Ziegler catalysts, copolymers from

The second type of solution polymerization concept uses mixtures of supercritical ethylene and molten PE as the medium for ethylene polymerization. Some reactors previously used for free-radical ethylene polymerization in supercritical ethylene at high pressure (see Olefin POLYMERS,LOW DENSITY polyethylene) were converted for the catalytic synthesis of LLDPE. Both stirred and tubular autoclaves operating at 30—200 MPa (4,500—30,000 psig) and 170—350°C can also be used for this purpose. Residence times in these reactors are short, from 1 to 5 minutes. Three types of catalysts are used in these processes. The first type includes pseudo-homogeneous Ziegler catalysts. In this case, all catalyst components are introduced into a reactor as hquids or solutions but form soHd catalysts when combined in the reactor. Examples of such catalysts include titanium tetrachloride as well as its mixtures with vanadium oxytrichloride and a trialkyl aluminum compound (53,54). The second type of catalysts are soHd Ziegler catalysts (55). Both of these catalysts produce compositionaHy nonuniform LLDPE resins. Exxon Chemical Company uses a third type of catalysts, metallocene catalysts, in a similar solution process to produce uniformly branched ethylene copolymers with 1-butene and 1-hexene called Exact resins (56). 

This deficiency in the Ziegler catalyst to produce block copolymers and the abilities of anionic initiators to produce it kept the interest in anionic initiators active in many industrial laboratories. This interest in anionic research in these laboratories paid off handsomely in the areas of block and random copolymers. In this review major emphasis will be focused on the major products from both homo and block copolymers currently being manufactured by anionic technique and future trends in this area. HOMOPOLYMERIZATION... 

IV to VIII metals and base metal alkyls of Group II or III metals (Penczek and Premia, 2012 Boor, 1979 Ciardelli, 1992). It arose from the spectacular discovery of Ziegler et al. (1955) that mixtures of titanium tetrachloride and aluminum alkyls polymerize ethylene at low pressures and temperatures and from the equally spectacular discovery by Natta (1955) that the Ziegler catalysts can stereospecifically polymerize monoolefins to produce tactic, crystalline polymers. As can be imagined, these systems can involve many combinations of catalyst components, not all of which are catalytically active or stereospecific. However, we shall be concerned here only with polymerizations involving the commercial elastomers, principally polyisoprene, polybutadiene (Duck and Locke, 1977 Zohuri et al., 2012 Teyssie et al., 1988), and the ethylene-propylene copolymers (Schobel et al., 2012 Ver Strate, 1986 Davis et al., 1996 Noordermeer, 2003 Baldwin and Strate, 1972). 

For reviews on NMR analysis of EP copolymers see, for example (a) Bovey, F. A. Mirau, P. A. NMR of Polymers. Academic Press San Diego, 1996. (b) Randall, J. C. A review of high-resolution liquid carbon-13 nuclear magnetic resonance characterizations of ethylene-based polymers. 7. Mac-romol. Set, Rev. Macromol. Chem. Phys. 1989, C29, 201-317. (c) Randall, J. C. Polymer Sequence Determination. Academic Press New York, 1977. (d) For NMR analysis on EP copolymers from metallocenes see, for example Tritto, I. Fan, Z. Q. Locatelli, P Sacchi, M. C. Camurati, I. Galimberti, M. NMR studies of ethylene-propylene copolymers prepared with homogeneous metallocene-based Ziegler-Natta catalysts. Macromolecules 1995, 28, 3342-3350. 

Two methods for preparing the calibration curve have been reported. Both methods were done by performing Crystaf analysis in a series of narrow-CCD copolymer samples with known comonomer contents with crystallizabilities covering a broad range of crystallization temperatures. The only difference between these two methods is the type of samples used in the calibration. The first method uses a series of polymer samples synthesized with single-site-type catalysts [58,68], while the second method uses a series of fractions from broad-CCD Ziegler-Natta copolymers obtained with P-Tref [1,49]. After the whole series of samples has been analyzed, the relationship between Crystaf peak temperature and CC is used as the cahbration curve. 

Modifications to the Ziegler-Natta catalyst system have led to the preparation of alternating copolymers from olefins, such as ethylene and propylene, and diolefins, such as butadiene and isoprene. In typical systems (Furukawa 1972, 1974a, b) the catalyst is prepared at very low temperatures (-70°C or below) from three components a... 

These data explain the relatively heterogeneous branching distribution of ethylene/1-olefin copolymers prepared with Ti/Mg Ziegler catalysts. For example, the catalyst prepared without silica contained at least five active sites that exhibited a range of r values from 19 to -180, which varies by a... 

Block copolymers from cyclooolelins have been prepared by various experimental techniques [52]. Some interesting methods use living ROMP catalysts, which allow ready synthesis of new products having controllable structures and properties. Other methods apply cross-metathesis between unsaturated polymers and/or polyalkenamers [3], polymerization of cycloolelins in the presence of unsaturated polymers [4], polymerization of two or more cycloolelins of quite different reactivities with classical ROMP catalysts [4], and copolymerization of cycloolefins with other monomers, effected by changing the polymerization mechanism from ROMP to anionic, cationic, Ziegler Natta, and group transfer, and vice versa [6-8, 52]. 

The weight percent propylene in ethylene-propylene copolymers for different Ziegler-Natta catalysts was measuredt for the initial polymer produced from identical feedstocks. The following results were obtained ... 

Figure 19 (a) Peak melting temperature as a function of the branch content in ethylene-octene copolymers (labelled -O, and symbol —B (symbol, ) and -P (symbol, A) are for ethylene-butene and ethylene-propylene copolymers, respectively) and obtained from homogeneous metallocene catalysts show a linear profile, (b) Ziegler-Natta ethylene-octene copolymers do not show a linear relationship between peak melting point and branch content [125]. Reproduced from Kim and Phillips [125]. Reprinted with permission of John Wiley Sons, Inc. 

Buna [Butadien natrium] The name has been used for the product, the process, and the company VEB Chemische Werke Buna. A process for making a range of synthetic rubbers from butadiene, developed by IG Farbenindustrie in Leverkusen, Germany, in the late 1920s. Sodium was used initially as the polymerization catalyst, hence the name. Buna S was a copolymer of butadiene with styrene Buna N a copolymer with acrylonitrile. The product was first introduced to the pubhc at the Berlin Motor Show in 1936. Today, the trade name Buna CB is used for a polybutadiene rubber made by Bunawerke Hiils using a Ziegler-Natta type process. German Patent 570, 980. 

It is important to note that the tendency of a monomer towards polymerization and therefore also towards copolymerization is strongly dependent on the nature of the growing chain end. In radical copolymerization the composition of the copolymer obtained from its given monomer feed is independent of the initiating system for a particular monomer pair, but for anionic or cationic initiation this is normally not the case. One sometimes observes quite different compositions of copolymer depending on the nature of the initiator and especially on the type of counterion. A dependence of the behavior of the copolymerization on the used catalyst is often observed with Ziegler-Natta or metallocene catalysts. 

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