Spontaneous Alternating Zwitterion Copolymerizations

This type of copolymerization results from spontaneous interactions of nucleophilic and electrophilic monomers (My and ME, respectively) without any additions of catalysts. Zwitterions form in the process that subsequently leads to formation of polymers [214-226]. The mechanism is a step-growth polymerization. It can be illustrated as follows:

Repeated additions of the charged species and the resulting zwitterionic products lead to high polymers:

The initial zwitterion that forms upon combination of a nucleophilic with an electrophilic monomer is called a genetic zwitterion [214]. Intramolecular reactions can produce "macrocycles":

The contribution of the cyclization reaction, however, is, apparently, small [214]. A reaction can also take place between a free monomer and any zwitterion at one of the ionic sites:

Such reactions disturb the alternating arrangements of the units -M-M in the products. The reactivity of the monomers determines whether homopropagations occur as well. Alternating propa- gation depends upon dipole-dipole interactions between MN and My monomers in preference to ion- dipole reactions between ion centers of zwitterions and monomers in homopropagations [214].

An example of an alternating copolymerization via zwitterion intermediates is a copolymerization of 2-oxazoline with ẞ-propiolactone. It takes place in a solution in a polar solvent like dimethyl- formamide at room temperature over a period of a day to yield quantitative conversions [215]:

ot

A zwitterion that forms first is the key intermediate for the polymerization. The onium ring from 2- oxazoline is opened by a nucleophilic attack of the carboxylate anion at carbon [214]:

In this reaction the number of copolymer molecules increases at first, then reaches a maximum and finally decreases as the conversion becomes high [214-226]. When the concentration of both monomers is high then the formation of "genetic" zwitterions is favored. As the concentration of macro-zwitterions becomes high and the monomer concentration decreases, the macro zwitterions react preferentially with each other. When stoichiometry is not observed and ẞ-propiolactone molecules predominate in the reactions mixture, the carboxylate end groups can react in various ways. They can react not only with the cyclic onium sites of the zwitterions, but also with free B-propiolactones and incorporate more than 50% of the propiolactone units [214].

Another example of such copolymerization is that of 2-oxazoline with acrylic acid. The reaction can be carried out by combining the two in equimolar quantities and then heating the reaction mixture to 60°C in the presence of a free radical inhibitor. Such an inhibitor can be p-methoxy phenol. The reaction mixture becomes viscous as an alternating copolymer forms [218]:

This copolymer is identical to the one obtained from reacting 2-oxazolone with ẞ-propiolactone. The acrylic acid is converted into the same repeat unit as the one that forms from ring-opening of B-propiolactone shown in the previous example. The suggested reaction mechanism involves a nucleophilic attack by oxazolone on acrylic acid and is followed by proton migration [214]:

A similar proton migration takes place in copolymerizations of acrylamide with cyclic imino ethers. The proton migration is part of the propagation process [219]. Other examples are copolymer- izations of a nucleophilic monomer, 2-phenyl-1,2,3-dioxaphospholane with electrophilic monomers [224, 225]. Here too the electrophilic monomers can be either acrylic acid or propiolactone. Identical products are obtained from both reactions [223]:

The opening of the phosphonium ring requires higher temperatures (above 120°C) and follows the pattern of the Arbusov reaction [224, 226]. Examples of some other monomers that can also act as nucleophiles in the above reaction are p-formyl benzoic acid [214], acrylamide [224], and ethylene sulfonamide. All three react in the same manner [224]:

It is reasonable to expect that some compounds can act at one time as My monomers and at other times as ME, depending upon the comonomer. This is the case with salicylyl phenyl phosphonite [224]. In the presence of bezoquinone it behaves as an My monomer and produces a 1:1 alternating copolymer at room temperature [224]:

where, X = Y = H; X = Y = Cl; X = Y = CH3; X = Cl; and Y = CN.

The above reaction is called a redox copolymerization reaction [224]. The trivalent phosphorus in the monomer is oxidized to the pentavalent state in the process of polymerization and the quinone structure is reduced to hydroquinone. The phosphonium-phenolate zwitterion is the key intermediate:

Nucleophilic attack of the phenoxide anion opens the phosphonium ring due to enhanced electro philic reactivity of the mixed anhydride and acid structures [224]. Salicylyl phenyl phosphonite, however, in combination with 2-methyl-2-oxazoline behaves as an ME monomer [224]. Ter polymerizations by this mechanism of sequence ordered 1:1:1 components can also take place. The following is an example [224]:

In addition 2:1 binary copolymerizations were also observed. Following is an example of a binary copolymerization [226]:

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مواضيع ذات صلة

Copolymerization of Cyclic Monomers

Ring-Opening Polymerizations of Cyclic Sulfides

Cationic Polymerization of Lactams

Polymerizations of Lactams

Special Catalysts for Polymerizations of Lactones

Special Catalysts for Polymerizations of Lactones

Polymerization of Lactones by Coordination Mechanism

Polymer-Based Solar Cells

Photo-Conducting Polymers That Are Not Based on Carbazole

Photoconductive Polymers Based on Carbazole

Photo-Conducting Polymers

Liquid Crystalline Alignment