Vanadium copolymers

The copolymers of ethylene and propylene (OCVP) are obtained by coordination catalysis using a derivative of vanadium and a derivative of an aluminum alkyl. Molar compositions of ethylene and propylene are usually on the order 45 and 55%. 

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). 

When mixed with Et2AlCl, the vanadium(III) complex (87) polymerizes propylene at —78 °C in a living manner.241,242 Poor initiator efficiency ( 4%) and low activities were improved by employing complex (88) activities of lOOgmmol h bar 1 were reported and the polymerization of propylene remained living (Mw/Mn= 1.2-1.4) up to 40 °C.243 244 The synthesis of end-functionalized PP and PP copolymers has also been achieved using these initiators. 

Ethylene-propylene (30-60 mole per cent) copolymers produce substances which are rubbery in nature. They are prepared by using Ziegler catalysts based on vanadium oxychloride/aluminium trihexyl by solution process at 40°C using chlorobenzene or pentane as a solvent. These can be vulcanised with peroxides. Ethylene-propylene-hexa 1, 4-diene terpolymers are rubbers which can be vulcanised with sulphur. 

The polymer chain end control model is supported by the observation that highly syndiotactic polypropene is obtained only at low temperatures (about —78°C). Syndiotacticity is significantly decreased by raising the temperature to —40°C [Boor, 1979]. The polymer is atactic when polymerization is carried out above 0°C. 13C NMR analysis of the stereoerrors and stereochemical sequence distributions (Table 8-3 and Sec. 8-16) also support the polymer chain end control model [Zambelli et al., 2001], Analysis of propene-ethylene copolymers of low ethylene content produced by vanadium initiators indicates that a syndiotactic block formed after an ethylene unit enters the polymer chain is just as likely to start with an S- placement as with an R-placement of the first propene unit in that block [Bovey et al., 1974 Zambelli et al., 1971, 1978, 1979]. Stereocontrol is not exerted by chiral sites as in isotactic placement, which favors only one type of placement (either S- or R-, depending on the chirality of the active site). Stereocontrol is exerted by the chain end. An ethylene terminal unit has no preference for either placement, since there are no differences in repulsive interactions. 

W. Wang and K. Nomura, Remarkable effects of aluminum cocatalyst and comonomer in ethylene copolymerizations catalyzed by (arylim-ido)(aryloxo) vanadium complexes efficient synthesis of high molecular weight ethylene/norbomene copolymer, Macromolecules, 38(14) 5905-5913, July 2005. 

Soluble catalysts denved from oiganoaluminum compounds and vanadium halides promote formation of atactic elastomeric propylene polymers and copolymers such as ethylene-propylenediene terpolynier rubbers (EPDM). Amorphous adhesive propylene homopolymers have some commercial use. Syndiotaclic or DLDL propylene polymers have been reported but their structures and properties have not been completely established. 

Block copolymers of propylene with ethylene have been produced in commercial polymerization processes using heterogeneous Ziegler-Natta catalysts. In all processes the block copolymers are produced in small concentrations, and the major products are homopolymers. Well-defined block copolymers free of homopolymer impurities can be prepared with catalysts exhibiting a living polymerization character. In this section we deal with the synthesis of well-defined block copolymers using the living polypropylene which has been prepared with soluble vanadium-based catalysts. 

The Ziegler-Natta catalysts have acquired practical importance particularly as heterogeneous systems, mostly owing to the commercial production of linear high- and low-density polyethylenes and isotactic polypropylene. Elastomers based on ethylene-propylene copolymers (with the use of vanadium-based catalysts) as well as 1,4-cz s-and 1,4-tran.y-poly(l, 3-butadiene) and polyisoprene are also produced. These catalysts are extremely versatile and can be used in many other polymerisations of various hydrocarbon monomers, leading very often to polymers of different stereoregularity. In 1963, both Ziegler and Natta were awarded the Nobel Prize in chemistry. 

With the sole exception of the random ethylene-propylene copolymers, for industrial applications heterogeneous catalysts have been used for alkene polymerisations. Ethylene-propylene statistical copolymerisation has been carried out using homogeneous vanadium-based catalysts [28]. 

Furthermore, studies of the microstructure of copolymers formed by the low-temperature copolymerisation of cis-1 -(2 H)-propene (or trans isomer) and perdeuteropropene in the presence of soluble vanadium-based Ziegler-Natta catalysts showed syndiospecific propagation to involve a monomer insertion of the cis type [27]. 

Let us recall also that vanadium-based soluble Ziegier-Natta catalysts have found widespread industrial application for the manufacture of elastomeric ethylene/propylene copolymers and ethylene/propylene/diene terpolymers [319-322]. The most commonly used vanadium-based catalysts for random ethylene/propylene copolymerisation are those prepared from VCI4, VOCI3, V(Acac)3, VO(OEt)Cl2, VO(OEt)2Cl or VO(OEt)3 as precursors and AlEt3, AlEt2Cl or Al(z-Bu)2Cl as activators, with an Al/V molar ratio not exceeding 3 1 [37, 72],... 

The same conclusion as in the case of propylene homopolymerisation has been drawn considering IR [396] and NMR [389,395] spectra of ethylene/propylene copolymers obtained with vanadium-based syndiospecific catalysts. The type of propylene insertion depends on the kind of last inserted monomer unit secondary insertion [scheme (40)] occurs more frequently when the last monomeric unit of the growing chain is propylene, while primary propylene insertion [scheme (39)] is more frequent when the last monomeric unit of the growing chain is ethylene [2]. The above explains the microstructure of ethylene/propylene copolymers obtained with vanadium-based Ziegler-Natta catalysts. These copolymers contain both m and r diads when the sequence of propylene units is interrupted by isolated ethylene units i.e. a propylene insertion after an ethylene insertion is substantially non-stereospecific [327,390,397], The existence of a steric interaction between the incoming monomer molecule and the last added monomer unit is also confirmed by the fact that the propagation rate for the secondary insertion of propylene in syndiospecific polymerisation is lower than for primary insertion in non-stereospecific polymerisation [398],... 

Of particular interest are random copolymers of ethylene and propylene, which are obtainable with soluble vanadium-based Ziegler-Natta catalysts such as V(Acac)3—AlEt2Cl [453], VOCl3-AlEt2Cl [72], VOC13-A1(/-Bu)2C1... 

Alternating copolymers have been obtained by copolymerisation of ethylene and cycloolefins (using a large excess of cycloolefin) in the presence of vanadium-based Ziegler-Natta catalysts such as V(Acac)3-AlEt2Cl and VC14 AlEt2Cl ... 

For instance, in the field of elastomers, alkyllithium catalyst systems are used commercially for producing butadiene homopolymers and copolymers and, to a somewhat lesser extent, polyisoprene. Another class of important, industrial polymerization systems consists of those catalyzed by alkylaluminum compounds and various compounds of transition metals used as cocatalysts. The symposium papers reported several variations of these polymerization systems in which cocatalysts are titanium halides for isoprene or propylene and cobalt salts for butadiene. The stereospecificity and mechanism of polymerization with these monomers were compared using the above cocatalysts as well as vanadium trichloride. Also included is the application of Ziegler-Natta catalysts to the rather novel polymerization of 1,3-pentadiene to polymeric cis-1,4 stereoisomers which have potential interest as elastomers. 

Of great industrial interest are the copolymers of ethene and propene with a molar ratio of 1/0.5, up to 1/2. These EP-polymers show elastic properties and, together with 2-5 wt% of dienes as third monomers, they are used as elastomers (EPDM). Since they have no double bonds in the backbone of the polymer, they are less sensitive to oxidation reactions. As dienes, ethylidenenorbomene, 1,4-hexadiene, and dicyclopentadiene are used. In most technical processes for the production of EP and EPDM rubber in the past, soluble or highly disposed vanadium components are used [69]. Similar elastomers can be obtained with metallocene/MAO catalysts by a much higher activity which are less colored [70-72]. The regiospecificity of the metallocene catalysts toward propene leads exclusively to the formation of head-to-tail enchainments. The ethylidenenor-bornene polymerizes via vinyl polymerization of the cyclic double bond and the tendency to branching is low. The molecular weight distribution of about 2 is narrow [73]. 

Vanadium-containing coordination centres producing syndiotactic polypropylene at 195 K can be transformed to radical centres simply by raising the temperature to 298 K [252]. In this way, Japanese authors have prepared the copolymer poly(propene)-Wocfc-poly(methylmethacrylate). The radical end is probably formed by homolytic splitting of the C—V bond, and it can be stabilized by the V ion. The authors state that, in this way, two-component blocks of polypropylene with various polymers propagating by the radical mechanism can be prepared. 

Maleic anhydride (MA) is an important intermediate in the production of chemicals such as unsaturated polyester resins, lube oil additives, maleic copolymers and others [124]. Currently, MA is predominantly produced by the oxidation of butane, and all industrial catalysts for this reachon are based on vanadium phos-... 

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