Polyolefin branching

Isotactic polyaldehydes melt (with some decomposition) at a somewhat higher temperature than the corresponding olefin polymers (Table VIII) (7), particularly in the higher members of the homologous series. Like isotactic polyolefins, branched isotacting polyaldehydes melt substantially higher than the corresponding unbranched polymers. 

The major goal of this study was to understand the factors controlling polyolefin branching and the relationship between the catalyst structure, temperature, pressure and the polyolefin topology. The DFT calculations were carried out for the elementary reactions in the polymerization of ethylene and propylene catalyzed by Pd-based diimine catalysts [13c,d] and the ethylene polymerization catalyzed by the Ni-anilinotropone catalyst [28]. The polymer growth in these processes was modeled by a stochastic approach [27-29]. Further, the model simulations were performed by systematically changing insertion barriers to model the influence of catalyst, beyond the diimine systems [29]. 

We have presented a comprehensive review of our recent theoretical studies on the polymerization and copolymerization processes catalyzed by the late transition metal complexes. The results of these studies show that a combined DFT/stochastic approach can be successfully used to model the elementary reactions in the polymerization processes and the influence of the reaction conditions on the polyolefin branching. Such an approach makes it possible to understand the microscopic factors controlling the branching of polyolefins and explain the differences between the Pd- and Ni-catalysts. The results also demonstrate that a wide range of microstructures can be potentially obtained from the ethylene polymerization. Thus, a rational design of the catalyst producing the desired polymer topology should be possible. 

However, there are still a number of important polymer properties that can only be measured by laborious and time-consuming off-line analyses. In this category one can include the MWD (especially in dispersed systems and/or for polyolefins), branching and crosslinking density, the gel content, the PSD, among other properties. (Despite the several examples reported in the scientific literature, at present no commercial equipment can ensure the fast and robust on-hne measurement of the entire size distribution of polymer particles in industrial reactors.)... 

Thermal, Thermooxidative, and Photooxidative Degradation. Polymers of a-olefins have at least one tertiary C-H bond in each monomer unit of polymer chains. As a result, these polymers are susceptible to both thermal and thermooxidative degradation. Reactivity in degradation reactions is especially significant in the case of polyolefins with branched alkyl side groups. For example, thermal decomposition of... 

Polyolefins such as polyethylene and polypropylene contain only C—C and C—H bonds and may be considered as high molecular weight paraffins. Like the simpler paraffins they are somewhat inert and their major chemical reaction is substitution, e.g. halogenation. In addition the branched polyethylenes and the higher polyolefins contain tertiary carbon atoms which are reactive sites for oxidation. Because of this it is necessary to add antioxidants to stabilise the polymers against oxidation Some polyolefins may be cross-linked by peroxides. 

Polyolefins with branched side chains other than P4MP1 have been prepared Figure 11.14). Because of their increased cohesive energy, ability for the molecules to pack and the effect of increasing chain stiffness some of these polymers have very high melting points. For example, poly-(3-methylbut-l-ene) melts at about 240°C and poly-(4,4-dimethylpent-l-ene) is reported to have a melting point of between 300 C and 350°C. Certain cyclic side chains can also... 

As is the case in the polyolefins, the polymethacrylates with branched side chains have higher softening points and are harder than their unbranched isomers. The effect of branching of Vicat Softening point is shown in Table 75.5.> ... 

The molecular structure and properties of polyolefins have been explained by several workers in the past [10-14]. This chapter deals with the primary molecular parameters and their effect on processability and ultimate properties of PEs. Since molecular parameters are closely interrelated, it is not possible to discuss one without referring to the other. Hence, in the section relating to the effect of chain branching, reference has also been made to MW and MWD and vice versa. 

Most commercial polymers are substantially linear. They have a single chain of mers that forms the backbone of the molecule. Side-chains can occur and can have a major affect on physical properties. An elemental analysis of any polyolefin, (e.g., polyethylene, polypropylene, poly(l-butene), etc.) gives the same empirical formula, CH2, and it is only the nature of the side-chains that distinguishes between the polyolefins. Polypropylene has methyl side-chains on every other carbon atom along the backbone. Side-chains at random locations are called branches. Branching and other polymer structures can be deduced using analytical techniques such as NMR. 

Favorable rheological properties are an essential requirement for the commercialization of polyolefins like polyethylene. The ease of processability of the polymer melt, obtained through modifications in the microstructural features, is as important as the end use mechanical properties of these polymers. Presence of long-chain as well as short-chain branching, LCB and SCB, respectively, more or less dictates the rheological behavior of most commercial... 

A viscosity online detector in a size exclusion chromatography (SEC) instrument allows for a universal calibration for polymers with known K- and a-values. For polymers that are only soluble at high temperature, e.g., polyolefines, high-temperature detectors are available, which can be operated up to 200°C. In addition to molar mass measurements, viscosity detectors have also been employed successfully to obtain structural information of branched polymers [28]. 

A combination of infrared spectroscopy with size exclusion chromatography has a wide application range in the characterization of copolymers, adhesives, impurity profiling in polymers and branching in polyolefines [60-65]. Commonly, the solvent used as a mobile phase absorbs strongly in the... 

One of the most defining characteristics of the late metal a-diimine polymerization systems is the uniquely branched polyolefins that they afford. This arises from facile p-hydride elimination that late transition metal alkyl complexes undergo. The characteristics of the isomerization process have been the subject of much investigation, particularly with the more easily studied Pd(II) a-diimine system. The process is initiated by P-hydride elimination from the unsaturated alkyl agostic complex 1.17, followed by hydride reinsertion into olefin hydride intermediate 1.18 in a non-regioselective manner (Scheme 5). In doing so, the metal center may migrate... 

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. 

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