Propylene syndiospecific

In the model proposed for the active centre of the propylene syndiospecific polymerisation, the V(III) atom is pentacoordinated [327]. Its ligands include three chlorine atoms (two of which are bridge-bonded to the aluminium atom), the chiral carbon atom of the last monomer unit of the growing chain and the coordinated propylene molecule. Prior to its coordination and after its insertion, the vanadium atom is tetracoordinated. In the alternative similar model, two chlorine atoms are substituted by a bidentate dionate, and the chlorine atom is bridge-bonded to the aluminium atom in dimeric A1R2C1 [2]. 

In contrast to the case of Cp2ZrX2/MAO giving atactic poly(alkene)s, Cp MCl2/MAO, M = Zr (139) and Hf (140), are the catalyst precursors of the syndiotactic polymerization of 1-butene and propylene [176]. Triad distribution indicated that this is chain-end controlled syndiospecific polymerization. The syndiospecificity is attributed to the increase of steric encumbrance around the metal center. Thus, Cp HfX2 is the most effective syndiospecific catalyst component in this system. 

With MAO activation, Zr- and Hf-FI catalysts 1 and 3 exhibit fairly high reactivity toward propylene and produce propylene oligomers [64, 65], Conversely, the corresponding Ti-FI catalyst/MAO 2 forms semicrystalline PP (1 °C polymerization), which displays a peak melting temperature of 97 °C, indicative of the formation of a stereoregular polymer. To our surprise, microstructural analysis by 13C NMR indicates that the resultant polymer is syndiotactic (rr 19%), and that a chain-end control mechanism is responsible for the observed stereocontrol, regardless of the C2 symmetric catalyst ([28] for the first report on syndiospecific propylene... 

We and others have revealed that syndiospecific propylene polymerization is exclusively initiated by 1,2-insertion followed by 2,1-insertion as the principal mode of polymerization [64]. This is the first example of a predominant 2,1-insertion mechanism for chain propagation exhibited by a group 4 metal-based catalyst. The unusual preference for 2,1-regiochemistry displayed by the Ti-FI catalysts compared with the Zr- and Hf-FI catalysts is apparently inconsistent with the crys-tallographically characterized structures, which indicate that the Ti is shielded more by the phenoxy-imine ligands and thus possesses higher steric compression. The reason for the unusual preference in the regiochemistry of Ti-FI catalysts is unclear at the present time. 

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... 

The structure of ethylene-propylene copolymers shows that in the case of syndiospecific polymerization the steric control is due to the chirality of the asymmetric carbon of the last unit of the growing chain end.337... 

Another class V catalyst, derived from 2,2-dimethylpropylidene(cyclopenta-dienyl) (9-fluorenyl)zirconium dichloride [t-BuCH(Cp)(Flu)ZrCl2] (Table 3.1), appeared to be an excellent catalyst for syndiospecific propylene polymerisation [107,137]. 

Although low-temperature syndiospecific polymerisation of propylene with soluble Ziegler-Natta catalysts, based on soluble vanadium compounds and dialkylaluminium chlorides as activators, was first carried out successfully as... 

The syndiospecific polymerisation of propylene with soluble vanadium-based Ziegler Natta catalysts is not completely regiospecific [389 392], i.e. the monomer unit enchainment is not entirely head-to-tail. In addition to syndiotactic stereoblocks, the polymer also contains sterically irregular stereoblocks. The whole polymerisation can be thus described as a copolymerisation with four head-to-tail and tail-to-tail stages [2,379]. 

In view of the data concerning propylene polymerisation in the presence of homogeneous vanadium-based Ziegler-Natta catalysts, the syndiospecificity of the polymerisation is believed [387,395] to arise from steric repulsions between the last inserted monomer unit of the growing chain and the methyl group of coordinated propylene molecule, i.e. chain end stereocontrol is postulated to play the essential role in the stereoregulation. 

The same conclusion as in the case of propylene homopolymerisation has been drawn considering IR [396] and NMR [389,395] spectra of ethylene/propylene copolymers obtained with vanadium-based syndiospecific catalysts. The type of propylene insertion depends on the kind of last inserted monomer unit secondary insertion [scheme (40)] occurs more frequently when the last monomeric unit of the growing chain is propylene, while primary propylene insertion [scheme (39)] is more frequent when the last monomeric unit of the growing chain is ethylene [2]. The above explains the microstructure of ethylene/propylene copolymers obtained with vanadium-based Ziegler-Natta catalysts. These copolymers contain both m and r diads when the sequence of propylene units is interrupted by isolated ethylene units i.e. a propylene insertion after an ethylene insertion is substantially non-stereospecific [327,390,397], The existence of a steric interaction between the incoming monomer molecule and the last added monomer unit is also confirmed by the fact that the propagation rate for the secondary insertion of propylene in syndiospecific polymerisation is lower than for primary insertion in non-stereospecific polymerisation [398],... 

Figure 3.33 Possible diastereoisomeric catalytic complexes for the soluble vanadium-based syndiospecific catalysts, with a si chain, presenting suitable orientation of the chain and of the propylene molecule. The corresponding minimum energies are indicated below the models, o - V or Al O Cl C O H - CH3 or CH2. Reproduced by permission from Ref. 328. Copyright 1985 American Chemical Society...

Figure 3.41 Models for the primary insertion of propylene into a polypropylene growing chain in a syndiospecific polymerisation with the Me2C(Cp)(Flu)ZrX2-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 C/j atom of the last monomeric unit in the chain (the monomer methyl group is directed towards the mouth of the two O, rings). 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-VCH Weinheim...

It is worth mentioning that a rar.- / ,v -titanocene methylaluminoxane catalyst, such as rac.-Ph2C(Cp)(Ind)TiCl2—[Al(Me)0]x, which yields an isotactic polymer in propylene polymerisation, promotes the syndiospecific polymerisation of styrene [73,100]. This is the first example where two different stereoregular polymers, isotactic and syndiotactic, can be obtained using the same catalyst in the case of two different monomers. 

Figure 19 Stereoselective insertions of propylene (grey) under catalytic-site control, mediated by the oc,[3 segment of the growing polymer chain (black), for isospecific polymerization by a C2-symmetric catalyst (A, left) and for syndiospecific polymerization by a Cs-symmetric catalyst (B, right).

Zam belli and Tosi have extensively studied the stereochemistry of the propagation step in propylene polymerization on Ziegler-Natta catalysts. Specific features of this process are shown in Table 4. Cis-addition of the olefin to the active metal-carbon bond has been observed both in isospecific and syndiospecific polymerization. The olefin addition to the active bond proceeds with the participation of the primary (L,(Mt—CH2—CHR—P) and secondary (L,Mt—CHR—CH2—P) carbon atoms of the growing polymer chain using isospecific and syndiospecific catalysts, respectively. 

In an effort to enhance the activity and syndiospecificity of 6 -symmetric ansa-Cp-Flu metallocene complexes in the MAO-co-catalyzed polymerization of propylene, a large number of fluorenyl-substituted ansa-Cp-Flu metallocene complexes have been prepared,879-881 including the di-/z r/-butyl-substituted derivative 1129.882 The hafnocene... 

Figure 10 Transition states for primary insertion of propylene (a) with the isospecific Me2Si(1 -lnd)2Zr system and (b) with the syndiospecific Me2C(Cp)(9-Flu)Zr systems.

Figure 11 Favored transition states for the secondary insertion of propylene with (a) the isospecific Me2Si(1 -lnd)2Zr system and with (b) the syndiospecific Me2C(Cp)(9-Flu)Zr system. High-energy transition states for the secondary insertion of propylene with (c) the isospecific Me2Si(1 -lnd)2Zr system and (d) the syndiospecific Me2C(Cp)(9-Flu)Zr system.

Table 19 Syndiospecific propylene polymerization catalyzed by bis(phenoxy-imine) catalysts... 

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