Ethylene Polymerization with Polar Comonomers

A number of industrial polymers are made from vinyl monomers of the general formula (CH =CHX (X = CO H, OCOCH, Cl, CN, etc.) that have polar groups. These polymers cannot be made by Zeigler-Natta type of catalysts mainly because titanium is highly oxophilic, and the resultant Ti-0 or Ti-X bonds are too strong. 

Currently ethylene-acrylate and ethylene-vinyl acetate copolymers are commercially produced by radical polymerizations in high-pressure reactors. In recent times much effort has gone into developing singlesite catalysts for the copolymerization of ethylene and polar vinyl monomers. 

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 

When 6.35 is used as the precatalyst, the copolymer of ethylene and MA is found to be highly branched. As shown by 6.37, in the resultant polymer the acrylate comonomers are located mainly at the end of some of the branches. Note that in 6.37 ethylene monomers are bonded to MA, but there is no MA-MA bond. 

The intermediate 6.39 by a series of )3-elimination followed by readdition, i.e., chain walk type of mechanism, is eventually converted to 6.35. Like in 6.40 and 6.35, an interaction between palladium and oxygen is present in 6.39, but not shown. Steric and electronic factors are considered to be the reasons behind predominant 2,1 insertion. 

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In Chapter 1, it was mentioned that highly branched low density polyethylene and copolymers made with polar comonomers are produced only by free radical polymerization at very high pressure and temperature. (All other forms of commercially available polyethylene are produced with transition metal catalysts under much milder conditions see Chapters 3, 5 and 6.) In this chapter we will review how initiators achieve free radical polymerization of ethylene. Low density polyethylene and copolymers made with polar comonomers are produced in autoclave and tubular processes, to be discussed in Chapter 7,... 

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). 

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. 

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... 

It is assumed that the monomers are present as aluminates. Although separately pretreating the alcoholic monomers with MAO prior to polymerization did not improve comonomer conversion of incorporation (ethylene uptake was improved), the polymerization procedure involves mixing of the polar comonomer with MAO prior to zirconocene addition, and higher total levels of MAO (ca. 10 000 equiv per Zr) were found to have a more favorable effect on conversion. 

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]. 

In contrast to the vinyl-Si(CH3)3, the copolymerization of allyl-Si(CH3)3 and ethylene proceeded better because the comonomer uptake was clearly higher (Table 7, runs 5-9). However, these polymerizations also suffered from low molar mass copolymer and reduced catalyst activity. Both these disadvantages are in agreement with the above explanation of the formation of stable polar transition states that reduce the propagation rate and allow the chain termination reaction to take place. This hypothesis was further supported by the H-NMR analysis, which showed an overwhelming concentration of chain-end ally lie silane groups (Fig. 20a), which were obviously formed after primary 1,2-insertion of allyl-Si(CH3)3 (Scheme 3b) [23]. 

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