Transfer products, polymer-metal

Other processes also contribute to chain growth termination under special conditions. In particularly crowded catalysts, fi-methyl transfer to the metal centre can occur instead of p-H transfer. When other reaction paths are blocked, a-bond metathesis, i.e. transfer of an H atom from a monomer to the metal-bound alkyl C atom can release a polymer with a saturated chain end with formation of a new unsaturated metal-bound chain start. Saturated chain ends will also result when H2 gas is added to a catalyst system thus leading to the production of shortened polymer chains. Such an H2 addition will often also cause an increase in overall catalyst activity, since H2 will predominantly react with species - such as occasional 2,1-inserted units - which are rather... 

Historically, the most important application of the electron transfer initiation involved the production of stereoregular diene rubbers by lithium metal initiation. The lithium was used as a fine dispersion with a large surface area to speed up the initiation reaction and the process was carried out in hydrocarbon solvents because polar solvents increase the generally undesired vinyl side chain content of the product polymer.)... 

By analysing the variations in the integral and differential MWD—curves for the original PE and its transfer products, the peculiarities of wear and transfer for the polymer sliding against steel (Rq = 0.04 yum) have been studied as influenced by loads and sliding velocities, chemical nature of the organic additives present in the polymer, and the counterface metal. The results obtained helped to substantiate the ways of wear control for polyethylene in terms of transfer. 

The deposition of Cr on two fluorinated poly(aryl ether) (rPAE) polymers has been investigated with x-ray photoelectron spectroscopy. Fluorine moieties were observed to be highly reactive towards the deposited Cr. Differences in polymeric fluorine chemistry (aliphatic vs. aromatic) did not affect the reaction pathway or the final reaction products. Interfacial deposition products form in a step-wise fashion dependent upon sietal coverage. A model ie proposed whereby the formation of reaction products is initiated by electron transfer from the metal to tha polymer followed by the formation of Cr-fluorldes and finally Cr-carbldes prior to the formation of a continuous unreacted metal overlayer. 

As a result of the particularities of the MAO activation method noticed so far, it has been concluded that, besides chain back-skip, a different mechanism occurs when using this cocatalyst. One possible explanation might be a reversible chain transfer reaction between the cocatalyst and the active species. As a result of the intrinsic chirality at the metal center, the catalytic system consists of two enantiomers (S and/ . Scheme 9.3). Under different polymerization conditions (i.e., different Al/Zr ratios), the coordination and insertion of the monomer can take place at the metal center of either of the two enantiomers. At higher Al/Zr ratios, a unidirectional transfer of polymer chains from Zr (enantiomer / , for example) to aluminum can be suggested, because reduced molecular weights of the polymer products have been found. Relocation of the chain from aluminum to the other enantiomer of the Ci-symmetric catalyst species (enantiomer 5, Scheme 9.3) and then back... 

The mechanisms described above tell us how heat travels in systems, but we are also interested in its rate of transfer. The most common way to describe the heat transfer rate is through the use of thermal conductivity coefficients, which define how quickly heat will travel per unit length (or area for convection processes). Every material has a characteristic thermal conductivity coefficient. Metals have high thermal conductivities, while polymers generally exhibit low thermal conductivities. One interesting application of thermal conductivity is the utilization of calcium carbonate in blown film processing. Calcium carbonate is added to a polyethylene resin to increase the heat transfer rate from the melt to the air surrounding the bubble. Without the calcium carbonate, the resin cools much more slowly and production rates are decreased. 

As stated above, we postulated that fast, reversible chain transfer between two different catalysts would be an excellent way to make block copolymers catalytically. While CCTP is well established, the use of main-group metals to exchange polymer chains between two different catalysts has much less precedent. Chien and coworkers reported propylene polymerizations with a dual catalyst system comprising either of two isospecific metallocenes 5 and 6 with an aspecific metallocene 7 [20], They reported that the combinations gave polypropylene (PP) alloys composed of isotactic polypropylene (iPP), atactic polypropylene (aPP), and a small fraction (7-10%) claimed by 13C NMR to have a stereoblock structure. Chien later reported a product made from mixtures of isospecific and syndiospecific polypropylene precatalysts 5 and 8 [21] (detailed analysis using WAXS, NMR, SEC/FT-IR, and AFM were said to be done and details to be published in Makromolecular Chemistry... 

Since their discovery over a decade ago, late transition metal a-diimine polymerization catalysts have offered new opportunities in the development of novel materials. The Ni(II) catalysts are highly active and attractive for industrial polyolefin production, while the Pd(II) catalysts exhibit unparalleled functional group tolerance and a propensity to form unusually branched polymers from simple monomers. Much of the success of these catalysts derives from the properties of the a-diimine ligands, whose steric bulk is necessary to accelerate the insertion process and inhibit chain transfer. 

Other than in polymer matrix composites, the chemical reaction between elements of constituents takes place in different ways. Reaction occurs to form a new compound(s) at the interface region in MMCs, particularly those manufactured by a molten metal infiltration process. Reaction involves transfer of atoms from one or both of the constituents to the reaction site near the interface and these transfer processes are diffusion controlled. Depending on the composite constituents, the atoms of the fiber surface diffuse through the reaction site, (for example, in the boron fiber-titanium matrix system, this causes a significant volume contraction due to void formation in the center of the fiber or at the fiber-compound interface (Blackburn et al., 1966)), or the matrix atoms diffuse through the reaction product. Continued reaction to form a new compound at the interface region is generally harmful to the mechanical properties of composites. 

While high polymers of /3-lactones can also be formed by cationic polymerization, most of the commercial production seems to be by the anionic route. Carboxylate salts such as sodium acetate or benzoate are commonly the initiators, but other nucleophiles, such as triethylamine, betaine, potassium f-butoxide, aluminum and zinc alkoxides, various metal oxides and tris(dimethylamino)benzylphosphonium chloride (the anion of which is the initiator), are of value. Addition of crown ethers to complex the counter cation increases the rate of reaction. When the reaction is carried out in inert but somewhat polar organic solvents, such as THF or ethyk acetate, or without solvent, chain propagation is very fast and proceeds without transfer reactions. 

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