Polyolefins monomer units

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... 

Referring to the ADMET mechanism discussed previously in this chapter, it is evident that both intramolecular complexation as well as intermolecular re-bond formation can occur with respect to the metal carbene present on the monomer unit. If intramolecular complexation is favored, then a chelated complex, 12, can be formed that serves as a thermodynamic well in this reaction process. If this complex is sufficiently stable, then no further reaction occurs, and ADMET polymer condensation chemistry is obviated. If in fact the chelate complex is present in equilibrium with re complexation leading to a polycondensation route, then the net result is a reduction in the rate of polymerization as will be discussed later in this chapter. Finally, if 12 is not kinetically favored because of the distant nature of the metathesizing olefin bond, then its effect is minimal, and condensation polymerization proceeds efficiently. Keeping this in perspective, it becomes evident that a wide variety of functionalized polyolefins can be synthesized by using controlled monomer design, some of which are illustrated in Fig. 2. 

Finally, there is need for a systematic study of the synthesis of different types of well-defined block copolymers free of homopolymer impurities. In particular the block copolymers consisting of polyolefin blocks and polar monomer unit blocks are expected to exhibit new characteristic properties owing to the effect of microphase separation. 

A major breakthrough in polymer production occurred with the discovery of metallocene catalysts [1]. We are now able to make polyolefins with a controlled level of branching (and tacticity). The simplest object is a statistically branched polymer, with a certain overall degree of polymerisation X, and a certain distance (monomer units) between successive branch points, which we shall call b. The basic goal of characterisation is to measure X and b from a minimum number of experiments in dilute solutions. 

There have been recent efforts to predict, or at least rationalize, the x parameters of these and other polyolefin-polyolefin blends. Bates et al. (1992) and Fredrickson et al. (1994) suggest that the x parameter is correlated to a difference in statistical segment length of the polymer molecules, on a volume-normalized basis. The volume normalization is required because the definition of the statistical segment length depends on how the monomer unit... 

In most blend systems of PA/PO compatibilized with olefin polymers and copolymers, are grafted with MAH, therefore, the concentration of the latter does not exceed 1-3 wt%. The main aim of grafting is (8) to ensure compatibility of polyolefins with polar polymers, which can be reached at the level of PO grafting of 0.1-0.5 moles of polar groups per 100 monomer units. 

The relative configuration of the monomer units can be controlled by the structure of the catalyst or by the configuration of the last inserted unit. These two scenarios are called site control and chain-end control. The isotactic polypropylene generated by the types of stereodefined metallocene catalysts presented in this chapter results from site control. More sophisticated architectures are possible by site-control mechanisms than chain-end control mechanisms, as illustrated by the variety of polyolefins prepared by homogeneous catalysts. 

There are four possible combinations of regiochemistry and stereochemistry within diad units. The olefins can join in a head-to-head or head-to-tail fashion. Most polyolefins formed by early metal catalyts are formed by strict head-to-tail enchainment, but this head-to-tail enchainment can occur by a series of 1,2-insertions in which the a-olefin substituent is located P to the metal in tihe insertion product or 2,1-insertions in which the a-olefin substituent is located a to the metal in the insertion product. In addition, the olefins can join to give rise to a diad unit containing identical (meso, m) or opposite (racemo, r) stereochemical relationships to the last inserted monomer unit. Site control of polymerization to form isotactic polymer gives rise to rr-defects m the polymer from stereoerrors, but cham-end control of polymerization to form isotactic polymer gives rise to r-defects from stereoerrors. 

Lamellas of crystalline phase of the surface layer of polyolefin blends studied are thicker than present in the surface layer of their components, what suggests cocrystallization of ethylene monomer unit from EPDM. Linear LDPE facilitates the phenomenon, especially when takes place in amorphous elastomer matrix. Branched plastomer recrystallizes to the same lamellar liiickness, no matter the structure of elastomer matrix. 

FIGURE 12.1 Polypropylene is a polyolefin comprised of repeating olefin monomer units. ... 

X-ray and melting-point studies of these copolymers have been carried out 14S). All the copolymers studied are crystalline over the whole range of composition (the total crystallinity diminishes from 55-51 % for polybutene-1 to 30% for 1 1-1 2 copolymers, and then rises again to 61-65% for another polyolefin). The melting point variations are also small. True cocrystallization does not occur in these cases owing to the difference in monomer unit sizes. These facts led the authors (145) to conclude that there was strong compositional inhomogeneity. 

Most monomers are above their free-radical ceiling temperature and only a single monomer unit is often added onto the polymer backbone. This is notably the case with maleic anhydride, acrylic acid, and methyl methacrylate. Styrene is an exception and one graft polymerize long polystyrene chains onto polyolefins [223]. 

Low- and high-density polyethylene, polypropene, and polymers of other alkene (olefin) monomers constitute the polyolefin family of polymers. All except LDPE are produced by coordination catalysts. Coordination catalysts are also used to produce linear low-density polyethylene (LLDPE), which is essentially equivalent to LDPE in structure, properties, and applications (Sec. 8-1 lc). The production figures given above for LDPE do not include LLDPE. The production of LLDPE now exceeds that of LDPE, with about 10 billion pounds produced in 2001 in the United States. (Copolymers constitute about one-quarter of all low density polyethylenes see Sec. 6-8b.)... 

Figure 8.14 Flow diagram showing the use of hydrocarbon-permeable membranes to recover unreacted monomers from a polyolefin plant resin degassing unit. The photograph is of a system installed by MTR in Qatar in 2007.

Release of the unsaturated chain end of a polyolefin can occur by fi-H transfer to the metal or to a monomer molecule (see Appendix 1 for backgound material). A metal-alkyl species, i.e. the starting unit for a new polymer chain, arises from the metal-hydride species formed in the first case by insertion of an olefin, or it can be formed directly by f-H transfer to a monomer (Figure 20). While the results are thus identical, the two reaction paths differ in their respective kinetics In the first case, the rate-limiting p-H transfer is independent of the olefin concentration, while the rate of p-H transfer to a monomer requires the formation of an olefin-containing reaction complex and will thus increase linearly with olefin concentration. 

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