Insertion styrene

The products have been described as pseudo-random copolymers. The reason for this is that successive head-to-tail styrene-styrene insertions do not occur. The structure may thus be formally represented as... 

Our calculations suggest that the stereoselectivity of the hydrosilylation is determined by the thermodynamic stability of the ri3-allylic complex that forms after styrene insertion. This opens up the possibility of improving the enantioselectivity by modifying the catalyst framework to alter the stability of the exo versus the endo T 3-allylic intermediate. 

Although the mechanism of steric control that operates in the isospecific styrene polymerisation system with homogeneous nickel-based [(MeAll)(Cod)Ni]+[PF6] /P(Chx)3 catalyst is not completely clear, it is evident that re-benzyl nickel species play an important role in determining the mode of the styrene insertion. 

Other approaches include the use of difunctional olefins such as 1,7-octadiene, 1,9-decadiene, or para-(3-butenyl)styrene.875 While the former method also generates chain cross-linking (thus unprocessable polymer gels), the latter leads only to LCB formation through hydrogenolysis after a secondary styrene insertion. Tandem Zr/Fe catalysis has been used as well.876 The preparation of iPP with PS branches has been achieved by co-polymerization of propylene with allyl-terminated PS macromonomers.877... 

PH3 ligands were followed by force field calculations on the real system. In this manner, it was concluded that both transition states TS I and TS II show lower energies for re-face than si-face styrene insertion. An interesting issue of this work is that the RhH(CO) (i ,i )-BINAPHOS would result in si-face selection by TS I and re-face selection by TS II. This matches the experimental results that the Rh-(P,P)-BINAPHOS complex gives aldehydes in much lower ees than the Rh-(R,S)-BINAPHOS complex. Similarly, the enantioselectivity observed for (Z)-2-butene was nicely explained with the same model. 

SCHEME 14.4 Two regiochemical pathways of styrene insertion into an active metal-carbon bond. [Ti] represents the metal fragment of the active species R can be hydride, alkyl gronps, and growing polymer chain. 

Izzo, L. Napoh, M. Ohva, L. Regiochemistry of the styrene insertion with CH2-hridged ansa-zirconocene-based catalysts. Macromolecules 2003, 36, 9340-9345. 

Propylene/CO and styrene/CO copolymers possess side chains (methyl and phenyl groups, respectively) thus, multiple possibilities for regioisomers and stereoisomers exist. Several mechanistic studies have proposed that the key step which determines the regio- and stereochemistry of olefin/CO copolymers is olefin insertion into the acylpalladium species. Polymers with high regioregularity are produced if the olefin insertion reaction is regioselective to either 1,2- or 2,1-addition. As shown in Scheme 22.2, propylene insertion is mostly 1,2- whereas styrene insertion is usually 2,1-. 

Aeby, A. Consiglio, G Ethene and styrene insertion into the Pd-acyl bond of [Pd(COMe)(P N) (solv)]03SCp3 and its role in the copolymerization of olefins with carbon monoxide. J. Chem. Soc., Dalton Trans. 1999, 655-656. 

STEREOCHEMISTRY OF THE STYRENE INSERTION 45 TABLE 4.1 Chemical Shifts and Coupling Constants of Copolymers A and B... 

Zambelli et al. [12] independently studied the stereochemistry of the styrene insertion catalyzed by the Ti(benzyl)4/MAO system. The C-NMR spectrum of the polymer obtained showed that the chain end of the polymer was consistent with a secondary insertion mechanism both in the initiation steps and in the propagation steps even when the former occurs via M-H bonds. They also showed that most of the polymer chains were initiated by M-H bonds. 

Although ordinary metallocenes are inactive for styrene polymerization, certain metallocenes can be effective for ethylene/styrene copolymerization [20-23]. Oliva et al. reported that the regiospecificity of styrene insertion into the Zr- CHs bonds is predominantly secondary in ethylene/styrene copolymerization using the [Me(Ph)C(fluorenyl)(Cp)]ZrCl2-MAO catalyst system [20]. Oliva later presented that some styrene units can be introduced in an isotactic polypropylene sequence (1,2-insertion) by using a small amount of ethylene to reactivate the catalyst after the styrene insertion (2,1-insertion) in propylene/styrene copolymerization using a r c-[Et(indenyl)2]ZrCl2-MAO catalyst system [21]. 

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