Homogeneous vinyl polymerization

Dimethylformamide [68-12-2] (DME) and dimethyl sulfoxide [67-68-5] (DMSO) are the most commonly used commercial organic solvents, although polymerizations ia y-butyrolactoae, ethyleae carboaate, and dimethyl acetamide [127-19-5] (DMAC) are reported ia the hterature. Examples of suitable inorganic salts are aqueous solutioas of ziac chloride and aqueous sodium thiocyanate solutions. The homogeneous solution polymerization of acrylonitrile foUows the conventional kinetic scheme developed for vinyl monomers (12) (see Polymers). 

The monomers dealt with can be polymerized by various mechanisms, not only by ROMP. For example, a rapid polymerization of norbornadiene occurs using a homogeneous catalytic system consisting of nickel acetylacetonate or a nickel-phosphine complex, such as nickel bis-(tri-n-butylphosphine) dichloride (NiCl2(TBP)2) or nickel bis-(tricyclohexylphosphine) dichloride (NiCl2(TBP)2). Nickel acetylacetonate as catalyst is known to initiate rather a classical vinyl polymerization (7). The classical vinyl polymerization... 

Although benzophenone derivatives have found application in a number of photopolymerization processes (10), these have usually involved cross-linking reactions induced by energy transfer from photo-exdted benzophenones to groups (e.g. cinnamate) attached to the polymer chains. In such cases, the benzophenones act as sensitisers and not as photoinitiators. An e>fample in which hydrogen abstraction by photoexdted benzophenone has been utilized for photoinitiation is the ultra-violet induced grafting of styrene on to polyethylene (8,45). Recently, extensive studies concerned with photoinitiation of vinyl polymerization by benzophenone derivatives in homogeneous media have been carried out, and are described in detail below. 

Several fluorinated monomers have been polymerized via homogeneous cationic polymerization in compressed carbon dioxide. For example, vinyl ethers with fluorinated side chains were polymerized in SCCO2 at 40 °C using adventitious water initiation with ethylaluminum dichloride as the Lewis acid coin-... 

Scheme 4.5-4 Homogeneous cationic polymerization of fluorinated vinyl ethers in supercritical CO2 [77],...

Membranes which may be used in the removal of alkali metal ions by electrodialysis are those which are impermeable to anions, but which allow the flow therethrough of cations. Such cation-selective membranes should, of course, possess chemical durability, high resistance to oxidation and low electrical resistance in addition to their ion-exchange properties. Homogeneous-type polymeric membranes are preferred, for example, network polymers such as phenol, phenosulfonic acid, formaldehyde condensation polymers and linear polymers such as sulfonated fluoropolymers and copolymers of styrene, vinyl pyridine and divinylbenzene. Such membranes are well known in the art and their selection for use in the method of the invention is well within the skill of the art. 

The homogeneous solution polymerization of acrylonitrile follows the conventional kinetic scheme developed for vinyl monomers [36,40,41]. This kinetic scheme can be presented as follows ... 

In homogeneous anionic polymerizations of simple vinyl monomers, steric placement is also temperature-dependent, just as it is in cationic polymerizations. Syndiotactic placement is favored in polar solvents at low temperature. In nonpolar solvents, however, isotactic placement predominates at the same temperatures. Here, too, this results mainly from the degree of association with the counterion. 

N-Vinyl-2-pyrrolidone-acrylamide copolymers were prepared by aqueous homogeneous solution polymerization in either distilled water or synthetic seawater. The initial monomer level in the polymerization solutions was either 9.1% or 20% (w/w). The solutions were initiated at either room temperature or 50 C by either 0.50phm or O.lOphm commercial free radical initiators. A temperature controlling water bath was used with the 50 C polymerizations. 

Peucker, U. Heitz, W. Vinylic polymerization and copolymerization of norbornene and ethene by homogeneous chromium(III) catalysts. Macromol. Chem. 2001, 202, 1289-1297. 

Radical polymerization is initiated by a free radical, which subsequently adds to a vinyl or diene monomer to produce a propagating radical. To obtain information about the structure and concentrations of initiating and propagating radicals in radical polymerizations, use of ESR spectroscopy has called the interest of physical or polymer chemists. However, ESR measurements on these radicals in solution poly merizati on were found to be difficult, except for the case where polymers precipitated, because otherwise the concentrations of the radicals were too low. Thus, these measurements had to be limited to polymerization systems in highly viscous solutions or in the solid state, where the disappearance of free radicals by bimolecular reactions is suppressed. Bresler et al. -i7) succeeded for the first time in obtaining ESR spectra of free radicals which were produced in homogeneous bulk polymerization of methyl methacrylate (MMA), methyl acrylate (MA) and vinyl acetate (VAc) at conversions of 50-60% (in the gel state). 

Both batch and continuous reactors are used in industrial vinyl polymerization processes. Agitated kettles, tower reactors, and linear flow reactors are just a few examples of industrially used polymerization reactors. The choice of reactor type depends on the nature of polymerization systems, (homogeneous versus heterogeneous), the quality of product, and the amount of polymer to be produced. Sometimes, multiple reactors are used and operated at different reaction conditions. Whichever reactor system is used, it is always necessary to maximize the process productivity by reducing the reaction time (batch time or residence time) while obtaining desired polymer properties consistently. 

The P-H-elimination is impeded in the norbomene polymerization by the reformation of a highly strained ring system. The catalysts required for this reaction have a non-coordinating anion and weak nitril ligands as a characteristic. Such a pallatUum complex was described 1981 by Senn (5). The modification of this catalyst by Risse et al. resulted in a homogeneous reaction in the polymerization of norbomene (4). Vinyl polymerized norbomene is an example of conformationally restricted polymers having typically very high transition temperatures. 

Conducting polymer composites have also been formed by co-electrodeposition of matrix polymer during electrochemical polymerization. Because both components of the composite are deposited simultaneously, a homogenous film is obtained. This technique has been utilized for both neutral thermoplastics such as poly(vinyl chloride) (159), as well as for a large variety of polyelectrolytes (64—68, 159—165). When the matrix polymer is a polyelectrolyte, it serves as the dopant species for the conducting polymer, so there is an intimate mixing of the polymer chains and the system can be appropriately termed a molecular composite. 

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