Spontaneous zwitterion polymerization

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... 

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. 

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. 

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. 

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 —... 

Spontaneous polymerization of 4-vinyl pyridine in the presence of polyacids was one of the earliest cases of template polymerization studied. Vinyl pyridine polymerizes without an additional initiator in the presence of both low molecular weight acids and polyacids such as poly(acrylic acid), poly(methacrylic acid), polyCvinyl phosphonic acid), or poly(styrene sulfonic acid). The polyacids, in comparison with low molecular weight acids, support much higher initial rates of polymerization and lead to different kinetic equations. The authors suggested that the reaction was initiated by zwitterions. The chain reaction mechanism includes anion addition to activated double bonds of quaternary salt molecules of 4-vinylpyridine, then propagation in the activated center, and termination of the growing center by protonization. The proposed structure of the product, obtained in the case of poly(acrylic acid), used as a template is ... 

The above subject has been selected as an example of the design of new polymerization reactions. It is concerned with a concept of copolymerization by spontaneous initiation and subsequent propagation via zwitterion intermediates. This copolymerization, which was invented and has been developed by the author, is called no catalyst copolymerization . It is very characteristic since usual polymerization reactions require either an initiator or a catalyst. 

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. ... 

This type of copolymerization results from spontaneous interactions of nucleophilic and electrophilic ihonomers (Mn and Me, respectively) without any additions of catalysts. Zwitterions form in the process that subsequently leads to formation of polymers. The mechanism is a step-growth polymerization. It can be illustrated as follows ... 

The zwitterion, which can be isolated, spontaneously polymerizes to an alternating iminoester at 150 C ... 

The addition of a Lewis acid to a polymerization recipe involving free radical initiators makes possible copolymerizations which would not otherwise occur e.g. Isobutene + methyl acrylate. The copolymerizations may also be unaffected by the addition of inhibitors or transfer reagents. Spontaneous alternating copolymerization may also occur between nucleophilic (Mn) and electrophilic (Mb) monomers through zwitterion intermediates (5) which are capable of propagation (6). 

Tertiary amines, unlike their phosphine analogues, enter into a variety of reactions depending upon their overall structure. Van Alphen examined the reaction of triethylamine with MA. An impure black substance was isolated which was soluble in water and released triethylamine on treatment with alkali. Mayahi and El-Bermani have observed a very exothermic reaction on mixing the same reactants neat. However, they also observed a clear yellow solution initially a dark brown solid was isolated. Based on spectroscopic evidence such as IR, NMR, and UV of the product, they rationalized their observations as follows At first, a charge-transfer complex 1 forms on mixing the reactants (a yellow solution). The tt complex 1 then collapses to the dark-brown product 3 through an intermediate cr complex 2. The intermediacy of 1 is important since succinic anhydride forms no complex or coloration with TEA. Spontaneous polymerization of MA in the presence of pyridine and trialkylamine has been reported.Zwitterionic cyclic intermediates are proposed. 

It should be noticed that pure TMC undergoes spontaneous polymerization in bulk at above 100 °C. In the plausible formulation of a TMC cleavage, a zwitterion intermediate with the trialkoxycarbenium ion, well stabilized by delocalization, and an alkoxide ion are formed (Scheme 38). The alkoxide ion can initiate chain growth according to the anionic mechanism. 

Various explanations were advanced to explain the unpredictable behavior of 1. Kabanov [32] attributed the spontaneous polymerization of 1 to an initiation-propagation sequence that began with nucleophilic attack at the vinyl group by counter-ion X , (Eq. 6.2.3). The zwitterionic intermediate 12 was... 

The isomers of co-methylbenzofulvene derivative BF3y were lacking of the tendency to polymerize shown by BFl. In other words, the presence of a methyl on the exocyclic methylene appeared to prevent the interaction and the consequent formation of the zwitterionic intermediate, which was assumed to initiate the spontaneous... 

Hindered amide derivative BF3n did not show the tendency towards spontaneous polymerization and the crystal structure of the complex BF3n-PTSA showed the interaction of electropositive =CH2 with the it-orbital of PTSA (Fig. 13, the distance between the centroid of PTSA toluene ring and vinylene carbon of BF3n was 3.802 A). This particular cation-x interaction was considered to support the proposed initiation mechanism in view of the fact that a similar interaction between the activated methylene group and the indene it-orbital could be the basis for the formation of the postulated zwitterionic dimer. 

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