Long-chain branching comonomer incorporation

Molar mass distribution is a dominant microstracture parameter that, in copolymers, needs to be measured with additional information to account for long chain branching, comonomer incorporation, or ethylene propylene combinations (in the case of EP copolymers). The combination of GPC and IR spectroscopy has been shown to be of great value in the characterization of copolymers. The importance of automation and sample care, especially in the case of polypropylene, has been discussed as well as the significant improvement in sensitivity by the use of IR MCT detectors. There are big expectations for the analysis of ultrahigh molar mass polyolefins by the new AF4 technology. 

We use carbon-13 NMR spectrometry to identify the monomer units present in copolymers, their absolute concentrations, the probability that two or more monomer units occur in proximity, and long chain branching concentrations. For instance, in the case of polyethylene, we can not only distinguish and quantify ethyl, butyl, and hexyl branches, but we can also determine whether branches are present on carbon backbone atoms separated by up to four bonds. We can compare the observed adjacency of branches to a theoretical value calculated for random comonomer incorporation. By this method, we can determine whether comonomers are incorporated at random, as blocks, or in some intermediate fashion. 

In addition to the metallocenes described previously, so-called halfsandwich compounds or constrained-geometry catalysts (Fig. 3) such as dimethylsilyl-t-butylamido cyclopentadienyl titanium dichloride are used. These catalysts are excellent for producing polyethylenes with long-chain branching and can incorporate high amounts of comonomers such as 1-octene... 

The main advantages for the high-pressure process compared to other PE processes are short residence time and the ability to switch from homopolymers to copolymers incorporating polar comonomers in the same reactor. The high-pressure process produces long-chain, branched products from ethylene without expensive comonomers that are required by other processes to reduce product density. Also, the high-pressure process allows fast and efficient transition for a broad range of polymers. 

Chain branching in low density versions of polyethylene is common. Extent and length of branching stem primarily from the mechanism of polymerization and incorporation of comonomers. Branching is classified as long chain branching (LCB) or short chain branching (SCB). By convention, SCB implies branches of 6 or fewer carbon atoms. LDPE contains extensive LCB and branches can contain hundreds of carbon atoms. Branches on branches are also common in LDPE. 

CGC also has excellent reactivity with a-olefin comonomers. The latter attribute enables production of copolymers containing large amounts of uniformly distributed a-olefin (VLDPE). Further, by a mechanism wherein a long chain a-olefin is eliminated and subsequently incorporated as a "comonomer", a small amount of long chain branching is introduced into the polymer. 

Ethylene-vinyl acetate copolymers can be thought of as modified high pressure polyethylenes. Because of the free-radical polymerization process they have structural characteristics such as short-chain and long-chain branching in addition to the effects due to the incorporation of the vinyl acetate comonomer. Ethylene and vinyl acetate have a reactivity ratio which is close to 1 and as a result EVA copolymers contain vinyl acetate which is homogeneously distributed among the polymer chains. The major effect of the VA on polymer properties is to reduce... 

The use of metallocenes for the production of polyolefin-clay nanocomposites has several well-known advantages [31, 32]. Metallocenes can produce polyolefins with narrow MWD and uniform comonomer incorporation. In addition, terminal groups, stereochemistry, short and long chain branching can be controlled depending on the metallocene structure employed [32]. 

It becomes evident that with a reasonably simple monomer (chloroprene being a substituted butadiene), one must consider four structures for the incorporation of monomer units, two types of interunit connections, two potential comonomers, long-chain branching, and cross-linking, as well as molecular weight distribution. The sum of all of these structural variables determines, for the most part, the end-use properties of the polymer and aU are strong functions of polymerization conditions. 

The long chain branching (LCB) already known from LDPE is not typical for LLDPE. While LDPE has high levels of LCB, there is little LCB in LLDPE however, there are high levels of short-chain branching (SCB) contributed by the incorporated comonomer. Molecular weight distribution is narrow (LDPE and HDPE tend to be broader). 

The level of short-chain (SCB) and long-chain (LCB) branches control the solid resin density of a PE resin. For example, the level of SCB is controlled by the amount of alpha olefin comonomer incorporated into LLDPE resin as a pendant group. The random positioning of the pendant groups disrupts the crystailization process when the polymer is cooled from the molten state, causing the level of crystallinity to decrease with increasing amounts of alpha olefin comonomer. 

To obtain the LCB distribution shown in Figure 81B, no special mechanistic assumptions are required. It is consistent with the simple view described in Scheme 24. By this reasoning, low-MW sites are known to be the most reactive with comonomer, and therefore they should also be the most reactive with macromer. Because most of the vinyl end-groups are also associated with short chains (as a result of statistical considerations only), and because the high-MW sites do not incorporate branches well, short backbones tend to receive short branches, and long chains receive none. Thus, the segregation of LCB is possible in this case. 

This inhibition is actually a useful trait. As noted in Section 7.6, Cr/ silica tends to concentrate branching into the low-MW part of the MW distribution, which can impair some polymer properties. Because the sites associated with titania tend to produce low-MW polymers [435], inhibition of comonomer incorporation by these sites tends to place proportionally more of the branching into the other (higher-MW) regions of the MW distribution for a given density. Thus, Cr/silica-titania catalysts tend to produce better polymer properties than Cr/silica, because they create first, a broader MW distribution with more long chains, and second, a flatter branch profile, in which the long chains receive more of the branches. 

Another family of low density polyethylenes (LDPE) can be obtained by high-pressure free radical polymerization, resulting in complex microstructures where side chain branches (mainly ethyl and butyl) are obtained through chain transfer reactions without the need of comonomer incorporation. The presence of long... 

Such sequence structures are required to reduce the crystallinity of the polyethylene sample as the comonomer n-butyl branch from incorporation of 1 -hexene into the polymer backbone interrupts the chain-folding mechanism responsible for the crystallinity in polyethylene. Long units of ethylene along the polymer backbone are able to crystallize the polymer sample and are undesirable in polyethylene products that require a large degree of elasticity for particular applications. 

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