electrophilic Monomer Pairs 

Zwitterions are the propagating species in a number of spontaneous polymerizations that occur upon mixing electron donor and acceptor monomers. Several reviews of spontaneous polymerizations via zwitterion intermediates are available [317-323]. In this novel type of polymerization reaction there is no need to add an initiator or catalyst. Alternating copolymers have been obtained in some cases when at least one of the monomers contains a heterocycle. In these polymerizations a nucleophilic monomer (Mn) and an electrophilic monomer (Me) react to form a zwitterion [Eq. (89)]. This zwitterion is referred to as a genetic zwitterion.  

Polymer chain growth is initiated by the dimerization reaction of two molecules of zwitterion 487 to form oligomeric zwitterion 488 [Eq. (89)]. Polymer chain growth continues by the addition of other zwitterions. These zwitterions can be the genetic zwitterion 487, oligomeric zwitter- 

Zwitterionic Polymerizations. Zwitterionic intermediates also appear to be responsible for the spontaneous specific 1 1 copolymerizations of some acrylate-type monomers with cyclic nucleophilic species reported by Saegusa and his CO-workers.In many ways these resemble the spontaneous polymerizations of strong donor and acceptor vinyl moities mentioned earlier. s — 

Johnston and D. C. Pepper, Proceedings of the lUPAC International Symposium on Macromolecules , Dublin, 1977, Vol. I, p. 121. 

209 -j- Saegusa, S. Kobayashi, and Y. Kimura, Pure Appl. Chem., 1976, 48, 307. 

Pascault, G. Merle, R. Salle, J. Gole, Q. T. Pham, I. Panayotov, and I. Rashkov, Abstracts 1st European Discussion Meeting on Polymer Science, Strasbourg, 1978, p. 146. 

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Ziegler-Natta (or Natta-Ziegler) catalyst (ZNC) Able to produce stereoregular pol5uners. zwitterionic polymerization Copolymerization between nucleophilic and electrophilic comonomers. 

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]. 

Fig. 20 Zwitterionic polymerization of pivalolactone initiated by tertiary amines...

Kricheldorf HR, Garaleh M, Schwarz G (2005) Tertiary amine-initiated zwitterionic polymerization of pivalolactone - a reinvestigation by means of MALDI-TOF mass spectrometry. J Macromol Sci, Part A Pure Appl Chem 42 139-148... 

Certain combinations of nucleophiles and electrophiles undergo polymerization without the need for initiator [Culbertson, 2002 Kobayashi and Saegusa, 1985 Kobayashi and Uyama, 2002 Saegusa, 1977, 1979, 1981]. The polymerization, referred to as zwitterion polymerization, proceeds via zwitterion intermediates. For example, polymerization between 2-oxazo-line and P-propiolactone involves nucleophilic attack of 2-oxazoline on P-propiolactone to form the dimer zwitterion LXXXIX, which reacts with itself to form the tetramer zwitterion LXXXX. The latter reacts with itself and with LXXXIX to form octamer and hexamer... 

A donor-acceptor zwitterionic polymerization has been mentioned 2-ethyl-oxazoline nucleophilic monomer reacts with the electrophilic N-phenyl maleimide giving a 1 1 adduct which polymerizes upon heating (Fig. 20). A similar reaction was observed between bismaleimide and bis-oxazoline giving a crosslinked network stable up to 300 °C [72]. 

Poly(alkyl-cyanoacrylates) As poly(alkyl-cyanoacrylates) form strong bonds with polar substrates including the skin and living tissues, they exhibit bioadhesive properties. These polymers are synthesized by free-radical, anionic, or zwitterionic polymerization. As detailed in a recent review, poly(alkyl-cyanoacrylate) nanoparticles are prepared by emulsion polymerization, interfacial polymerization, nanoprecipitation, and emulsion-solvent evaporation methods [102],... 

A variety of monomer pairs have been used in spontaneous zwitterion polymerizations. Examples of nucleophilic and electrophilic monomer pairs, genetic zwitterions, and polymer structures are shown in Table 2. 

Table 2 Examples of nucleophilic and electrophilic monomers that participate in spontaneous zwitterionic polymerization... 

Stable zwitterions containing ammonium and carboxylate ionic centers have also been polymerized. The bulk zwitterionic polymerization of... 

Two types of polymerization can be distinguished in this category, those which require other molecules or ions as initiators and those which do not. Pure cyclic sul-fonium zwitterions polymerize when heated. The literature gives no indication that diazoalkanes or nitrile oxides will spontaneously polymerize in a similar manner. 

Evidence to support hypothesis (ii) comes from Schmidt s observation that when salt is added, fewer macrocycles are formed during cyclic sulfonium zwitterion polymerizations. 

Those authors who have studied zwitterionic polymerizations in more than one solvent find that the more polar the solvent the faster the polymerization. 

Objections to the zwitterion polymerization mechanism because of the high Coulombic energy of charge separation are not necessarily valid because the propagating chains need not be linear, with increasing distance between the two ionic end groups. They may be cyclized or paired with another chain throughout the polymerization. 

Mixed cyclic anhydrides (e g. 3-hydroxy-l-propanesulfonic acid sulfone 2,)) provide the zwitterionic polymerization of l-(2-cyanoethyl)azetidine 22) and l-(2-cyanoethyl)-aziridine 231. The sulfonate anions are sufficiently stable for the formation of high-molecular-weight polyamines. 

Random copolymerization of different oxazolines is described in Section 15.1,2.2. The reactivity ratios increase as expected with monomer basicity 18). Statistical copolymers of cyclic imino ethers with other groups of monomers are not known, although oxazolines and oxazines form readily alternating copolymers with a number of electrophilic monomers by spontaneous zwitterionic polymerization. The mechanism and examples of this process are discussed in Section 15.2.1. 

Zwitterionic Polymerization by Classical and Frustrated Lewis Pairs... 

Scheme 14 Generalized scheme for the zwitterionic polymerization of conjugated polar vinyl monomers by classical or frustrated Lewis pairs (CLP or FLP) through zwitterionic active specie...

Anionic polymerizations are initiated in polar systems by bases and Lewis bases. For example, alkali metals, alcoholates, metal ketyls, metal alkyls, amines, phosphines, and Grignard compounds act as initiators. However, the polymerization mechanism does not depend on the nature of the initiator alone. For example, tertiary amines and phosphines do not only initiate anionic polymerizations under certain conditions, they can also initiate zwitterion polymerizations. In addition, polyinsertions can proceed in less polar systems. Thus, anionic polymerizations are often carried out in polar solvents. Ethers and nitrogen compounds, such as tetrahydrofuran, ethylene glycol dimethyl ether (glyme), diethylene glycol dimethyl ether (diglyme), pyridine, and ammonia are most commonly used. 

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