Agents, polymeric counterion

Tlte cationic polymerization of thiiranes is initiated by proton adds, Lewis acids, and alkylating agents. " The counterions of the cationic propagating species must have a nudeophilidty lower than that of the monomer in order to prevent termination reaction with the counterion. Typical nonnucleophilic counterions are tetrafluoroborate, hexafluorophosphate, tri-fluoromethanesulfonate, and perchlorate. 

Wang GM, van Beylen M (2003) Influence of 7t-complexing agents on the anionic polymerization of styrene with lithium as counterion in cyclohexane. 1. Effect of durene. Polymer 44 6205-6210... 

For any specific type of initiation (i.e., radical, cationic, or anionic) the monomer reactivity ratios and therefore the copolymer composition equation are independent of many reaction parameters. Since termination and initiation rate constants are not involved, the copolymer composition is independent of differences in the rates of initiation and termination or of the absence or presence of inhibitors or chain-transfer agents. Under a wide range of conditions the copolymer composition is independent of the degree of polymerization. The only limitation on this generalization is that the copolymer be a high polymer. Further, the particular initiation system used in a radical copolymerization has no effect on copolymer composition. The same copolymer composition is obtained irrespective of whether initiation occurs by the thermal homolysis of initiators such as AIBN or peroxides, redox, photolysis, or radiolysis. Solvent effects on copolymer composition are found in some radical copolymerizations (Sec. 6-3a). Ionic copolymerizations usually show significant effects of solvent as well as counterion on copolymer composition (Sec. 6-4). 

The Effects of Complexing Agents. Cai et al.66 replaced the sodium counterion of MX-DNA with various aliphatic amine cations, e.g. spermine cation, and alkyltrimethylammonium cation as well as polymeric amine cations, such as poly-L-lysine and polyethylenimine to vary the separation between DNA duplexes. The radiation-produced electrons from the complexing agents readily transfer to the more electron affinic DNA. 

As discussed in Section II.C, in any system in cationic ring-opening polymerization, a reaction of active species with polymer repeating unit may lead to chain transfer to polymer or termination (if the resulting branched or cyclic onium ions are not active), whereas recombination with counterion leads to termination in the case of irreversible reaction. The later reaction may be avoided by the proper choice of counterion. As the onium ions are generally inherently stable there is no other termination reaction, provided that impurities that may act as terminating agents are eliminated. 

Polymerization was initiated with alkylating agents such as CH3I, CeHsC Br, or CH3OSO2CF3. Depending on the nucleophilicity of the counterion, propagation proceeds on phosphonium ion active species (triflate counterion) or on covalent alkyl bromide species. 

Two-phase systems, usually consisting of a strongly alkaline aqueous rfiase and methylene chloride, are also effective and convenient. Trialkylsulfonium salts with methylsulfate counterions can be used alone under these conditions, whereas those with halide counterions generally benefit from added phase transfer catalysts (equation 2). Dodecyldimethylsulfonium chloride was found to be an effective phase transfer reagent. A sulfonium salt covalently attached to a polymeric resin was found most effective when a phase transfer agent was also employed. ... 

This high tendency of poly(ethylene oxide) to solvate cations and from the polymeric shell around the counterion leads to autoacceleration in polymerization (polymerization faster on solvated ion-pairs), and increase of conductivity with monomer conversion. Moreover, polymerization is not sensitive to the "external" solvating agents, e.g. crown ethers. 

The use of halogens as oxidizing agents has also been reported,75 resulting in a PPy/X product containing halide (X-) ions as the dopant counterions. These same workers used chemical polymerization to produce PPy-polymethylpyrrole copolymers. 

Hanks has investigated the chemical polymerization of complexes 253-256.143144 With appropriate oxidizing agents (e.g., phosphomolybdic acid or /-butyl ammonium persulfate), film-forming conductive (10 2 to 10 3Scm 1) polypyrroles were generated. The polymer from Pd11 complex 255 was luminescent, while the other polymers were nonemissive. Dodecylbenzenesulfonate counterions improved the film quality of cast polymers. 

A key to the success of the living cationic polymerization of vinyl ethers is the stabilization of the unstable carbocations via suitable nucleophilic counterion. There are two ways to stabilize the carbocations (1) generation of suitable nucleophilic counterion resulted from the initiator and the catalyst, and (2) addition of nucleophilic agents to the polymerization media. In the first way, Bronsted acids such as hydrogen iodide are employed as the initiators, while Lewis acids such as zinc iodide are employed as the catalysts (Scheme 11.41) [140-143],... 

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