Syndiospecific systems

The Cg-symmetry of the syndiospecific system can be easily disturbed by placing a methyl group on 3-position of the cyclopentadlenyl ring (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. 

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 authors conducted a similar investigation of precatalysts 7 and 11 using TiBA and trityl tetrakis(pentafluorophenyl)borate as the cocatalyst. They concluded that this material contained no fraction that could be characterized as blocky. It was therefore proposed that reversible chain transfer occurred only with MAO or TMA and not with TiBA. This stands in contrast to the work of Chien et al. [20] and Przybyla and Fink [22] (vida supra), who claim reversible chain transfer with TiBA in similar catalyst systems. Lieber and Brintzinger also investigated a mixture of isospecific 11 and syndiospecific 12 in attempts to prepare iPP/sPP block copolymers. Extraction of such similar polymers was acknowledged to be difficult and even preparative temperature rising elution fractionation (TREF) [26, 27] was only partially successful. 

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. 

It is worth noting that the lower syndiospecificity of catalytic systems based on 31, with respect to those based on 30,9 is accounted for by these calculations. This is easily rationalized in the framework of the enantioselective mechanism which imposes to the growing chain (both in the preinsertion intermediate and in the approximated transition state) a chiral orientation toward... 

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. 

One of the best catalysts for the syndiospecific polymerisation of styrene appeared to be that derived from CpTiCl3 and methylaluminoxane. The polymerisation rate for this system decreases with increasing polymerisation time such behaviour is very similar to that of other Ziegler Natta catalysts. A maximum polymerisation rate is achieved at 50 °C [6]. Other catalysts such as CpTi(OBu)3 [Al/Me/O] are also reported to exhibit a very high activity and syndiospecificity in the polymerisation of styrene [50,51]. 

Catalysts of the Ti(OR)4—[Al(Me)0]x type show greatly inferior activity and syndiospecificity in the polymerisation of styrene by comparison with catalysts of the CpTi(OR)3—[Al(Me)0]x type [54,70]. The activity and syndiospecificity of Ti(OR)4—[Al(Me)0]x catalysts increases when the Al/Ti molar ratio in the polymerisation system is increased. The maximum activity of Ti(OR)4—[A1 (Me)0]x catalysts is observed at an Al/Ti molar ratio of ca 100 [54,55]. It is worth mentioning that, under the same polymerisation conditions, these catalysts yield syndiotactic polystyrene with a higher molecular weight than does the CpTiCl3—[Al(Me)O]x catalyst [71],... 

In view of these results, it was suggested that syndiospecific and isospecific polymerisations with heterogeneous catalysts containing a chlorine atom in the titanium compound or in the support are promoted by homogeneous species formed in the polymerisation system and by heterogeneous species respectively. 

In the case of chlorine-free catalysts such as Mg(OH)2/Ti(OBu)4—[A1 (Me)0]x and Si02/Ti(0Bu)4—[Al(Me)0]x, heterogeneous species are assumed to promote the syndiospecific polymerisation of styrene [67,68]. In a polymerisation system with the latter catalyst, best results were achieved when treating the carrier with [AI(Mc)0]x prior to supporting Ti(OBu)4 (no further activation with methylaluminoxane was needed). The polymerisation rate reaches a maximum at an Al/Ti molar ratio of 20 this is much lower than the value of the Al/Ti molar ratio required to reach the maximum polymerisation rate in the respective homogeneous system, i.e. the system without a carrier [54]. 

The different orientation of the monomer in systems with Ziegler-Natta catalysts based on Ti or Nd and Co or Ni precursors has been suggested [41] to be determined by the presence in the isospecific catalysts (Ti- and Nd-based ones) of anionic ligands bound to the transition metal and the absence of any anionic ligand in the syndiospecific catalysts (Co- and Ni-based ones). In the... 

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. 

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. 

Both the early data and the more precise values given in Table 20A differ significantly from published estimates based on monomer and polymer composition (e.g. rj r2 = 0.60 in Table 19). As all the data relate to the, in general, more consistent soluble vanadium systems, this work reinforces doubts concerning the accuracy of much of the published information. A complication is that since C2 and C4 sequences are observed in ethylene/propene copolymers inverted head to head prop-ene units must be present and this will reduce the accuracy of analyses of EP sequences. In copolymers prepared by VC /AlEtj 4% of head to head propene sequences have been reported with the catalyst VCl4/AlEt2Cl which is syndiospecific for polypropene 8% of head to head sequences was found [295]. ... 

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.

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