Narrow composition distribution

Acetone, methyl ethyl ketone, methyl isobutyl ketone, dimethylformamide, ethyl acetate, and tetrahydrofuran are solvents for vinyhdene chloride polymers used in lacquer coatings methyl ethyl ketone and tetrahydrofuran are most extensively employed. Toluene is used as a diluent for either. Lacquers prepared at 10—20 wt % polymer sohds in a solvent blend of two parts ketone and one part toluene have a viscosity of 20—1000 mPa-s (=cP). Lacquers can be prepared from polymers of very high vinyhdene chloride content in tetrahydrofuran—toluene mixtures and stored at room temperature. Methyl ethyl ketone lacquers must be prepared and maintained at 60—70°C or the lacquer forms a sohd gel. It is critical in the manufacture of polymers for a lacquer apphcation to maintain a fairly narrow compositional distribution in the polymer to achieve good dissolution properties. 

Narrow composition distribution Absence of high comonomer fraction ease processesing and comonomer effective in reducing m.p. sharp m.p. Heat seal layers in composites films... 

Tagata and Homma47 analyzed in the aforementioned manner the compositional heterogeneity of two typical commercial SBR samples, E-SBR and S-SBR, which had 23.5 and 20.0 average styrene wt.%, and were produced by an emulsion polymerization and by a solution polymerization with an organometallic catalyst, respectively. The result was that the former gave a distinctly narrower compositional distribution than the latter. GPC experiments on these samples were also carried out, which indicated that the above situation was just the opposite for the molecular-weight distributions. 

Einally, as per Equation 18.27b, if a copolymer has a very narrow composition distribution, will be very close to (M) using a single solvent the same applies to mixed solvents if we follow the procedure mentioned earlier. This can be the case, for instance, of a radical copolymerization in a true azeotrope composition or of a block copolymer synthesized by a controlled anionic or living radical polymerization. 

This new family of polyolefin copolymers hcis a significantly different chain microstructure than conventional LLDPE.The single site characteristics of metallocenes, with the catalyst site being identical, are known to produce materials having the most probable molecular weight distribution /M 2.0), with essentially a random comonomer distribution and narrow composition distribution. The comonomers most frequently used commercially are butene, hexene, and octene. Copolymerization of ethylene with 4-methyl-l-penetene (4-MP-l) and cyclic and bicylic groups has been also reported in the literature. Several terpolymers are also commercially available.Exxon Chemical Company manufactures ethylene-butene copolymers, ethylene-hexene copolymers, and terpolymers of butene and hexene comonomers. Exxon markets these mLLDPEs under the trademark name of... 

Some of the principles as well as problems involved in the melting of random copolymers are found in olefin type copolymers. The melting temperatures of a large number of random type ethylene copolymers, as determined by differential scanning calorimetry, are plotted as a function of the mole percent branch points in Fig. 5.11. The samples represented in this figure are either molecular weight and compositional fractions or those with a narrow composition distribution with a most probable molecular weight distribution.(74) These samples were crystallized and heated rapidly. In this set of data there are two different copolymers that contain ethyl... 

Even in experiments performed at strictly constant F, the copolymers obtaiied with heterogeneous catalysts sometimes have a significant compositional distribution (32,129,138,145,179,180). The possible explanation is that different active sites on the catalyst sur ce have different activities in respect to comonomers, i.e. different aixl rj values (66, 145, 169). On the other hand, when soluble catalytic systems are used, a relatively narrow compositional distribution is observed (32,179,180) showing ore type of active site. 

The distribution broadness is inversely proportional to chain length, suggesting smaller variations in longer chains. It is also a function of rit2. Monomer pairs with small rir2 values tend to produce alternating copolymers that have narrow composition distributions. 

Natta and Ziegler found early on that vanadium halides could be treated with aluminum alkyls to form catalysts competent for alkene polymerizations. A variety of simple precursors in various oxidation states, for example, VCI3, VCI4, and VOCI3. can be treated with aluminum alkyl halides to produce active catalysts. Unlike the titanium and chromium systems, the vanadium catalysts can be single sited with narrow MWDs and more importantly narrow composition distributions. Also unlike the other systems, vanadium incorporates a-olefms at rates somewhat slower but comparable to ethylene. For this reason, vanadium catalysts are used commercially to make EPMs and EPDMs that can have at least 50 mol.% ethylene with the balance being predominantly other olefins and much smaller amounts of diolefins (Figure 9). 

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