Charge polymerization initiated

Reaction Mechanism of Vinyl Polymerization with Amine in Redox and Photo-Induced Charge-Transfer Initiation Systems... 

A substantial number of photo-induced charge transfer polymerizations have been known to proceed through N-vinylcarbazole (VCZ) as an electron-donor monomer, but much less attention was paid to the polymerization of acrylic monomer as an electron receptor in the presence of amine as donor. The photo-induced charge-transfer polymerization of electron-attracting monomers, such as methyl acrylate(MA) and acrylonitrile (AN), have been recently studied [4]. In this paper, some results of our research on the reaction mechanism of vinyl polymerization with amine in redox and photo-induced charge transfer initiation systems are reviewed. 

Photoinduced copolymerization of donor-acceptor monomer pairs (Scheme 1) can be either self initiated by excitation of the charge transfer complex (charge transfer initiation) or by polymerization of the charge transfer complex/monomer equilibrium... 

A Kjeldahl flask was charged with ,/V -dimethyl-1,6-diaminohexane (5.75 mmol) dissolved in 10 ml of THF and then treated with the dropwise addition of -butyl-lithium (5.75 mmol) while vigorously stirring. This solution was then treated with chlorotrimethylsilane (5.75 mmol) and stirred at ambient temperature for 30 minutes and then filtered off through a poly(tetrafluoroethylene) (PTFE) filter and 15 mL of the filtrate charged into a 150-ml glass bottle. This aliquot was then treated with tetramethylethylene diamine (4.23 mmol) and n-butyllithium (4.23 mmol) and used immediately as a polymerization initiator. 

Unlike cationic polymerization initiated by a conventional catalyst, the propagating species in the present system would bear different type of counter-ion or would be much more free. The counter-anion obtained in this entirely organic system would be large and unstable. The problem of the counterion in charge transfer ionic polymerization certainly requires further study. 

Hirooka (29) has proposed that copolymerizations of this type be named "complex copolymerization. Russian workers (55) have suggested that polymerizations initiated by radical catalysts in the presence of a complexing agent be called "complex-radical processes. Both polymerizations are more appropriately considered as polymerizations through activated charge transfer complexes, in which the Lewis acid or metal halide catalyzes the formation of the complex, and free radicals may or may not be necessary to initiate the homopolymerization of the complex. 

The copolymerization with alkyllithium to produce uniformly random copolymers is more complex for the solution process than for emulsion because of the tendency for the styrene to form blocks. Because of the extremely high rate of reaction of the styryl-lithium anion with butadiene, the polymerization very heavily favors the incorporation of butadiene units as long as reasonable concentrations of butadiene are present. This observation initially was somewhat confusing because the homopolymerization rate of styrene is seven times that for butadiene. However, the cross-propagation rate is orders of magnitude faster than either, and it therefore dominates the system. For a 30 mole percent styrene charge the initial polymer will be almost pure butadiene until most of the butadiene is polymerized. Typically two-thirds of the styrene charged will be found as a block of polystyrene at the tail end of the polymer chain ... 

The high rate constants for chemical quenching of triplet ketones by amines provide two sidelights of considerable importance. Photoinitiation of polymerization has received widespread and varied industrial applications. One problem is that many vinyl monomers quench triplet ketones very rapidly either by charge transfer or energy transfer mechanisms, without forming any radicals. Most solvents cannot compete with the olefins for the triplet ketone. However, tri-ethylamine quenches at rates close to diffusion-controlled, so that radical formation and polymerization initiation are quite efficient 158>. 

Unlike ordinary chain reactions, chain-growth polymerization need not involve free radicals. The reactive center may instead be a carbanion or carbocation generated by intermolecular transfer of a proton or electron. Depending on the sign of the ionic charge on the chain carriers, the overall reaction is called anionic or cationic polymerization. As in free-radical polymerization, initiation is required. 

This review deals with polymerizations initiated by a molecule, in which at least one author believes macrozwitterions are formed. The majority are of the simple addition type and are subdivided into vinyl, carbonyl, and strained ring. The other types are all charge cancellations. Monomers are either preformed zwitterions, highly polarised molecules (i.e. CH3CNO) or zwitterions are formed in situ by nucleophile/electrophile pairs. 

Processes involved Same reaction involved with all reacting species present No initiator required (catalyst sometimes) Monomer consumed early in polymerization molecular wt. increases slowly Initiation, propagation, and termination mechanisms different Suitable radical, or charged (+, or —) initiator required Only monomer and high polymer present each polymer molecule forms very rapidly... 

Termination will occur when the carbocation undergoes reaction with nucleophilic species other than monomer to produce a dead chain and no re-initiation. Since cationic polymerizations are carried out with high-purity reagents and under rigorous conditions this reaction is much less likely than chain transfer to monomer. The mutual repulsion of the charged polymerization sites ensures that bimolecular termination cannot occur (unlike in free-radical polymerization, where this is the most probable termination route). Recombination of the cation with the counter-ion will occur, and these termination reactions are often very specific to the chemistry of the initiator. 

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