Propylene isospecific

A detailed molecular mechanics analysis of model catalytic sites has rationalised these observed behaviours for homogeneous propylene isospecific polymerisation catalysts based on bridged rac.-metallocene [357], It is interesting that, in contrast to the discussed case of enantioselectivity of the models for the primary insertion of propylene, the enantioselectivity of the models for the secondary insertion of propylene is due to direct interactions of the... 

Zirconocene dichloride 121 derived from (l-phenylethyl)cyclopentadienyl ligand is formed as a mixture of diastereomers from which the racemic form can be isolated by fractional crystallization. This complex was studied by X-ray diffraction methods and revealed a virtually chiral C2-symmetrical conformation in which the chiral ring-substituents are arranged in a synclinal position relative to the five-membered ring. It was proposed that this conformation is preserved in solution. Using 121 as catalyst the influence of double stereodifferentiation during isospecific polymerization of propylene (Eq. 32) was demonstrated for the first time [142],... 

The isotacticities and activities achieved with nonbridged metallocene catalyst precursors were low. Partially isotactic polypropylene has been obtained by using a catalyst system of unbridged (non-ansa type) metallocenes at low temperatures [65]. A chiral zirconocene complex such as rac-ZrCl2(C5H4 CHMePh)2 (125) is the catalyst component for the isospecific polymerization of propylene (mmmm 0.60, 35% of type 1 and 65% of type 2 in Scheme Y) [161]. More bulky metallocene such as bis(l-methylfluorenyl)zirconium dichloride (126) together with MAO polymerized propylene to isotactic polypropylene in a temperature range between 40 and 70°C [162]. 

The Tj, and the isospecificity obtained with these FI catalysts represent some of the highest values for iPPs ever synthesized. The isospecific propylene polymerization proceeds via a 1,2-insertion with a site-control mechanism. 

As stated above, we postulated that fast, reversible chain transfer between two different catalysts would be an excellent way to make block copolymers catalytically. While CCTP is well established, the use of main-group metals to exchange polymer chains between two different catalysts has much less precedent. Chien and coworkers reported propylene polymerizations with a dual catalyst system comprising either of two isospecific metallocenes 5 and 6 with an aspecific metallocene 7 [20], They reported that the combinations gave polypropylene (PP) alloys composed of isotactic polypropylene (iPP), atactic polypropylene (aPP), and a small fraction (7-10%) claimed by 13C NMR to have a stereoblock structure. Chien later reported a product made from mixtures of isospecific and syndiospecific polypropylene precatalysts 5 and 8 [21] (detailed analysis using WAXS, NMR, SEC/FT-IR, and AFM were said to be done and details to be published in Makromolecular Chemistry... 

Shiono and co-workers developed a unique method for the preparation of polymers with pendant vinyl groups with an isospecific metallocene catalyst from propylene and BD in the presence of Hz- This method is introduced in Section... 

Isospecific polymerization. Important features of stereoregular isospecific polymerization of propylene, and in general, terminal alkenes, are as follows 125,254,32 328... 

It means that after the incorporation of an ethylene monomer, the addition of propylene is still isospecific. 

It is worth noting that, to obtain efficient supported Ziegler-Natta catalysts of the third and fourth generation, it is TiCU rather than preformed TiCL that is supported. When TiCl3 was supported on MgCl2 (e.g. by deposition from the solution of a TiCl3.3Py complex) and then activated with an alkylaluminium compound, the obtained catalyst appeared to be aspecific for propylene polymerisation [69], However, it could be converted into an isospecific catalyst by adding a Lewis base such as ethyl benzoate [70],... 

Cationic [ZrBz3]+ species are capable of polymerising oc-olefins such as propylene, 1-butene and 1-pentene at elevated temperature. It is worth emphasising that the non-metallocene [Zr(CH2Ph)3] + [PhCH2B(C6F5)3] catalyst, which is a cationic arene zirconium complex [189], is capable, at least partially, of isospecific a-olefin polymerisation at relatively high temperature [162,189,190],... 

A number of models for catalytic active sites have been proposed to explain the isospecific polymerisation of propylene in the presence of TiC -based catalysts [68]. It was Natta who proposed [366], for the first time, that the steric control in isospecific propylene polymerisation is caused by the structure of the active species located on the borders of crystal layers of violet titanium trichloride. Arlman and Cossee [277] suggested that the isospecific active sites are located on lateral crystal surfaces, which in violet titanium trichloride such as oc-TiCh correspond to (110) planes. Titanium atoms present on the above-defined fracture surface have a vacant octahedral site bonded to five chlorine atoms (Figure 3.26) [1,68]. 

For the discussed model sites, the two situations presenting the outward and inward propylene coordination are enantioselective and non-enantioselective respectively. Hence, these model sites are isospecific only under the assumption that propylene always coordinates at a given coordination position (i.e. that the chain skips back to the starting position after each monomer insertion and prior or simultaneously to the coordination of the new propylene molecule) [345]. 

Figure 3.37 Models for the primary insertion of propylene into a polypropylene growing chain in isospecific polymerisation with the (R, R)-rac.-(IndCH2)2ZrX2-based catalyst. The growing alkyl chain occupies an open sector of the ligand framework propylene enters the reaction complex with its methyl substituent away from the Cp atom of the last monomeric unit in the chain (the monomer methyl group is directed towards another open sector of the ligand framework). For the sake of clarity, only the C C bonds are sketched for the n ligands. O - Zr O - C or CH3 o - H. Reproduced by permission from Ref. 30. Copyright 1995 Wiley-YCH Weinheim...

Figure 3.38 Possible model site for isospecific propylene polymerisation with the (R, A>)-/Y/r.-(ThindCH2)2 ligand-containing catalyst. The broken lines indicate the forbidden conformation of the growing chain and the favoured coordination of propylene corresponding to this forbidden chain conformation. For the sake of clarity, hydrogen atoms are omitted. O Zr O C, CH, CFF or CH3. Reproduced (by permission from Elsevier Science) from Ref. 1. Copyright 1989 Pergamon Press...

At the end of considerations of the stereoregulation mechanism in the isospecific polymerisation of a-olefins with homogeneous catalysts obtained from chiral stereorigid metallocenes of class III with C2 molecular symmetry, let us address isospecific propylene polymerisation catalysts, which are methy-laluminoxane-activated non-bridged metallocenes (Table 3.2). 

Many attempts have been made to synthesise ethylene/propylene block copolymers, referred to as polyallomers, with isospecific Ziegler-Natta catalysts. However, true block copolymers can hardly be synthesised. This is due to the short life of the growing polymer chains [68,241]. Therefore, only in a few cases, when the copolymers are synthesised by adding two comonomers sequentially and under very specific conditions in order to reduce chain transfer reactions, does unambiguous evidence for true block copolymer formation with isospecific catalysts exist [457]. 

Although the isospecific polymerisation of styrene monomers has much less steric demands for the Ziegler-Natta catalysts than that of x-oldins, it proceeds with much lower propagation rate constants by comparison with the polymerisation of x-olefins for example, on a molar basis, styrene is ca 100 times less reactive than propylene in the polymerisation [30,33], Also, compare the relatively slow polymerisation of styrene and other vinylaromatic monomers with the relatively fast polymerisation of vinylcyclohexane [20,31,34-36]. 

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