Syndiospecific systems insertions

Syndiospecific catalytic systems based on metallocenes are highly regioreg-ular. As a consequence, their stereoselectivity in possible regioirregular insertions has been experimentally investigated for propene copolymers only.78,79 However, an analysis of the stereoselectivity of possible secondary propene insertions on syndiospecific catalytic models based on -symmetric metallocenes is reported here, also due to its relevance to the rationalization of the dependence of regiospecificity on the type of stereospecificity (see Section 3.1.4.1).80... 

Since the 1960s the syndiospecific chain-end controlled polymerization of propene in the presence of homogeneous vanadium-based catalytic systems has been known. For these systems, it has been well established by the work of Zambelli and co-workers that the polymerization is poorly regioselective and the stereoselective (and possibly syndiospecific) step is propene insertion into the metal secondary carbon bond with formation of a new secondary metal-carbon bond.133134... 

Recently, bis(imino)pyridyl Fe(II)-based catalysts have been reported to afford isospecific chain-end controlled propene polymerization occurring through secondary monomer insertion.138 139 Even more recently, catalytic systems based on the octahedral bis(salicylaldiminato)Ti complex have been reported to afford syndiospecific chain-end controlled propene polymerization,140 which possibly occurs through secondary monomer insertion.141... 

Nonbonded energy interactions are able to rationalize not only the stereospecificities observed for different metallocene-based catalytic systems (isospecific, syndiospecific, hemi-isospecific, and with oscillating stereocontrol) but also the origin of particular stereodefects and their dependence on monomer concentration as well as stereostructures associated with regioirregular insertions. Nonbonded energy analysis also allowed to rationalize the dependence of regiospecificity on the type of stereospecificity of metallocene-based catalysts. 

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.

Czv-Symmetric Catalysts. Syndiotactic polymers have been formed using metallocene catalysts where the polymer chain end controls the syndiospecificity of olefin insertion. Resconi has shown that Cp 2MCl2 (M = Zr. Hf) derived catalysts produce predominantly syndiotactic poly(l-butene) with an approximate 2 kcal/mol preference for syndiotactic versus isotactic dyad formation." At —20 °C. Cp 2HfCl2/MAO produces poly(l-butene) with 77% rr triads. Pellecchia had reported that the diimine-ligated nickel complex 30 forms moderately syndiotactic polypropylene at —78 °C when activated with MAO ([rr] = 0.80)." " Olefin insertion was shown to proceed by a 1.2-addition mechanism." in contrast to the related iron-based systems which insert propylene with 2.1-regiochemistry. ... 

The Two Coordination Positions Are of Similar Energy. In the hypothesis that the chain migratory insertion mechanism is still prevailing, the model of these catalytic systems would be isospecific or syndiospecific, if the two situations originated from propene coordination on the two coordination positions are enantioselective in favor of the same or opposite propene enantiofaces, respectively. If only one situation is enantioselective, the corresponding catalytic system is hemiisospecific. 

Syndiospecific catalytic systems based on Cj-sym-metric metallocenes are more regioselective than the C2-symmetric metallocenes of class II. As a consequence, the enantioselectivity in regioirregular insertions have been experimentally investigated for propene-based copolymers only. In particular, NMR characterization of ethene-l- C/propene copolymers suggests that the very low amount (0.03—... 

Isotactic sequences are here due to successive insertions upon the same side of the catalyst (rather than from alternate sides as is believed to occur in C2-S5unmetric systems), which are made possible by windshield-wiper motions of the chain relative to the catalyst. The dibridged catalyst 55 switches from being predominantly syndiospecific to being moderately isospecific as temperature is raised and propylene concentration is lowered, probably due to a similar phenomenon (54). The fact that the catalyst with the chiral substituent projecting... 

Mechanisms of Stereocontrol. Stereochemistry of the olefin insertion step can be controlled by both the steric environment of the active site (enantiomorphic-site control) as well as the growing polymer chain (chain end control). In chain end stereocontrol, stereospecificity arises from the chiral )3-carbon atom of the last enchained monomer imit, which in turn influences the stereochemistry of monomer addition. Chain-end control is usually less effective than site control and has been observed for some achiral metallocenes at low polymerization temperatures. Partially iPP resulting from chain end stereocontrol has been obtained with Cp2TiPh2/MAO (56,272). The syndiospecific polymerization of 1-butene using the Cp 2MCl2/MAO (M = Zr, Hf) catalyst systems has been described (273). Predominantly sPP has been obtained under chain end control, using Brookhart s diimine nickel catalysts (274-277). 

In 1962, Natta and Zambelli reported a heterogeneous, vanadium-based catalyst mixture which produced partially syndiotactic polypropylene at low polymerization temperatures.45 jjie regiochemistry of the insertion was determined to be a secondary insertion of propylene, and a chain-end control mechanism determined the syndiospecificity of monomer insertion. This system suffered from both low activity and low stereospecificity. 

For parent syndiospecific catalyst systems, the propylene jc-complexes in which the Zr center has an S configuration insert the rntmomer in a l,2- fashitm, and the ctmfiguration of the chiral center on the alkyl chain closest to the metal center is R. Alternatively, when the configuratimi of the Zr center is R, the closest chiral center (m the chain is of S configuration and a 1,2-re insertion occurs. After each insertion, the alkyl chain migrates frinn one coordination site to the other. Due to the enantiotopic nature of the metallic active center, to have a complete picture it is sufficient to study only one of the two possibilities. 

The phenyl group of styrene is a bulky group and is an electron-withdrawing substituent. Styrene behaves differently from an alkyl-olefin with respect to its polymerization reaction. The possible insertion process for styrene into a metal-methyl bond, which is presumed to arise from the methylation of the titanium by MAO in syndiospecific catalyst systems, may be formulated as a primary insertion (Eq. 4.3) or as a secondary insertion (Eq. 4.4) ... 

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