Free-Radical Additions Polyethylene

Solution Work backward. The double bond in the product was a single bond in the starting diene. Therefore, 

PROBLEM 3.26 Draw the structure of the product of each of the following cycloaddition reactions. 

Some reagents add to alkenes by a free-radical mechanism instead of by an ionic mechanism. From a commercial standpoint, the most important of these free-radical additions are those that lead to polymers. 

A polymer is a large molecule, usually with a high molecular weight, built up from small repeating units. The simple molecule from which these repeating units are derived is called a monomer, and the process of converting a monomer to a polymer is called polymerization. 

The free-radical polymerization of ethylene gives polyethylene, a material that is produced on a very large scale (more than 150 billion pounds worldwide annually). The reaction is carried out by heating ethylene under pressure with a catalyst (eq. 3.38). 

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Hydroboration of Alkenes Addition of Hydrogen Additions to Conjugated Systems Free-Radical Additions Polyethylene Oxidation of Alkenes A WORD ABOUT... Ethylene Raw Material and Plant Hormone... 

Addition to double bonds may also occur by a free-radical mechanism. Polyethylene can be made in this way from the monomer ethylene. 

This expression for the overall polymerization rate is found to be generally true for such practical examples of free radical addition polymerization as polyethylene, and others. 

Ethylene reacts by addition to many inexpensive reagents such as water, chlorine, hydrogen chloride, and oxygen to produce valuable chemicals. It can be initiated by free radicals or by coordination catalysts to produce polyethylene, the largest-volume thermoplastic polymer. It can also be copolymerized with other olefins producing polymers with improved properties. Eor example, when ethylene is polymerized with propylene, a thermoplastic elastomer is obtained. Eigure 7-1 illustrates the most important chemicals based on ethylene. 

Low-density polyethylene (LDPE) is produced under high pressure in the presence of a free radical initiator. As with many free radical chain addition polymerizations, the polymer is highly branched. It has a lower crystallinity compared to HDPE due to its lower capability of packing. 

Addition polymers, which are also known as chain growth polymers, make up the bulk of polymers that we encounter in everyday life. This class includes polyethylene, polypropylene, polystyrene, and polyvinyl chloride. Addition polymers are created by the sequential addition of monomers to an active site, as shown schematically in Fig. 1.7 for polyethylene. In this example, an unpaired electron, which forms the active site at the growing end of the chain, attacks the double bond of an adjacent ethylene monomer. The ethylene unit is added to the end of the chain and a free radical is regenerated. Under the right conditions, chain extension will proceed via hundreds of such steps until the supply of monomers is exhausted, the free radical is transferred to another chain, or the active site is quenched. The products of addition polymerization can have a wide range of molecular weights, the distribution of which depends on the relative rates of chain grcnvth, chain transfer, and chain termination. 

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. 

Bohm, the methylene free radical, —CHoCCHL—, may be produced momentarily in polyethylene by eliminating molecular hydrogen during the irradiation. This process could replace or exist in addition to that represented by Equation 1. Such a biradical (or Lewis acid) would not be expected to be stable but could revert to the vinvlene group... 

Alkanes can be simultaneously chlorinated and chlorosulfonated. This commercially useful reaction has been applied to polyethylene (201—203). Aromatics can be chlorinated on the ring, and in the presence of a free-radical initiator alkylaromatic compounds can be chlorinated selectively in the side chain Ring chlorination can be selective. A patent shows chlorination of 2,5-di- to 2,4,5-trichlorophenoxyacetic acid free of the toxic tetrachlorodibenzodioxins (204). With alkenes, depending on conditions, the chlorination can be additive or substitutive. The addition of sulfuryl chloride can occur to form a 2-chloroalkanesulfonyl chloride (205). 

One method is to measure chain-transfer coefficients with low-MW analogues of the polymer. Thus Gilchrist (140) measured the rate at which 14C labelled decane was incorporated into polyethylene in the free-radical polymerization, and hence obtained an estimate of the transfer coefficient with methylene groups this was in fair agreement with another estimate obtained from the effect of the addition of fractions of linear polyethylene on the Mn of the branched polyethylene, which could be separated from linear polymer plus grafted branched polymer by column extraction. Low MW polymer may be used as a transfer agent Schulz and co-workers (189) obtained chain-transfer coefficients in styrene polymerization from the effect of added low MW polymer on Mn. 

Isolated butyl branches in low-density polyethylene are formed by an intrachain radical rearrangement that is followed by repeated addition of ethylene without further rearrangement. Here, stereochemical selectivity during the formation of CH2R—CH2-CHR—CH2-branches in the free radical initiated polymerization of monosubstituted vinyl monomers is Investigated. The configuration partition functions are denoted by Zm and Zn respectively. They can be written as 2 Um Up Iv1 v2 v3]T and Zr = U U2 Up Ur Up [v3 v2 v3lT. Numerical values... 

Figure 16 shows the absorption spectrum obtained by additive-free polyethylene [67], At ambient temperature the absorption observed on nanosecond time-scale increased continuously from 500 to 200 nm without showing any maximum. The absorption in UV is similar to that obtained by y-irradiation. Considering the results obtained by liquid alkanes, the absorption seems to be comprised of several different free radicals. At 95 K additional absorption due to the trapped electron was observed at wavelengths longer than 600 nm the band was observable even at ambient temperature in the picosecond time-domain [96]. The electron decays presumably by the hole-electron recombination. The decay of the trapped electron was independent of the presence of carbon tetrachloride, suggesting that the additives reacted with a mobile electron but not with the trapped electron. On adding naphthalene, the radiation-induced spectrum showed the bands due to the first excited triplet state and the radical... 

Synthetic methods targeting amino acid incorporation into functional materials vary widely. Free-radical polymerization of various amino acid substituted acrylates has produced many hydrocarbon-amino acid materials [161, 162]. In separate efforts, MorceUet and Endo have synthesized and meticulously characterized a library of polymers using this chain addition chemistry [163- 166]. Grubbs has shown ROMP to be successful in this motif, polymerizing amino add substituted norbornenes [167-168]. To remain within the scope of this review, the next section wiU focus only on ADMET polymerization as a method of amino add and peptide incorporation into polyethylene-based polymers. 

The polyethylene polymers illustrate one of the major types of polymerization reactions, called addition polymerization, in which the monomers simply add together to produce the polymer. No other products are formed. The polymerization process is initiated by a free radical (a species with an unpaired electron) such as the hydroxyl radical (HO ). The free radical attacks and breaks the tt bond of an ethylene molecule to form a new free radical,... 

As a result of the advances in catalyst discovery for aqueous ethylene polymerization, silica-polyethylene nancomposites have been prepared with structures that vary with changing catalyst structure and silica composition." It is likely that many more advances in the area of high-tech composites with potential biological and nanotechnology applications will be made in the next few years through aqueous polymerization processes. In addition to free radical polymerizations and catalytic polymerizations, it should be noted that oxidative polymerizations can also be performed in aqueous media to yield conducting polymers. Recently, this has been used to prepare polypyrrole-coated latex particles that are expected to be interesting synthetic mimics for micrometeorites. 

As previously mentioned, in addition to free radical initiation, ethylene may be polymerized by use of transition metal catalysts. To place the importance of these catalysts in proper perspective, one must recognize that transition metal catalysts were used to produce about 73% of the global industrial output of polyethylenes in 2008 or about 56 million tons (124 billion pounds). 

Chapter 1 is used to review the history of polyethylene, to survey quintessential features and nomenclatures for this versatile polymer and to introduce transition metal catalysts (the most important catalysts for industrial polyethylene). Free radical polymerization of ethylene and organic peroxide initiators are discussed in Chapter 2. Also in Chapter 2, hazards of organic peroxides and high pressure processes are briefly addressed. Transition metal catalysts are essential to production of nearly three quarters of all polyethylene manufactured and are described in Chapters 3, 5 and 6. Metal alkyl cocatalysts used with transition metal catalysts and their potentially hazardous reactivity with air and water are reviewed in Chapter 4. Chapter 7 gives an overview of processes used in manufacture of polyethylene and contrasts the wide range of operating conditions characteristic of each process. Chapter 8 surveys downstream aspects of polyethylene (additives, rheology, environmental issues, etc.). However, topics in Chapter 8 are complex and extensive subjects unto themselves and detailed discussions are beyond the scope of an introductory text. 

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