Ziegler-Natta vanadium-based

Metallocene catalysis is an alternative to the traditional Ziegler-Natta vanadium-based catalysis for commercial polyolefin production, e.g. the use of metallocene-catalyzed ethylene alpha-olefin copolymers as viscosity index modifiers for lubricating oil compositions [23]. The catalyst is an activated metallocene transition metal, usually Ti, Zr or Hf, attached to one or two cyclopentadienyl rings and typically activated by methylaluminoxane. Metallocene catalysis achieves more stereo-regularity and also enables incorporation of higher alpha-olefins and/or other monomers into the polymer backbone. In addition, the low catalyst concentration does not require a cleanup step to remove ash. 

Solution processes use autoclave, tubular, or loop reactors. As compared to slurry and gas-phase polymerization, solution processes are commonly operated at a much higher temperature to keep the polymer dissolved in the reaction medium, and at much lower average residence times (5-20 min, as opposed to 1-4 h). Since polymerization conditions are more uniform in solutions reactors - there are no inter- and intraparticle heat- and mass-transfer resistances, for instance - this configuration is commonly used for the production of EPDM rubbers with soluble Ziegler-Natta vanadium-based catalysts. Composition homogeneity is a require-... 

Although low-temperature syndiospecific polymerisation of propylene with soluble Ziegler-Natta catalysts, based on soluble vanadium compounds and dialkylaluminium chlorides as activators, was first carried out successfully as... 

Cycloolefins having rings with more than four carbon atoms do not homo-polymerise in the presence of Ziegler-Natta catalysts based on titanium or vanadium compounds as precursors and alkylaluminium activators. However, these cycloolefins may copolymerise with ethylene via the double bonds while preserving the cycloolefin ring ethylene is able to compensate the steric hindrance at the Ca atom of the growing chain after and before the 1,2-insertion of the cycloolefin [2],... 

Four different stereoisomers are possible for polymer XLII, poly (cyclobutane-1,2-diyl) (Sec. 8-lf). Cis and trans isomers are possible for pol3mier XLin, poly (but-1-ene-1,4-diyl). (XLni is the same polymer obtained by the l,4-pol3fmerization of 1,3-butadiene— Sec. 8.10). Traditional Ziegler-Natta initiators based on vanadium and metallocene initiators yield polymerizations almost exclusively through the double bond. Titanium, tungsten, and ruthenium initiators yield predominantly ROMP with varying amounts of cis and trans placements. 

Table 7). In an early application, VF was polymerized with a catalyst composed of diethylalnminnm bromide-titanium tetrachloride-carbon tetrachloride (82). A Ziegler-Natta system based on vanadyl acetylacetonate and AIR(OR)Cl compounds gives good yields (82) at polymerization temperatures up to 50°C. A modified catalyst prepared from vanadium oxytrichloride, triisobutylaluminum, and tetrahydrofnran was foimd to be the most effective Ziegler-Natta system... 

The copolymerizations between monoolefins and dienes have been considered to be of practical and theoretical importance. As reported in the literatures ethylene-butadiene and propylene-butadiene copolymers can be prepared with conventional Ziegler-Natta titanium-based or vanadium-based catalysts. The copolymer composition and monomer sequence distribution strongly depend on the catalyst system and polymerization conditions. Alternating copolymers were synthesized when the catalyst components were mixed at the... 

Polymerization of bicyclo[3.2.0]hepta-2,6-diene has been carried out with Ziegler Natta catalysts based on titanium and vanadium salts [140]. Structure investigation showed that titanium catalysts gave mainly ring-opened polymers [Eq. (75)]... 

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. 

The Mw/Mn ratio is usually equal to 5-10 for polyethylene [49,64,66,67, 123,244-247], A much lower polydispersity is displayed by polymers obtained in polymerisation with homogeneous metallocene catalysts the Mw/Mn ratio usually does not significantly exceed a value of 2 [22,95,101,112,138,140], By polymerising propylene with soluble vanadium-based Ziegler-Natta catalysts at low temperature, a very narrow molecular weight distribution of the polypropylene has been found (the Mw/Mn ratio usually reaches values of 1.15-1.25) and a linear increase in its Mn with time has been observed, indicating a noticeable living character of the polymerisation [75,76,241],... 

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]. 

Syndiospecific Propagation Reaction Stereocontrol in the Presence of Soluble Vanadium-based Ziegler-Natta Catalysts... 

The syndiospecific polymerisation of propylene with soluble vanadium-based Ziegler Natta catalysts is not completely regiospecific [389 392], i.e. the monomer unit enchainment is not entirely head-to-tail. In addition to syndiotactic stereoblocks, the polymer also contains sterically irregular stereoblocks. The whole polymerisation can be thus described as a copolymerisation with four head-to-tail and tail-to-tail stages [2,379]. 

In view of the data concerning propylene polymerisation in the presence of homogeneous vanadium-based Ziegler-Natta catalysts, the syndiospecificity of the polymerisation is believed [387,395] to arise from steric repulsions between the last inserted monomer unit of the growing chain and the methyl group of coordinated propylene molecule, i.e. chain end stereocontrol is postulated to play the essential role in the stereoregulation. 

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 ... 

Czaja, K. and Bialek, M., Vanadium-based Ziegler-Natta Catalyst Supported on MgCl2(THF)2 for Ethylene Polymerisation , Macromol. Rapid Commun., 17, 253-260 (1996). 

The understanding of direct-initation mechanisms. In this context, tire use of metal salts of strong acids has opened a new area of fundamental studies. Indeed, we believe that the chemical insertion of the olefin between the metal cation and the anion following its adsorption on the salt surface closely resembles recent mechanistic proposals put forward by Zambelli and collaborators to rationalise initiation in stereospecific polymerisation (propene with vanadium-based catalysts). We are inclined to conclude that the cationic nature of stereospecific polymerisations of the Ziegler-Natta type is supported by our findings , viz. electrophilic attack by... 

In addition to titanium-based Ziegler-Natta catalysts, vanadium-based systems have also been developed for PE and ethylene-based co-polymers, particularly ethylene-propylene-diene rubbers (EPDM). Homogeneous (soluble) vanadium catalysts produce relatively narrow molecular mass distribution PE, whereas supported V catalysts give broad molecular mass distribution.422 Polymerization activity is strongly enhanced by the use of a halogenated hydrocarbon as promoter in combination with a vanadium catalyst and aluminum alkyl co-catalyst.422,423... 

Among known Ziegler-Natta catalytic systems catalysts on the base of V and Ti compounds combination with chloroaluminumalkyles are effective for ethylene and propylene copolymerization [176, 177]. It is particularly convenient to use systems on the base of vanadium compounds (tetrachloride, trichloroxide, triacetylacetonate) and diisobutylaluminum chloride. 

The 1,2-disubstituted olefmic monomers will usually not homopolymerize with the Ziegler-Natta catalysts. They can, however, be copolymerized with ethylene and some a-olefins. Due to poorer reactivity, the monomer feed must consist of higher ratios of the 1,2-disubstituted olefins than of the other comonomers. Copolymers of cw-2-butene with ethylene, where portions of the macromolecules are crystalline, form with vanadium-based catalysts. The products have alternating structures, with the pendant methyl groups in erythrodiisotactic arrangements. Similarly, vanadium-based catalysts yield alternating copolymers of ethylene and butadiene, where the butadiene placement is predominantly rm/w-1,4. ... 

Many ot-olefins were polymerized by the Ziegler-Natta catalysts to yield high polymers and many such polymers were found to be stereospecific and crystalline. Polymerizations of a-olefins of the general structure of CH2 = CH — (CH2) — R, where x is 0-3 and R denotes CH3, CH-(CH3)2, C(CH3)3, or CsHs, can be catalyzed by vanadium trichloride/triethyl aluminum [80]. The conversions are fairly high, though higher crystallinity can be obtained with titanium-based catalysts [81]. Addition of Lewis bases, such as ( 4119)20, (C4H9)3N, or ( 4119)3 , to the catalyst system further increases crystallinity [82]. 

---