Metallocene polyethylene constrained geometry catalysts

Constrained-geometry catalysts for C2H4 polymerization 88 that are counterparts of well-known ansa-metallocene systems have been prepared and shown to be active, in combination with MAO, toward polymerization of ethylene the product is almost entirely polyethylene, with ca. 1% of 1-octene obtained. The titanium complex was found to be four times as active as the zirconium species.1... 

These monocyclopentadienyl amidotitanium complexes, which are classified as constrained-geometry catalysts, are capable of producing low-density polyethylene (ethylene copolymers with C4, C(, or Cg 1-alkenes) that also contain long-chain branches, in contrast to strictly linear low-density polyethylene (ethylene copolymers with C4, C(, or Cg 1-alkenes) produced by bent metallocene-based catalysts [30,105,148,149]. 

In addition to the metallocenes described previously, so-called halfsandwich compounds or constrained-geometry catalysts (Fig. 3) such as dimethylsilyl-t-butylamido cyclopentadienyl titanium dichloride are used. These catalysts are excellent for producing polyethylenes with long-chain branching and can incorporate high amounts of comonomers such as 1-octene... 

Although ethylene/1-olefin copolymers were well documented in the late 1950s with the discovery of the chromium-based Phillips catalyst and the titanimn-based Ziegler catalyst, it was the discovery of metallocene-based single-site catalysts and the constrained geometry catalyst system that significantly increased the various types of new ethylene-based copolymers that are available for commercial applications. These new catalysts created new products, applications and markets for the polyethylene industry. 

The long-chain branched metallocene polyethylenes made with the constrained geometry catalysts described in Chapter 3 pose their own special problem, because the level of branching, 0.01 to 0.1 LCB per 1,000 carbon atoms, is too low to be detected by means of GPC-MALLS. For these materials the factor j8 in Eq. 2.104 has been established to be 0.5 by use of C-13 NMR [57], and this makes it possible to use GPC-LALLS-DV to determine the distribution of radii of gyration and the number of branches per 1000 carbon atoms. Then using the results... 

The first commercial process for making LLDPE was the Sclair technology developed by Dupont Canada and now implemented by NOVA Chemicals. This process involves high-temperature solution polymerization. Much LLDPE is now made in gas-phase reactors with butene or hexene as the co-monomer. The constrained-geometry catalyst (CGC) is a metallocene catalyst developed by Dow Chemical for the manufacture of linear, very-low density polyethylene resins by solution polymerization with octene as the comonomer. For a given co-monomer content, the solid-state density is lower for octene than for lower a-olefins. 

Figure 3.2 Algorithm for Monte Carlo simulations of branching distribution in branched metallocene polyethylenes made using a constrained-geometry catalyst. A large number of molecules are "created"according to rules based on the relative probabilities of propagation (pp) and monomer addition Ip). From Soares and Hamielec [871.

Figure9.21 Storage modulus versus frequency data [57] for a series of metallocene (constrained geometry catalyst) polyethylenes with progressively increasing levels of long-chain branching, as given in Table 9.1, compared to the predictions of the hierarchical model at 160 C.The parameter values used in the hierarchical model are M = 1167, G 5 = 2.0 10 Pa, Tg = 3.5 -10 s with a = 4/3.These values were obtained from fits of model predictions to rheological data for model hydrogenated 1,4-poly butadiene combs, except that was adjusted to account for a difference in temperature. From Park and Larson [59].

Wood-Adams, P. M. The effect of long-chain branching on the rheological behavior of polyethylenes synthesized using constrained geometry and metallocene catalysts. Doctoral Thesis (1998) McGill University... 

---