Syndiotactic Polypropylene: Discovery, Development, and Industrialization via Bridged Metallocene Catalysts

Abstract

This chapter covers the discovery that enabled, for the first time, the manufacturing of high molecular weight, highly stereoregular syndiotactic polypropylene polymers with narrow molecular weight distribution via the new class of metallocene molecules with bridged cyclopentadienyl-fluorenyl ligands. First, the salient crystal structural features of the neutral metallocene molecules and the crystal structure of the corresponding mono-alkyl-zirconocenium cation is introduced. Then, by employing the structural and geometrical data, a realistic working model for the active site precursor is developed and presented. The stereochemistry of the catalytic propylene poly-insertion reaction and the formation of syndiotactic polypropylene according to the enantiomorphic site control mechanism at the enantiotopic cationic active sites, formed after the activation of the prochiral metallocene dichloride with methylaluminoxane, MAO, is reviewed and further delineated. In the following sections, the impact of the introduction of substituents with proper size and composition at some selected positions of the ligand is discussed and the methods for increasing the molecular weight and stereoregularity of syndiotactic polypropylene polymers are elaborated. With the aid of computational calculations on real and hypothetical catalyst systems with substitutionally altered ligand structures, it is shown that the degree of enantioselectivity of syndiospecific catalyst systems can be determined and/or predicted for these systems quantitatively and with high precision. The calculations confirm that the introduction of substituent(s) reinforcing the preferential conformational orientation of the polymer chain enhances the overall enantioselectivity of the active site, and to certain extends stereoselectivity in general, by affecting the site epimerization processes. The computational calculations reveal that to account correctly for the frequency of the site epimerization-dependent m-type stereo-errors it is essential to include the counter-ion, as an integrated part of the A. Razavi (*) 35 Domaine de la Brisee, 7034 Obourg, Belgium e-mail: abbasrazavi@skynet.be catalyst system, into the calculations. The relevance of the symmetry and stereorigidity of the metallocene structure for the syndiospecificity of the final catalyst is debated by presenting a few "non-conforming" catalyst examples.Finally, the challenges involved in the manufacture of syndiotactic polypropylene in commercial scale continuous production processes are highlighted as well as the polymer's unique structural, physical, mechanical and rheological properties.

The Influence of Ziegler-Natta and Metallocene Catalysts on Polyolefin Structure, Properties, and Processing Ability

“…This provided organometallic and polymer chemists with a potent co-catalyst able to activate group 4 metallocenes (and a large number of other transition metal complexes, too) towards the polymerization of virtually any 1-olefins, as well as several cyclic olefins [ 65 ]. Over the past 30 years, these homogenous SSCs have dominated the literature due to a greater understanding of the mechanism of polymerization of ethylene leading to opportunities for designing and developing improved classes of catalysts [ 64 , 150 , 151 , 152 , 153 , 154 , 155 , 156 , 157 , 158 , 159 , 160 , 161 , 162 , 163 , 164 , 165 , 166 , 167 , 168 , 169 , 170 , 171 , 172 , 173 , 174 , 175 , 176 , 177 , 178 ]. However, the activity of Cp 2 -MtX 2 /MAO catalysts was moderate with propylene and, more importantly, did not produce stereo-regular polymers [ 65 ].…”

“…Both heterogeneous titanium catalysts [ 69 , 163 ] and titanium-based metallocenes [ 67 ] produce ethylene-propylene copolymers (EPs) with high fractions of odd-numbered methylene sequences ( n = 1, 3, 5) as a result of the highly regio-regular primary propylene insertion; resonances corresponding to the even-numbered sequences of length (two and four) are not observed (Ψ 2 = Ψ 4 ≈ 0). On the other hand, EPs synthesized using vanadium catalysts contain higher fractions of even-numbered methylene sequences (Ψ 2 = 0.14, Ψ 4 = 0.08) due to a high number of propylene inversions [ 164 , 165 , 166 , 167 , 168 , 169 ].…”

Assignment of Regioirregular Sequences in the 13C NMR Spectrum of Syndiotactic Polypropylene

“…Cat1 (in combination with methylalumoxane, MAO) was the first catalyst ever reported able to yield, under proper conditions (i.e., high monomer concentration and low temperature) highly stereoregular s-PP [14]. It is also one of the most regioselective zirconocene catalysts in the public domain: the favored propene insertion mode is 1,2 (primary), and the 13 C NMR spectra of the polymer recorded with standard probes do not reveal 2,1-inserted units.…”

“…Apart from a much lower technological impact of this polymer compared with i-PP, s-PP microstructure is more complex to elucidate. In the present paper, we report the first full 13 C NMR assignment of isolated regiodefects in s-PP samples produced with two well-known molecular catalysts (Chart 1), namely the C s -symmetric stereorigid ansa -zirconocene Cat1 [14,15], and the C 2 -symmetric fluxional bis(phenoxy-imine)Ti complex Cat2 [16].…”

Crystalline Isotactic Polar Polypropylene from the Palladium‐Catalyzed Copolymerization of Propylene and Polar Monomers

“…[3] Polypropylene with low levels of isotacticity shows good elastomeric properties, [4] while syndiotactic polypropylene displays other unique properties,s uch as high impact strength and optical clarity. [5] Therefore,c ontrol over regio-and stereoregularity is crucial in any type of propylene polymerization. Accordingly,i ti s hardly surprising that substantial efforts have been devoted to the understanding and control of mechanistic details in the metal-catalyzed polymerization of propylene.…”