Syndiospecific Polymerization

The addition of the monomer362 is syn, but in contrast to isotactic polymerization, secondary or type 2-1 insertion is characteristic of syndiotactic polymerization 358,359 

The different mode of insertion is attributed to the different polarities of the Ti—C and V—C bonds. The regioselectivity of the syndiotactic catalysts, however, is much lower than that of isospecific catalysts inversion, namely, head-to-head and tail-to-tail additions (type 1-2 addition), are more frequent.359 

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 

Considering Zambelli s model,327 346 syndiotactic propagation is ensured by the higher stability of the intermediate four-center complex 50, in which the methyl group of the chain end and the methyl group of the complexed monomer are in trans position, compared to 51 with cis methyl groups  

Soluble catalysts again may exhibit unique selectivities. Hafnium and zirconium fluorenyl metallocenes with methylaluminoxanes give syndiotactic polymers in high yields. The microstructure of the products indicates site stereochemical control with chain migratory insertion resulting in site isomerization with each monomer 

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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. 

Table 9. Syndiospecific polymerization catalyzed by metallocene catalysts...

The latter mechanism is supported by evidence obtained from the initiation and termination steps in the syndiospecific polymerization of styrene [190]. The 13C-enriched titanium catalyst afforded polystyrene with a CH(Ph)CH213CH3 end group, which indicates that the initiation step proceeded by secondary insertion (2,1-insertion) of styrene into the Ti-13C bond of the active species (Eq. 10). In contrast to this mechanism, termination by the addition of 13C-enriched methanol or tert-butyl alcohol afforded polymers without 13CH30 or tertbutoxy end groups. 

Section 4 will deal with catalytic systems whose stereospecificity is controlled principally by the chirality of the closest tertiary carbon atom of the growing chain (chain-end stereocontrol). In Section 4.1 possible mechanisms for chain-end controlled isospecific and syndiospecific propene polymerizations will be reviewed. In Section 4.2 informations relative to the mechanism of chain-end controlled syndiospecific polymerization of styrene and substituted styrenes will be reviewed. In Section 4.3 chain-end controlled mechanisms for the isospecific and syndiospecific cis-1,4 and 1,2 polymerizations of dienes will be presented. 

Polymerization of methyl methacrylate ( = MMA) has been effected with the organolanthanide precatalysts [(C5Me5)2Sm( -H)]2 and (C5Me5)2LnMe(THF). Both complexes have been found to initiate the living, highly syndiospecific polymerization of methyl methacrylate, Eq. (12) [62],... 

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).

Kirillov, E., Lehmann, C.W., Razavi, A. etal. (2004) Highly syndiospecific polymerization of styrene catalyzed by allyl lanthanide complexes. Journal of the American Chemical Society, 126, 12240. 

Some types of borate compounds act as co-catalysts for the syndiospecific polymerization of styrene in these catalyst systems. The active borate compounds have a tetraphenylborate anion. The effect of anions on the catalytic activity is summarized in Table 17.3. Fluorine substituents at the of 3,4,5-positions increase the catalytic activity and tetrapentafluorophenylborate showed the highest performance. 

Zambelli et al. reported on the mechanism of styrene polymerization [36]. They showed that the main chain of the syndiotactic polymer has a statistically trans-trans conformation. It was established then the double-bond opening mechanism in the syndiospecific polymerization of styrene involves a cis opening. The details in the control of the monomer coordination for this polymerization mechanism were examined by Newman and Malanga using detailed, 3C NMR. It was shown through the analysis of tacticity error (rmrr) that the tacticity in the polymer is chain-end controlled and that the last monomer added directs the orientation and coordination of the incoming monomer unit prior to insertion [37]. 

Highly syndiotactic polystyrene (SPS) was synthesized using a homogeneous catalytic system using a titanium compound and methylaluminoxane or borate [1]. The detailed syndiospecific polymerization of styrene is described in the previous chapter. 

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 syndiospecific polymerization using soluble vanadium catalysts, defects of the chemical addition are noticeable, their number being consistent with the Markoff first — order distribution 59-6i.63> jjj jjjjg regioselectivity as well as stereospecificity are greatly influenced by the possibility of the secondary insertion of monomer The chiratity of the last monomeric unit of the growing polymer determines the stereocontrol of syndiotactic polymerization. The introduction of an achiral (ethylene) unit results in the loss of stereocontrol As active center of syndio-... 

A series of 1- and 2-aryl substituted trichloro indenyltitanium complexes (ArInd)TiCl3 (Ar = Ph, ct-Naph, /3-Naph) have been synthesized and tested as catalyst precursors for the syndiospecific polymerization of styrene with MAO as a co-catalyst.409... 

Metallocene derivatives are the most extensively studied class of homogeneous catalysts for the polymerization of olefins. Less saturated and less hindered mono-Gp group 4 metal species of the type [Cp MR2]+ (Cp denotes Cp or a substituted cyclopentadienyl ring) may also behave as useful catalysts or initiators for olefin polymerization. Catalysts for syndiospecific polymerization of styrene based on mono-Cp titanium derivatives with different substituents on the Gp ligand and with various types of tetraphenylborates have been examined. A good relationship between the... 

Several series of mono-aryloxo and bis-aryloxo titanium derivatives have been prepared by the routes outlined in Scheme 349. Activated by MAO the compounds are used as efficient catalysts for the syndiospecific polymerization of styrene and the co-polymerization of ethylene and styrene. The molecular structures of some of these compounds have been determined by X-ray diffraction. The Ti-O-C bond angle in the structure of Cp TiCl2(0-2,6-Pr12C6H3) differs significantly from those that are observed for the other structures. The effect of the substituents, both on the Cp ring and the alkoxo group, plays an essential role for the catalytic activity and the properties of the polymer obtained.578,779,837-847... 

The monoalkoxo complexes CpTiCl2(OR)(R = methoxyethyl, methoxypropyl, methoxy-isopropyl, o-methoxyphe-nyl, tetrahydrofurfuryl) have been synthesized, characterized and, when activated with MAO, tested as catalyst precursors for the syndiospecific polymerization of styrene.853,854... 

The alkoxo complexes (Ind)TiCl2(OR) (R = Me, Et, Pr1, cyclohexyl) have been prepared and evaluated as catalysts for the syndiospecific polymerization of styrene when activated with MAO.880... 

Indenyl-alkoxo derivatives have been employed to catalyze the syndiospecific polymerization of styrene.985 Ab initio investigations of the synthesis of the tetramethyl-Gp-propoxo dichloro titanium complex Ti[C5Me4(CH2)30]Cl2 from Ti[G5Me4(CH2)30Me]Gl3 have been reported.986... 

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