Ethylene polymerization branching

EinaHy, in 1976, Kaminsky and Sinn in Germany discovered a new family of catalysts for ethylene polymerization. These catalysts (ie, Kaminsky catalysts) contain two components a metallocene complex, usually a zkconocene, and an organoaluminum compound, methylaluminoxane (8,9). These catalysts and thek various later modifications enable the synthesis of ethylene copolymers with a high degree of branching uniformity. Formally classified as MDPE, LLDPE, or VLDPE, the resins thus produced have a number of properties that set them apart from common PE resins in terms of performance... 

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). 

Most small olefins produced in the chemical industry are used to make polymers, with a global production of the order of 100 million tons per year. Polymers are macromolecules with molecular weights of typically lO" to 10 and consist of linear or branched chains, or networks built up from small monomers such as ethylene, propylene, vinyl chloride, styrene, etc. The vast majority of polymers are made in catalytic processes. Here we concentrate on ethylene polymerization over chromium catalysts as an example [M.P. McDaniel, Adv. Catal. 33 (1985) 47]. 

When less bulky ancillary ligands are used /3-hydride elimination leads to the formation of Q-olefins. As a consequence iminopyridine complexes are typically much less active than the diimine catalysts and afford lower-molecular-weight PE.321-324 For example, MAO/(122) polymerizes ethylene to branched oligomers with Mn < 600, and 240 branches per 1,000 carbons.325 Complex (123), is highly active for ethylene polymerization (820gmmol 1 h bar ).326 As with the diimine systems, reduction in the steric bulk of the ligand substituents results in reduced activity and lower-molecular-weight products. 

PE is obtained by anionic polymerization from hydrogenating a precursor 1,4-PB polymer. Since 1,4-PB contains a small fraction of 1,2-nnits the PE stndied here has two ethylene side branches per 100 backbone carbons. alt-PEP Essentially alternating poly(ethylene co-l-butene). 

It might be expected that short-chain branches would be formed in the polymerization of vinyl acetate by a cyclic mechanism like that proposed by Roedel (6) for ethylene polymerization, and this possibility has been mentioned by several authors. There appears to be no evidence that such short-chain branches occur if they are absent this is perhaps attributable to steric repulsion between the substituents that might prevent the formation of the cyclic transition state required for the back-biting reaction. 

We will start by discussing the calculations on ethylene polymerization. In this case, the issues are mostly polymer length and the formation of linear or branched polymers. The first DFT/MM applications on this topic were published near simultaneously in 1997 and 1998 by the groups of Ziegler [36] and Morokuma [37, 38] on the reaction of ethylene polymerization catalyzed by cationic diimine Ni(II) complexes of the type (ArN=C(R)-C(R)=NAr)-Ni-CFl3+. Shortly before this, these species (one of them is... 

Complex 17a displayed moderate catalytic activity toward the polymerization of ethylene (3.3 x 105 g/molh 1). In addition, higher molecular weight distributions were observed (Mw/Mn = 12.8). The 13C NMR analysis of the polyethylene showed that methyl branches predominate (with ca. 3.4 methyl branches per 1000 carbon atoms), suggesting that chain walking does not affect polymerization to a high degree. When only the pyridine moiety (and not the imidazolium salt) is ligated (17b) [48], ethylene polymerization occurs twice as effectively (6 x 105 g PE/(mol of Ni) h 1) under similar conditions (only 30 min rather than 60 min). 

This second reaction leads to the small amount of branching (usually less than 5%) observed in the alcohol product. The alpha olefins produced by the first reaction represent a loss unless recovered (8). Additionally, ethylene polymerization during chain growth creates significant fouling problems which must be addressed in the design and operation of commercial production facilities (9). 

In a similar way, n-butyl acrylate was copolymerized by ATRP with methacrylate macromonomers containing highly branched polyethylene prepared by Pd-catalyzed living ethylene polymerization. The observed reactivity ratios depend on the molecular weight and concentration of the macromonomer. The resulting graft copolymers showed microphase separation by AFM [304]. 

Butene and other secondary olefins do not readily copolymerize with ethylene, and they may even tend to inhibit ethylene polymerization. This observation probably indicates steric crowding. For example, secondary olefins might be able to coordinate to the catalytic site, but not insert in the polymer chain. Alternatively, they may insert but give chains that are resistant to further insertion of ethylene. Similarly, a-olefins with a branch near the double bond, such as isobutylene or a-olefins with branches in the third position, also react poorly [403]. Examples of the differences in reactivity of the various a-olefins are shown in Table 8. 

An example of this behavior is shown in Table 11. In the reported experiments, a typical Cr/silica catalyst was tested for ethylene polymerization with small amounts of butene added to the reactor. Three different butene isomers were used in three series of experiments 1-butene, 2-butenes (cis and trans), and isobutylene. In the first series, as 1-butene was added to the reactor, the density of the polymer declined significantly, indicating the presence of ethyl branches on the chains from the incorporation of the comonomer (branching disrupts crystallinity and creates more amorphous polymer, which lowers the average density). The MI values of the polymers in this series went up as 1-butene was added, as would be expected from the greater ease with which a (3-hydride can be abstracted from the tertiary carbon resulting from 1-butene incorporation. This is the behavior typical of all a-olefin comonomers. 

An example illustrating this behavior is shown in Table 62 Cr(VI) and Cr(II) catalysts, otherwise identical in composition, were tested for ethylene polymerization in the absence and presence of BEt3 cocatalyst. The second column in the table shows the degree of branching found in the resultant polymer, and the third column shows the density of that polymer. The response by Cr(VI) to the cocatalyst was only slight, but the response by Cr(II) was remarkable. With the addition of BEt3, the density of the polymer dropped so severely that the product type changed from the class of HDPE (homopolymer) to LLDPE. 

However, H2 need not be added directly to the reactor to influence the catalyst performance. In several experiments, a Cr/silica catalyst was activated at 800 °C, followed by reduction with CO at 350 °C to yield Cr(II). The reduced catalyst was then treated with 2.2 MPa (300 psig) of H2 at 105 °C for 1 h, and the H2 was evacuated. When this catalyst was then tested for ethylene polymerization in a reactor containing no H2, it still produced a small amount of a-olefins as well as polymer in normal yield. The density of the polymer dropped from 0.968 g mL 1 before H2 treatment of the catalyst, to 0.958 g mL-1 afterward. Therefore H2 modifies some Cr(II) sites in a permanent way, and does not need to be in the reactor during polymerization to create a small amount of in situ branching in the polymer. 

V(L)Cl2(TpMs )] (L = N Bu L = O) were in situ supported onto SiC>2 and onto MAO and trimethylaluminum. All catalyst systems were shown to be active in ethylene polymerization. The systems were stable at different [A1]/[V] molar ratios and polymerization temperatures.21 Branched polyethylene/high-density polyethylene blends were prepared using the combined [NiChfa-diimine)] and V(T(Tp-vls )(N Bu) catalysts. The polymerization reactions were performed in hexane or toluene at three different polymerization temperatures (CPC, 30°C and 50°C) and several nickel molar fractions, using MAO as cocatalyst.22 TpMs- and TpMs -imido vanadium (V) were immobilized onto a series of inorganic supports All the systems were shown to be active in ethylene polymerization in the presence of MAO or TiBA/MAO mixture.23... 

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