Cation method

Since a thorough review of the scientific and patent literature failed to produce any information relative to die preparation of silicon-functional polyolefins by cationic methods, this untapped field appeared particularly attractive for systematic research. Added justification for this objective was provided by the possible exploitation of the vast synthetic possibilities offered by a combination of the chemistries of cationic polymerization and that of organosilanes24. ... 

It is possible, however, to avoid any violation of these fundamental properties, and derive a result on the local electron densities of non-zero volume subsystems of boundaryless electron densities of complete molecules [159-161]. A four-dimensional representation of molecular electron densities is constructed by taking the first three dimensions as those corresponding to the ordinary three-space E3 and the fourth dimension as that representing the electron density values p(r). Using a compactifi-cation method, all points of the ordinary three- dimensional space E3 can be mapped to a manifold S3 embedded in a four- dimensional Euclidean space E4, where the addition of a single point leads to a compact manifold representation of the entire, boundaryless molecular electron density. 

Similarly, in the case of cationic method, the initiation reaction between labile alkyl halide [68] and Lewis acid have been utilised for cationic grafting onto halogenated polymers as shown in Scheme 3. 

Reports of anionic polymerization reactions of oxetane monomers are less common in the literature, than the generally applied cationic method. While unsubstituted oxetanes have been successfully converted to their corresponding... 

The all-cis structure of natural rubber is vita) to its elasticity. The all-trans compound is known and it is hard and brittle. Though dienes such as isoprene can easily be polymerized by cationic methods, the resulting rubber is not all-cis and has poor elasticity and durability. However, polymerization of isoprene in the Ziegler-Natta way gives an all-cis (90-95% at least) polyisoprene very similar to natural rubber. 

Typical monomers that may be polymerized by cationic methods include styrene, isobutylene, and vinyl ethers. Unlike radical polymerizations, solvent polarity can influence the rate of polymerization. This is due to the presence of the counterion (see Fig. 15.13). For example, more polar solvents can increase the degree of separation between the growing end and the counterion during the propagation step, increasing the rate of propagation.17... 

A, Favorskii method B, dihalocarbene method C, trichlorocyclopropenyl cation method. 

By applying the fluorescent cation method, it was revealed that a filamentous mitochondria may represent an electrically united system operating as intracellular electric cables. A local damage of such a filament by very narrow (0.5 pm in diameter) laser beam was shown to cause efflux of the cation and, hence, the fluorescence decreases in the entire 50 pm mitochondrial filament in a human fibroblast cell. 

Isobutylene was copolymerized with methylbenzenes by cationic methods to give products containing 1-2% of the comonomer. Attachment of polyplvalolactone grafts was accomplished using the same chemistry as that for Isoprene block-graft copolymers. 

Cationic Methods. Cationic methods have also found utility in preparation of block polymers however, generally speaking these methods suffer from a variety of synthesis problems including monomer-chain end equilibria, facile chain end termination or transfer with certain counteranions, and difficult molecular weight control. Most examples of living cationic systems include the use of an oxonium cation with a carefully selected counterion (Reaction 8). 

Cationic Methods. Cationic grafting has been employed by Kennedy... 

Three-membered cyclic ethers (see Table 10.1) are known as epoxides, the simplest epoxides being ethylene oxide and propylene oxide. Their polymerization is a subject of considerable technological importance. The ring-opening polymerization of epoxides can be initiated by both anionic and cationic methods. 

The effect of high shear mechanical mixing and soni-cation methods on the physical properties of the nanocomposites has been analyzed [84] using modified clays with a quaternary ammonium salt and calcium carbonate [85] and silane-treated clays [31]. Although the nanoclay is usually chemically modified to make it organophilic and compatible with the polymer matrix, untreated MMT was... 

Potentially, there are greater numbers of monomers that are suitable for cationic polymerization than for anionic, but the cationic method is less successful in block copolymer synthesis because, in many systems, the existence of a living carbocationic species is doubtful. Consequently, the involvement of carbocations in block copolymer synthesis tends to be limited to mixed reactions, e.g., the couphng of poly(tetrahydro-furan) cations with polystyryl anions to give an (A - B) diblock (Equation 5.19). 

The preparation of well-defined sequential copolymers by an anionic mechanism has been explored and utilized commercially for some time now. Initially, the cationic methods received less attention until it was demonstrated by Kennedy that a large variety of block copolymers can be formed. The key to Kennedy s early work is tight control over the polymerization reaction. The initiation and propagation events must be fundamentally similar, although not identical ... 

Acetal polymers are formed from the polymerization of formaldehyde. They are also given the name polyoxymethylenes (POMs). Polymers prepared from formaldehyde were studied by Staudinger in the 1920s, but thermally stable materials were not introduced until the 1950s, when DuPont developed Dehin. Hompolymers are prepared from very pure formaldehyde by anionic polymerization as shown in Fig. 2.1. Amines and the soluble salts of alkali metals catalyze the reaction. The polymer formed is insoluble and is removed as the reaction proceeds. Thermal degradation of the acetal resin occurs by unzipping with the release of formaldehyde. The thermal stability of the polymer is increased by esterification of the hydroxyl ends with acetic anhydride. An alternative method to improve the thermal stabihty is copolymerization with a second monomer, such as ethylene oxide. The copolymer is prepared by cationic methods developed by Celanese and mar-... 

Optimizing the covalent cationization method for the mass spectrometry of polyolefins. Macromolecules, 35, 7149-7155. 

Byrd, H.C.M., Bencherif, S.A., Bauer, B.J., Beers, K.L., Brun, Y., Lin-Gibson, S., and Sari, N. (2005) Examination of the covalent cationization method using narrow polydisperse polystyrene. Macromolecules, 38,1564-1572. 

A very powerful but time-consuming sample preparation is the covalent cationization method. The polymer is dissolved in toluene at 110°C and it is reacted with bromine. The brominated polymer is then dissolved in xylene and reacted with triphenyl phosphine (TPP). Scheme 45.2 reports the structure of the reaction products. Eventually, all chains contain the same end group. 

Bauer, B.I, Wallace, W.E., Fanconi, B.M., Guttman, C M. (2001) Covalent cationization method for the analysis of polyethylene by mass spectrometry. Polymer, 42,9949-9953. 

---