Polyolefins reaction mechanisms

Acid-Catalyzed Cracking of Polyolefins Primary Reaction Mechanisms... 

C. Vasile, P. Onu, V. Barboiu, M. Sabliovschi, G. Moroi, Catalytic decomposition of polyolefins. II. Considerations about the composition and the structure of reaction products and the reaction mechanism on silica-alumina cracking catalyst. Acta Polym. 36, 543 (1985). 

As illustrated in Scheme 12, the metallocene-mediated copolymerization of a-olefin and reactive comonomer forms a copolymer containing several pendent reactive groups, and then serves as an intermediate for the transformation to functional polyolefins by various reaction mechanisms. In addition to the metallocene catalyst for effective copolymerization, the key factor in this approach is the design of a comonomer containing a reactive group that can simultaneously fulfill the following requirements. First, the reactive group must be stable to metallocene catalysts and soluble in hydrocarbon polymerization media. 

Despite the progress that has been made, the effect of the polyolefin composition on MA grafting is still not fully understood, due to the lack of true insight into the reaction mechanism. Actually, most grafting studies have been carried out using different grafting recipes (type and amount of peroxide levels for PP). 

SEM photomicrograph of the smoothed and toluene-etched surfece of a PET ultra-low-density polyethylene (ULDPE)-g-DEM blend obtained in a discontinuous mixer by adding Ti(OBu)4 as a transesterification catalyst. (From M. B. ColtelH, Catalysed Reactive Compatibilization of Polyolefin and Poly(ethylene terephthalate) Blends Reactions Mechanisms and Phase Morphology Development, Ph.D. thesis. University of Pisa, Italy, 2005.)... 

Based on the same three considerations (i.e., stability, solubility, and versatility) of the reactive comonomer, we also investigated p-methylstyrene (p-MS) [40 14]. The major advantages of p-MS are its commercial availability, easy incorporation into the polyolefin, and versatility in functionalization chemistry under various reaction mechanisms, including free radical, cationic, and anionic processes. The benzylic protons are known to be readily reactive in many chemical reactions (such as halogenation, metallation, and oxidation) to form a desirable functional group at the benzylic position tmder mUd reaction conditions, as illustrated in Scheme 4. 

Researchers [2, 40] have investigated the catalytic degradation of polyolefins, using TGA, as a potential method for screening catalysts and have found that the presence of a catalyst led to a decrease in the apparent activation energy. There are different steps in the carbonium ion reaction mechanism, such as H-transfer, chain/] scission. 

In order for a soHd to bum it must be volatilized, because combustion is almost exclusively a gas-phase phenomenon. In the case of a polymer, this means that decomposition must occur. The decomposition begins in the soHd phase and may continue in the Hquid (melt) and gas phases. Decomposition produces low molecular weight chemical compounds that eventually enter the gas phase. Heat from combustion causes further decomposition and volatilization and, therefore, further combustion. Thus the burning of a soHd is like a chain reaction. For a compound to function as a flame retardant it must intermpt this cycle in some way. There are several mechanistic descriptions by which flame retardants modify flammabiUty. Each flame retardant actually functions by a combination of mechanisms. For example, metal hydroxides such as Al(OH)2 decompose endothermically (thermal quenching) to give water (inert gas dilution). In addition, in cases where up to 60 wt % of Al(OH)2 may be used, such as in polyolefins, the physical dilution effect cannot be ignored. 

Degradation of polyolefins such as polyethylene, polypropylene, polybutylene, and polybutadiene promoted by metals and other oxidants occurs via an oxidation and a photo-oxidative mechanism, the two being difficult to separate in environmental degradation. The general mechanism common to all these reactions is that shown in equation 9. The reactant radical may be produced by any suitable mechanism from the interaction of air or oxygen with polyolefins (42) to form peroxides, which are subsequentiy decomposed by ultraviolet radiation. These reaction intermediates abstract more hydrogen atoms from the polymer backbone, which is ultimately converted into a polymer with ketone functionahties and degraded by the Norrish mechanisms (eq. 

Many of the structures for MAH-modifled polyolefins that appear in the literature are wholly speculative, and are based on a proposed mechanism for the grafting reaction rather than an analysis of the reaction or reaction products. In early work, product characterization took the form of determining overall grafting levels by titration or IR spectroscopy. In more recent work, with the availability of... 

However, Pacansky and his coworkers77 studied the degradation of poly(2-methyl-l-pentene sulfone) by electron beams and from infrared studies of the products suggest another mechanism. They claim that S02 was exclusively produced at low doses with no concomitant formation of the olefin. The residual polymer was considered to be essentially pure poly(2-methyl-l-pentene) and this polyolefin underwent depolymerization after further irradiation. However, the high yield of S02 requires the assumption of a chain reaction and it is difficult to think of a chain reaction which will form S02 and no olefin. 

A detailed study of the mechanism of the insertion reaction of monomer between the metal-carbon bond requires quantitative information on the kinetics of the process. For this information to be meaningful, studies should be carried out on a homogeneous system. Whereas olefins and compounds such as Zr(benzyl)4 and Cr(2-Me-allyl)3, etc. are very soluble in hydrocarbon solvents, the polymers formed are crystalline and therefore insoluble below the melting temperature of the polyolefine formed. It is therefore not possible to use olefins for kinetic studies. Two completely homogeneous systems have been identified that can be used to study the polymerization quantitatively. These are the polymerization of styrene by Zr(benzyl)4 in toluene (16, 25) and the polymerization of methyl methacrylate by Cr(allyl)3 and Cr(2-Me-allyl)3 (12)- The latter system is unusual since esters normally react with transition metal allyl compounds (10) but a-methyl esters such as methyl methacrylate do not (p. 270) and the only product of reaction is polymethylmethacrylate. Also it has been shown with both systems that polymerization occurs without a change in the oxidation state of the metal. 

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. 

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