Copolymerization Ziegler-Natta

Random copolymers of ethylene and a-olefins (l-aUcenes) can be obtained with Ziegler-Natta catalysts, the most important being those of ethylene and 1-butene (LLDPE) and of ethylene and propylene (EPM or EPR and EPDM). Some reactivity ratios are listed in Table 9.5. Tha ratios vary with the nature and physical state of the catalyst and in most instances, r r2 is close to unity. However, all these values show that ethylene is much more reactive than higher alkenes. Copolymers produced using Ziegler-Natta catalysts usually have a wide range of compositions. This may be due to the presence of different active sites in the catalyst giving rise 

More homogeneous copolymer cpmpositions are obtained with soluble Zieg-ler-Natta catalysts, especially where monomer compositions are maintained relatively constant during polymerization. Commercially important ethylene-propylene binary copolymers (EPM rubbers) and ethylene-propylene-diene ternary copolymers (EPDM rubbers) are made by this process. 

Ziegler-Natta catalysts have been used to prepare a number of block copolymers, though in most cases these are mixed with significant amounts of homopolymers. The Ziegler-Natta method is clearly inferior to anionic polymerization for preparing block copolymers of controlled compositions. Nevertheless, block copolymers of ethylene and propylene (Polyallomers), which are high-impact plastics exhibiting crystallinity characteristics of both isotactic polypropylene and linear polyethylene, have been made by this process as a commercial product. 

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The dihydrido complex trara,s-(silox)3Ta(H)2 ((198) Scheme 44) was prepared by reduction of the equivalent chloro species under H2.65,451 It underwent reversible cyclometalation and reacted with Mel and C2H4 to yield monohydrido complexes ((199)-(201) Scheme 44). This chemistry was extended as a model of Ziegler-Natta copolymerizations. 

Table 9.7 Representative Reactivity Ratios (r) in Ziegler-Natta Copolymerization...

Only a few papers describe the polymerization of unsaturated monomers with a covalent M-O bond. Ziegler-Natta copolymerization of the diisobutylaluminium-alkoxy-isopren derivative 35 with butadiene occurs by a neodynium catalyst in a hydrocarbon solvent [180]. Mainly the monomer 35 in, A-cis configuration is found in the copolymer. A chiral monomer based on ethyleneglycolmonomethacrylat being substituted by alkoxy derivatives of Ti(IV) and different chiralic substituents was polymerized [181]. Such polymers are interesting as chiralic catalysts. 

Chemical Properties. Higher a-olefins are exceedingly reactive because their double bond provides the reactive site for catalytic activation as well as numerous radical and ionic reactions. These olefins also participate in additional reactions, such as oxidations, hydrogenation, double-bond isomerization, complex formation with transition-metal derivatives, polymerization, and copolymerization with other olefins in the presence of Ziegler-Natta, metallocene, and cationic catalysts. All olefins readily form peroxides by exposure to air. 

Keli, T., Doi, Y. Synthesis of Living Polyolefins with Soluble Ziegler-Natta Catalysts and Application to Block Copolymerization. Vol. 73/74, pp. 201 —248. 

The sterically unencumbered catalyst active site allows the copolymerization of a wide variety of olefins with ethylene. Conventional heterogeneous Ziegler/Natta catalysts as well as most metallocene catalysts are much more reactive to ethylene than higher olefins. With constrained geometry catalysts, a-olefins such as propylene, butene, hexene, and octene are readily incorporated in large amounts. The kinetic reactivity ratio, rl, is approximately... 

Several copolymers of or-olelins are used as drag reducers. Suggested recipes are summarized in Table 12-1. Linear low-density polyethylene is a copolymer of ethylene and a-olefins. It is obtained by copolymerization utilizing Ziegler-Natta catalysts or metallocene catalysts. Concentrates may be prepared by... 

Ziegler-Natta polymerization leads to linear unbranched polyethylene, the so-called high density polyethylene (HDPE), which is denser, tougher and more crystalline. By copolymerization with other alkenes it is possible to obtain linear low density polyethylene (LEDPE) with better mechanical properties than LDPE. Blends of LLDPE and LDPE are used to combine the good final mechanical properties of LLDPE and the strength of LDPE in the molten state. 

Ziegler-Natta (or Natta-Ziegler) catalyst (ZNC) Able to produce stereoregular pol5uners. zwitterionic polymerization Copolymerization between nucleophilic and electrophilic comonomers. 

Pasquon, I., A. Valvassori, and G. Sartori, The Copolymerization of Olefins by Ziegler-Natta Catalysts, Chap. 4 in Stereochemistry of Macromolecules, Vol. 1, A. D. Ketley, ed., Marcel Dekker, New York, 1967. 

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. 

Propylene and 4-methyl- 1-pentene were copolymerized by Colin et al. (1) using a Ziegler-Natta catalyst and the product characterized as having at least one fraction obtained by that had a block index greater than about 0.3 and up to about 1.0 with a polydispersity greater than 1.3. 

The class of monocyclopentadienylamido (CpA) titanium complexes has attracted the interest for the polymerization of a-olefins with bulky side groups. This arises since conventional Ziegler-Natta catalysts are less effective in starting the copolymerization of ethene with 4-methyl-l-pentene. Homogeneous catalysts of the zirconium cyclopentadienyl type (Cp2M) with methylaluminoxane exhibit a low catalytic activity. 

All higher a-olefins, in the presence of Ziegler-Natta catalysts, can easily copolymerize both with other a-olefins and with ethylene. In these reactions, higher a-olcfins arc all less reactive than ethylene and propylene. 

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