Branching metallocene polymerization

Long-chain branching (LCB), generally less than 0.1 branch per 1000 carbons, has been observed in some metallocene polymerizations of ethylene and propene [Nele and Soares, 2002 Soares, 2002 Weng et al., 2002]. The presence of even small amounts of LCB improves melt strength and melt processability of narrow PDI polymers. Thus, it is often useful to choose conditions, such as the metallocene, temperature, and other reaction conditions, that deliberately introduce long chain branching. 

Rg. 9.8. Mixed-metallocene polymerization of ethylene in a semibatch reactor branching (constrained geometry) catalyst CGC-Ti linear catalyst Et[lnd]2ZrCl2. Reactor and kinetic data initial concentration CGC-Ti 8 x 10 kmol m initial concentration Et[lnd]2ZrCl2 3.2 X 10 kmol m monomer molar feed... 

Fig. 9.9. Mixed-metallocene polymerization of ethylene in a CSTR. Kinetic data are the same as in Figure 9.8. Residence time CSTR 300 s feed concentrations identical to initial concentrations in Figure 9.8. Bivariate chain length/number of branches distribution. Hydrogen present. Based on molecular weight distribution and branching distribution from...

K parameter in tydization or parameterin semibatdi control A parameter in free volume H particle volumetric growth rate or parameter in metallocene polymerization with branching parameter in fiee-volume equation Pm monomer density Pp polymer density p branching density p 6, ) CTOSS-fink density distribution pa 0, ) additional cross-Unk density distribution pj 0, ) cyclization density Pcs,a(0/ ) additional secondary cyclization density Pcp(0) primary cyclization density Paui(0) instantaneous secondary cyclization density Pt(0) instantaneous cross-link density reaction radius of the reacting species r ratio of reaction rates as defined by eqn [56] number fraction of type-i radicals monomer volume fraction polymer volume fraction chain length r number fraction < > s chain length s number fraction 0 present conversion... 

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). 

A recent new discovery is the fact that the hydrolysis of branched /3-alkyl-substituted aluminoxanes are, in some cases, as effective as co-catalysts in olefin polymerization as MAO.63,64 For example, when combined with the the metallocenes, Cp 2ZrCl2, the hydrolysis products (Al/HzO = 2) of R3A1 (R = Bu and Oct) produced akylated ion pairs with high polymerization activities.65 The same combinations with Cp2ZrCl2 did not produce active catalysts, a result interpreted as due to the inhibition of /3-hydride elimination in the substituted metallocene derivatives. 

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]. 

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