Polyolefins chain-branched

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 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... 

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. 

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... 

Free-radical polyolefin reactions form polymers with many mistakes in addition to the ideal long-chain alkanes because of chain-branching and chain-termination steps, as discussed. This produces a fairly heterogeneous set of polymer molecules with a broad molecular-weight distribution, and these molecules do not crystallize when cooled but rather form amorphous polymers, which are called low-density polyethylene. 

They are able to polymerize a large variety of vinyl monomers. The polymer microstructure can be controlled by the symmetry of the catalyst precursor. Prochiral alkenes such as propylene can be polymerized to give stereospecific polymers,554 572-574 allowing production of polyolefin elastomers. They can give polyolefins with regularly distributed short- and long-chain branches which are new materials for new applications. 

M. H. Wagner, H. Bastian, P. Hachmann, J. Meissner, S. Kurtzbeck, H. Miinstedt and F. Langouche, The Strain-hardening Behaviour of Linear and Long-chain-branched Polyolefin Melts in Extensional Flows, Rheol. Acta, 39, 97-109 (2000). 

The application of refractive index and differential viscometer detection in SEC has been discussed by a number of authors [66-68]. Lew et al. presented the quantitative analysis of polyolefins by high-temperature SEC and dual refractive index-viscosity detection [69]. They applied a systematic approach for multidetector operation, assessed the effect of branching on the SEC calibration curve, and used a signal averaging procedure to better define intrinsic viscosity as a function of retention volume. The combination of SEC with refractive index, UV, and viscosity detectors was used to determine molar mass and functionality of polytetrahydrofuran simultaneously [70]. Long chain branching in EPDM copolymers by SEC-viscometry was analyzed by Chiantore et al. [71]. 

Copolymers of ethylene with a-olefins, such as the short-chain branched LLDPE (linear low-density polyethylene) impact materials or the EPD (ethylene-propylene-diene copolymer) rubbers represent major percentages of the total polyolefin production, due to their desirable mechanical properties. Solid-state MgCl2-supported Ziegler-Natta catalysts however, have unfavourable reactivity... 

The dynamic cross-linking process is used to produce thermoplastic elastomers from mixtures of crystallizable polyolefins and various rubbers. Variations of basically the same method are employed to produce novel, stable polymer alloys by performing chemical reactions during extrusion of such mixtures. In that case, the cunent industrial term is reactive extrusion. Such processes are used, for example, to improve processability of LLDPE s into tubular film (by introducing long chain branches during extrusion with low levels of peroxides) or to... 

Vega, J. Aguilar, M. Peon, J. Pastor, D. Martinez-Salazar, J. Effect of long chain branching on linear-viscoelastic melt properties of polyolefins. E-Polymers 2002, 46, 1-35. 

Other catalytic systems that generate long chain-branched polyolefins have been reported such as Dow s half-sandwich constrained geometry titanium catalysts (CGC) [14e], and Bayer s novel donor/acceptor metallocenes [14f]. This polymer structural feature improves material and processing properties. 

Figure 11.13. Effect of side-chain branching on the melting point and glass transition temperature of polyolefins (—CHR—CH2—) — (R straight chain) (Ref 13)...

M.K. Reinking, G. Orf, D. McFaddin, Novel mechanism for the formation of long-chain branching in polyethylene, in Polyolefins SPE Polyolefins XII, International Conference on Polyolefins, Houston TX, (February 27-March 1, 2000). 

The polyolefins produced by transition metal catalysts are characterized by the absence of large amounts of long- or short-chain branching, which causes variability in density, crystallinity, and melting points. Most catalysts used are heterogeneous but some homogeneous systems are known. A two-step mechanism for catalysis is widely accepted (1) adsorption of the monomer, which may be activated by the configuration established in this step, and (2) insertion of the activated monomer into a metal-carbon bond. 

LCT (originally developed for di-block copolymers) was found to be particularly useful to explain miscibility of polyolefin blends where the two resins differ in the type and size of short chain branching. The stractural units of a polymer with two carbons in the main chain can be written as PE = (CH,-CH,), PP = [CH,-CH (CHj)], poly-2-butene (P2B) = [CH (CH3)-CH (CHj)], PIB = [CH -C (CHj) ]jj, poly(4,4-dimethyl 1-butene) (PDMB) = [CH -CH (C Hg)], etc. Three structural parameters (ratio of end to interior groups) have been used to distinguish PO structure r, p, and q. Their values for the model macromolecules discussed above are listed in Table 2.7. 

---