Ethylene/polar comonomers copolymerization

Ethylene may be copolymerized with a range of other vinylic compounds, such as 1-butene, 1-octene and vinyl acetate (VA). These are termed comonomers and are incorporated into the growing polymer. Comonomers that contain oxygenated groupings such as vinyl acetate are often referred to as "polar comonomers." Comonomer contents range from 0 to 1 wt% for HOPE up to 40 wt% for some grades of ethylene-vinyl acetate copolymer. 

Chain propagation during copolymerization of ethylene with polar comonomers can proceed in several ways depending on the nature of the macroradical end group and the monomer being added, illustrated with vinyl acetate in eq 2.1-2.4 ... 

Figure 6.6 Structure of a single-site catalyst described by Goodall. Catalyst is capable of copolymerizing ethylene with polar comonomers without cocatalysts (BL Goodall, NT Allen, DM Conner, TC Kirk, LH McIntosh III and H Shen, International Conference on Polyolefins, Society of Plastics Engineers, Houston, TX, February 25-28, 2007).

Goodall (12) disclosed late transition metal catalysts that are highly active and are capable of copolymerizing ethylene with polar comonomers, such as acrylic acid and methyl acrylate. Moreover, Goodall catalysts do not require cocatalysts. An example of a Goodall catalyst is provided in Figure 6.6. 

As mentioned in Chapter 1, ethylene is always the more reactive olefin in systems used to produce copolymers involving a-olefins (LLDPE and VLDPE). An important process consideration for copolymerizations is the reactivity ratio. This ratio may be used to estimate proportions needed in reactor feeds that will achieve the target resin. However, fine tuning is often required to obtain the density or comonomer content desired. Reactivity ratios were discussed previously (Chapter 2) in the context of free radical polymerization of ethylene with polar comonomers. Reactivity ratios are also important in systems that employ transition metal catalysts for copolymerization of ethylene with a-olefins to produce LLDPE. Discussions of derivations and an extensive listing of reactivity ratios for ethylene and the commonly used a-olefins are provided by Krentsel, et al. (1). 

The lower oxophilicity and the greater functional group tolerance of late transition metals relative to early metals such as Ti, Zr, and Hf make them likely targets for the development of catalysts for the homo- and copolymerization of ethylene with polar comonomers under mild conditions. 

Brookhart and co-workers [79-81] introduced catalysts based largely on chelating, nitrogen-based ligands that are active for the homopolymerization of ethylene and the copolymerization of ethylene with 1-olefins and polar comonomers (31). Ni, Co, Fe or Pd are used as late transition metals. The diimine ligands have big substituents to prevent 6-hydride elimination. Ni(II) or Pd(II) complexes form cations by combination with MAO and polymerize ethylene to highly branched polymers with molecular weights up to one million. The activities reach TON... 

CnRhMe(OH)2 is not a catalyst. One polyethylene sample formed in water had a Mv of 5100 and a polydispersity index of 1.6 the average turnover rate was 1 per day. It is possible to copolymerize ethylene with polar comonomers such as methyl acrylate with the rhodium catalysts. In addition to the Cn ligands, softer trithiocyclononane ligands support the polymerization of ethylene on both rhodium and platinum. 

Late transition-metal complexes can also polymerize nonpolar and polar comonomers, such as alkyl acrylates, acrylonitrile, or carbon monoxide. Polyketones have been efficiently produced through CO-ethylene copolymerizations [46],... 

Iron catalysts [20-24] have been used to make polyethylene-clay nanocomposites where the polyethylene had very broad molecular weight distribution (MWD). Ziegler-Natta [25, 26], organo-chromium (Phillips) [27], and bis(imino)pyridine iron and cobalt catalysts [28] have also been used to make polyolefin-late transition metal catalysts [29, 30], capable of producing highly branched polyethylene from only ethylene and of promoting the copolymerization of ethylene with polar comonomers, have also been apphed to make polyolefin-clay nanocomposites. 

Regarding synthesis of polyolefin nanocomposites with wider applications, copolymerization of ethylene with other olefinic monomers, including higher a-olefins and polar comonomers (with late transition metal catalysts) still needs to be investigated in more detail. For instance, exfohated polyolefin-using chain end functionalized polyolefins can potentially be used as the polymeric surfactants [112]. 

Recently, Ni- and Pd-based catalysts have been developed in order to copolymerize nonpolar ethylene, 1-olefins or cycloolefins with polar comonomers, such as carbon monoxide and methyl acrylate or 1-olefins, containing polar groups which are separated by at least two methylene... 

LDPE Copolymers. A variety of comonomers can be added to the polymerization of ethylene to make copolymers. The free-radical polymerization mechanism of LDPE production allows for the copolymerization of polar comonomers. At this time, the incorporation of polar comonomers is unique to LDPE. The transition metals used to catalyze HDPE and LLDPE production are generally poisoned by polar comonomers and therefore, only copolymers containing alpha-olefins like 1-butene, 1-hexene, and 1-octene can be made. Because the polar copolymers can be made only by the LDPE process, they command a premium in the market. The most common comonomers (and their corresponding copol5uners) are vinyl acetate (EVA), methyl acrylate (EMA), ethyl acrylate (EEA), and acrylic acid... 

Single-site catalysts enable a broader range of altemalive comOTumers in ethylene and propylene copolymerizations through better comonomer respraise, which allows the use of much lower polar comonomer concentrations. Dramatically new technical properties could be achieved in polyolefin materials when the properties of polyolefins and polar comonomers were combined [16-18]. 

Ethylene copolymerized with other olefins has already been covered above. Ethylene copolymers with polar comonomers are made by the high-pressure route the favoured monomer is vinyl acetate (VAC) which, under the usual reaction conditions, copolymerizes with ethylene ideally. Small amounts of VAC reduce crystallinity and melting point, but increase toughness and flexibility such copolymers (EVAC) are used for tough film, although the moisture permeability is increased by the inclusion of the polar groups. 

Cationic Pd complexes of the type 6.18 can catalyze the copolymerization of ethylene and polar comonomers such as methyl acrylates (MAs). Structure 6.35 shows another Pd-based cationic precatalyst. Complexes having the general structure 6.36 have also been found to be effective... 

Copolymerization of ethylene with polar comonomers results in such resins as ethylene-co-vinyl acetate, ethylene-co-vinyl alcohol, and ethylene-co-metha-crylic acid copolymers. The polar side groups so incorporated may interact with each other to endow the product with specific physical properties, or they may be used as sites for subsequent chemical reactions. A major family of polymers falling into this category are ionomers, which consist of ethylene-co-vinyl acid copolymers, the acid functions of which have been neutralized to form metal salts. 

The cationic Pd(II) catalysts exhibit effective copolymerizations of ethylene and other a-olefins with polar-functionalized comonomers, with the majority of insertions occurring at the ends of branches. Among the best tolerated monomers are those bearing fluorine or oxygen-containing functionalities, such as esters, ketones, and ethers. The copolymerization of ethylene and acrylates, attractive because the monomers are inexpensive and the copolymers exhibit unique physical properties, has been well-studied mechanistically [27,69], Examples of copolymerizations of ethylene and a-olefins with methyl acrylate are shown in Table 4. In general, the amount of comonomer incorporation varies linearly with its reaction concentration and... 

Tphis paper is concerned with the effect of ionizing radiation on the physical and mechanical properties of copolymers of ethylene with alkyl acrylates, such as ethyl acrylate, butyl acrylate, and 2-ethvlhexyl acrylate (J, 2, 3). These polymers are made by the free radical copolymerization of ethylene under high pressure with alkyl esters of acrylic acid (9). They are more flexible than polyethylene and because of the polar nature of the comonomer, they are more compatible with fillers and with other polymers than is polyethylene. 

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