Polymerization of Ethylene and 1-Olefins

The discoveries of several new types of catalysts for (non-aqueous) efhylene and 1-olefin polymerization based on late transition metals over fhe last decade have initiated great interest in fhis field [16, 53-55]. By comparison to early transition metal-based Ziegler or Phillips catalysts used for industrial polyolefin synfhesis [13— 

complexes of late transition metals are less oxophilic, and fhey can be much more stable towards polar media. 

It can be noted that latexes of low-density polyethylene (LDPE) are prepared via free-radical emulsion polymerization as a specialty product [56]. However, fhe low variability wifh respect to fhe polymer microstructure is disadvantageous for tailoring latex properties (e.g. film-forming properties), resulting in a narrow property profile. In addition, working at pressures in excess of 1000 bar in fhe presence of water is challenging for the equipment used. 

In 1993, Flood and co-workers reported ethylene polymerization in water using the rhodium complex [(N-N-N)RhMe(OH2)(OH)] as a catalyst precursor (N-N-N= l,4,7-trimethyl-l,4,7-triazacyclononane) [57]. After 90 days of reaction at 60 bar efhylene pressure and room temperature, some low-molecular-weight polyefhylene was obtained (M 5x10 g mol ). The amount of polymer obtained corresponded to 1 TO per day. 

In 1995 Brookhart and co-workers reported that cationic diimine-substituted palladium complexes of type 5 [Eq. (5)] can polymerize ethylene to high molecular-weight, highly branched material in organic solvents such as methylene chloride. 

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Since the aforementioned investigations, significant advances in aqueous catalytic insertion polymerization have only been made over the past decade. Alternating copolymerization of olefins with carbon monoxide, polymerization of ethylene and 1-olefins, and polymerizations of norbomenes and of butadiene have been studied. 

Ihara, E. Nodono, M. Yasuda, H. Kanehisa, N. Kai, Y. Single site polymerization of ethylene and 1-olefins initiated by rare earth metal complexes. Macromol. Chem. Phys. 1996,197, 1909-1917. 

Compound (CsH4SiMe2NBut)TiCl2 has been synthesized and used as pre-catalyst for ethylene polymerization. The activities and the properties of the polymers have been compared to similar zirconium and hafnium derivatives.720 The consequences of anion-cation interactions on the activity of GGG group 4 metal complexes in olefin polymerizations have been explored for a series of zirconocene derivatives as well as the cationic species [(C5Me4SiMe2NBut)TiMe]+ with the sterically congested tris(perfluorobiphenyl)fluoroaluminate as the counteranion.721 The co-polymerization of ethylene and 1-butene by (CsMe iMe Bu TiC in the presence of... 

The polymerization of ethylene and 1-hexene has been studied.750,751 The reaction of [C5Fl4SiMe2NCF[2-CFl2NMe2]Ti(CH2Ph)2 with B(C6F5)3 has been investigated in order to understand the nature of the cationic 12-electron species [(C5H4SiMe2NGH2CH2NMe2)Ti(GH2Ph)]+ and its behavior in the ct-olefin polymerizations.752... 

Johnson, L.K., Killian, C.M. and Brookhart, M. (1995) New Pd(II)-based and Ni(II)-based catalysts for polymerization of ethylene and alpha-olefins. Journal of the American Chemical Society 117,6414-6415. Wunderlich, B. and Poland, D. (1963) Thermodynamics of crystalline linear high polymers. 2. Influence of copolymer units on thermodynamic properties of polyethylene. Journal of Polymer Science Part a-General Papers 1, 357. 

Perfluorinated phenylboranes and perfluorinated phenylborates are well-established activators in the metallocene-initiated polymerization of olefins. With the increasing commercial importance of metallocene technology for the polymerization of ethylene and the copolymerization of ethylene and 1-alkenes, perfluorinated phenylboranes and perfluorinated phenylborates became more readily accessible. As a consequence, a few studies on the influence of these highly fluorinated activators on Nd-catalysis are available in literature. 

The polymerization tests with ethylene and 1-olefines as well as with dienes showed a good ability of the metallocene catalyst for copolymerization. Interesting results from practical and theoretical point of view could be gained in the copolymerization of ethylene and 1,5-hexadiene. During polymerization first a complexation of one of the double bonds of 1,5-hexadiene takes place at the vacant coordination side of the transition metal. After insertion into the polymer chain the complexation of the second double bond occurs followed by intramolecular cyclisation of the 5-membered ring. Analysis of the 13C-NMR spectra reveals an incorporation of 4.2 mole% 1,5-hexadiene and a predominance of trans rings caused by the diastereoselectivity of the cyclisation step. 

Rhodium chloride catalyzes the polymerization of butadiene with high stereospecificity to trarw-poly( 1,4-butadiene) (4) and also the dimerization of ethylene and other olefins (2). Although certain oligomerizations are catalyzed by solid palladium and rhodium catalysts (9), polymerization to high molecular weight products is not generally observed. 

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... 

The dual function of the precatalysts 4 opened the way to well-controlled block polymerization of ethylene and MMA (eq. (5)) [89, 90]. Homopolymerization of ethylene (Mn = 10000) and subsequent copolymerization with MAA (Mn 20000) yielded the desired linear AB block copolymers. Mono and bis(alkyl/silyl)-substituted flyover metallocene hydride complexes of type 8 gave the first well-controlled block copoymerization of higher a-olefins with polar monomers such as MMA or CL [91]. In contast to the rapid formation of polyethylene [92], the polymerization of 1-pentene and 1-hexene proceeded rather slowly. For example, AB block copolymers featuring poly( 1-pentene) blocks (M 14000, PDI = 1.41) and polar PMMA blocks (M 34000, PDI = 1.77) were obtained. Due to the bis-initiating action of samarocene(II) complexes (Scheme 4), type 13-15 precatalysts are capable of producing ABA block copolymers of type poly(MMA-co-ethylene-co-MMA), poly(CL-co-ethylene-co-CL), and poly(DTC-co-ethylene-co-DTC DTC = 2,2-dimethyltrimethylene carbonate) [90]. 

The three major classes of polyethylene are described by the acronyms HOPE. LDPE. and LLDPE. High-density polyethylene (HOPE) is a linear, semicrystalline ethylene homopolymer Tm 135 °C) prepared by Ziegler—Natta and chromium-based coordination polymerization technology. Linear low-density polyethylene (LLDPE) is a random copolymer of ethylene and a-olefins (e.g.. 1-butene. 1-hexene, or... 

Polyethylene (PE) is the most widely used plastic throughout the world, and high density PE (HDPE) is the most widely used type of PE. HDPE has generally been taken to mean the product of ethylene polymerization having density greater than about 0.935 (or 0.94). It includes ethylene homopolymers and also copolymers of ethylene and alpha-olefins such as 1-butene, 1-hexene, 1-octene, or 4-methyl-1-pentene. Other types of PE include low density PE (LDPE), made through a free-radical process, and linear low density PE (LLDPE). 

Yttrocene tetramethylaluminate complex 38a was found to be less reactive towards ethylene and a-olefins than the corresponding scandocene, 38b. This reactivity trend is consistent with the observed reactivity for 39/H2(g). Compounds 38a-c and 39/H2( ) were tested for the polymerization of propylene and 1-pentene. 1-pentene polymerizations were carried out in neat monomer, either at 0 °C or at room temperature ( 22 °C). Propylene polymerizations were carried out either in neat monomer or with a 50/50 (v/v) mixture with toluene, all at -5 °C. In general, scandocenes 38b and 38c provided higher polymer Mn values and were more active for polymerization than yttrocenes 38a and 39/H2(g). 

If reaction had proceeded in the same manner as Ziegler-Natta polymerization of ethylene and substituted ethylenes (Section 29.6B), a 1,2-addition polymer would have been formed. What is formed, however, is an unsaturated polymer in which the number of double bonds in the polymer is the same as that in the monomers polymerized. This process is called ring-opening metathesis polymerization, or ROMP, after the olefin metathesis involving reaction of acyclic alkenes and nucleophilic carbene catalysts described in Section 24.6. 

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