Metallocene-methylaluminoxane

Sivaram S, Srinivasa RS (1995) Homogeneous metallocene-methylaluminoxane catalyst systems for ethylene polymerization. Prog Polym Sci 20 309-367... 

Arndt, M. and Kaminsky, W., Microstructure of Poly(cycloolefin)s Produced by Metallocene/Methylaluminoxane (mao) Catalysts . Macromol. Symp., 97, 225-246 (1995). 

The range of polypropylene microstructures available by procatalyst ligand (and/or metal) variation in the metallocene-methylaluminoxane system is illustrated in Table 3.1 [22,23,101,105,107,112,113,124,127,132,137], The ligands shown in Table 3.1, representative of the particular symmetry and class of the catalyst, are given as examples only. There are a variety of other metallocenes that have been successfully used to obtain polypropylenes of various stereostructures. 

D-Limonene and ot-pinene have been used as renewable solvents and chain transfer agents in metallocene-methylaluminoxane (MAO) catalysed polymerization of ot-olefins. Chain transfer from the catalyst to the solvent reduces the achieved in limonene compared with toluene and also reduces the overall catalyst activity. This was confirmed, as in the ROMP studies, by performing identical reactions in hydrogenated limonene. However, an increase in stereospecificity was seen when D-limonene was used as the solvent. This is measured as the mole fraction of [mmmm] pentads seen in NMR spectra of the polymer. 100% isotactic polypropylene would give a value of 1.0. On performing the same propylene polymerization reactions in toluene and then in limonene, the mole fraction of [mmmm] pentads increased from 0.86 to 0.94, indicating that using a chiral solvent influences the outcome of stereospecific polymerizations. Unfortunately, when a-pinene was used, some poly(a-pinene) was found to form and this contaminates the main polymer product. 

Table 2. Ethylene polymerization with metallocene/methylaluminoxane catalysts.

Figure 2. Dependence of the weight average molecular weight and the density ofmetallocene polyethylene obtained in zirconium based metallocene (methylaluminoxane as co-catalyst) catalyzed polymerization of ethylene at 1500 bar at different temperatures in the range from 80 to 260°C. [Adapted from ref. 11]...

Koppl, A. Babel, A. L Alt, H. G. Homopolymerization of ethylene and copolymerization of ethylene and 1 -hexene with bridged metallocene/methylaluminoxane catalysts The influence of the bridging moiety. J. Mol. Catal. A Chem. 2000,153,109-119. 

Fischer, D. Miilhaupt, R. The influence of regio- and stereoirregularities on the crystallization behavior of isotactic poly(propylene)s prepared with homogeneous group IVa metallocene/methylaluminoxane Ziegler-Natta catalysts. MacromoZ. Chem. Phys. 1994,195, 1433-1441. 

When propylene is polymerized using homogeneous metallocene/methylaluminoxane (MAO) catalysts, several chain transfer mechanisms occur to release free polymer chains. Their relative frequencies are dependent on the polymerization conditions and the catalyst structure. Three chain transfer mechanisms are identified that form chain-end unsaturated polypropylene. These are (1) P-hydride transfer to metal after a primary (1,2-) propylene insertion, (2) f)-hydride transfer to monomer after a primary propylene insertion, and (3) f)-hydride transfer after a secondary (2,1-) propylene insertion. The formations of these chain ends and associated polymer head groups (chain starts) are shown in Scheme 10.1. Mechanism (2) is commonly referred to as chain transfer to monomer. 

Chien, J. C. W. Wang, B. P. Metallocene-methylaluminoxane catalysts for olefin polymerization. I. Tiimethylalnminnm as coactivator. J. Polym. Sci., Part A Polym. Chem. 1988, 26, 3089-3102. 

Yamaguchi, Y Suzuki, N. Fries, A. Mise, T. Koshino, H. Ikegami, Y Ohmori, H. Matsumoto, A. Stereospecific polymerization of 1 -hexene catalyzed by an a-metallocene/methylaluminoxane systems under high pressures. J. Polym. Sci., Part A Polym Chem. 1999, 37, 283-292. 

Ewen, J. A. Mechanisms of stereochemical control in propylene polymerization with soluble group 4B metallocene/methylaluminoxane catalysts. J. Am. Chem. Soc. 1984, 106, 6355-6364. 

Metallocene/methylaluminoxane (MAO) and other single site catalysts for olefin polymerization have opened a new field of synlhesis in polymer chemistry. Strained cyclic olefins such as cyclobutene, cyclopentene, norbornene (NB), and their substituted compounds can be used as monomers and comonomers in a wide variety of polymers." Much interest is focused on norbornene homo- and copolymers because of the easy availability of norbornene and the special properties of their polymers. Norbornene can be polymerized by ring opening metathesis polymerization (ROMP), giving elastomeric materials, or by double bond opening (addition polymerization). Homopolymerization of norbornene by double bond opening can be achieved by early and late transition metal catalysts, namely Ti, Zr, Hf, Ni, - ° and Pd (Scheme 16.1). 

Metallocene/methylaluminoxane (MAO) catalysts can be used to polymerize and copolymerize strained cyclic olefins such as cyclobutene, cyclopentene, norbornene, DMON and other sterically hindered olefins [205-210]. While polymerization of cyclic olefins by Ziegler-Natta catalysts is accompanied by ring opening [10], homogeneous metallocene [211], nickel [212,213], or palladium [214,215], catalysts achieve exclusive double bond opening polymerization. 

After Kaminsky, Brintzinger, and Ewen discovered homogeneous metallocene/ methylaluminoxane (MAO) catalysts for stereospecific a-olefin polymerizatiOTi (for reviews on olefin polymerization, see [13-21]), the first report [22, 23] rai addition cycloolefin polymerization without ROMP appeared. This stimulated a great interest in these polymers and in catalysts for cycloolefin polymerization (Fig. 1). Cycloolefins such as cyclopentene, cyclooctene, and norbomene can be polymerized via addition (Fig. 2). Polycycloolefins by metallocenes are difficult to process due to their high melting points and their low solubility in common organic solvents. However, metallocenes allow the synthesis of cyclic olefin copolymers (COC), especially of cyclopentene and norbomene with ethene or propene, which represent a new class of thermoplastic amorphous materials (Scheme 1) [24, 25]. 

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