Ethylene/polar comonomers

Since the reactivity ratios of ethylene-polar monomer pairs are quite different, the preparation of copolymers with precisely the same comonomer composition can be a challenging endeavor. Earlier in this chapter, we described the synthesis and characterization of precisely placed methyl groups on a polyethylene... 

Linear, Random Copolymers of Ethylene and Polar Comonomers. 168... 

After five decades of catalyst research there is slowly emerging a family of discrete late transition metal catalysts that are capable of generating high molecular weight, linear, random copolymers of ethylene and polar comonomers such as acrylates. Further advances in the efficiency of these catalysts will likely give rise to new families of commercial polyolefins with a wealth of new performance properties imparted by the polar groups attached to the polymer backbone. 

The main advantages for the high-pressure process compared to other PE processes are short residence time and the ability to switch from homopolymers to copolymers incorporating polar comonomers in the same reactor. The high-pressure process produces long-chain, branched products from ethylene without expensive comonomers that are required by other processes to reduce product density. Also, the high-pressure process allows fast and efficient transition for a broad range of polymers. 

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. 

The range of suitable comonomers depends upon the nature of the catalyst or initiator. For example, Ziegler-Natta catalysts are poisoned by polar comonomers. Hence, commercial copolymers of ethylene and vinyl acetate are currently produced only with free radical initiators. However, some single site catalysts are tolerant of polar comonomers (see section 6.2.1). 

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,... 

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... 

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. 

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],... 

The sheer size and value of the polyethylene industry ensure that there is continued research, progress, and development in catalysis, for their potential commercial impact. Although this whole subject is not within the scope of this chapter, we mention a couple of aspects of the progress, which offer the potential to impact this industry. In 1995, DuPont introduced work, carried out with them at the University of North Carolina—via the largest patent applicafion ever in the USA. They disclosed what are described as post-metallocene catalysts. These are transition and late transition metal complexes with di-imine ligands, which form part of the DuPont Versipol technology. Such catalysts create highly branched to exceptionally linear ethylene homopolymers and linear alpha-olefins. Late transition metals offer not only the potential for the incorporation of polar comonomers, which until now has only been possible in LDPE reactors, but also their controlled sequence distribution, compared to the random composition of free radical LDPE copolymers. Such copolymers account for over 1 million tons per annum [20]. Versipol has so far only been cross-licensed and used commercially by DuPont Dow Elastomers (a former joint venture, now dissolved) in an EPDM plant. 

However, the influence of polar comonomer units on polymer solubihty is in general neither Hnear nor necessarily monotonic. Fig. 2.6a shows the ethylene solubility of poly(ethylene-co-methyl acrylate) copolymers for different amounts of the methyl acrylate monomer in the copolymer from 0 mol% (corresponds to LDPE) to 44 mol%. For small amounts of the methyl acrylate monomer, favorable interactions of the methyl acrylate units of the copolymer with the quadru-pole moment of the ethylene enhance the solubility of the copolymer. Here, the copolymers first show a decreasing cloud point pressure. However, upon further increase of the methyl acrylate contents (above 13 mol%), the importance of the polar intermolecular interactions between the different methyl acrylate units of the copolymer molecules becomes dominant, leading to decreasing solubility. However, for the similar system poly(ethylene-co-propyl acrylate), very different behavior is observed. Here, the solubility of the copolymer increases with in-... 

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