Chain-end control syndiotactic polymers

As we have seen above polymerisation of all prochiral alkenes produces a new stereogenic centre for each monomer inserted. In a site-controlled 

The detailed kinetics determine how this happens precisely. We don t know whether the complexation reaction or the insertion reaction is rate-determining. Theoretical work on insertion reactions of early-transition metal catalysts indicates that the complexation is rate determining and that the migration reaction has a very low barrier of activation. If the complexation is irreversible, it also determines the enantioselectivity. 

Achiral catalysts derived from VCh/Al Hs or V(acac)3/A1(C2H5)2 are the best known examples, giving syndiotactic polymers [40]. Termination experiments have proved that this polymerization involves a 2,1-insertion instead of 

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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... 

Figure 10.8. Chain end control through 2,1 insertions leading to syndiotactic polymer...

The polymer chain end control model is supported by the observation that highly syndiotactic polypropene is obtained only at low temperatures (about —78°C). Syndiotacticity is significantly decreased by raising the temperature to —40°C [Boor, 1979]. The polymer is atactic when polymerization is carried out above 0°C. 13C NMR analysis of the stereoerrors and stereochemical sequence distributions (Table 8-3 and Sec. 8-16) also support the polymer chain end control model [Zambelli et al., 2001], Analysis of propene-ethylene copolymers of low ethylene content produced by vanadium initiators indicates that a syndiotactic block formed after an ethylene unit enters the polymer chain is just as likely to start with an S- placement as with an R-placement of the first propene unit in that block [Bovey et al., 1974 Zambelli et al., 1971, 1978, 1979]. Stereocontrol is not exerted by chiral sites as in isotactic placement, which favors only one type of placement (either S- or R-, depending on the chirality of the active site). Stereocontrol is exerted by the chain end. An ethylene terminal unit has no preference for either placement, since there are no differences in repulsive interactions. 

Both bridged and unbridged C2v-symmetric metallocenes, mostly the unsubstituted biscyclopentadienyl initiators, but also others such as (CH3)2SiFlu2ZrCl2, have been studied. These initiators are achiral, and their two coordination (active) sites are both achiral and homotopic. The result is that atactic polymer is formed via chain end control. Modest tendencies toward slight isotactic or syndiotactic placement are observed for some initiators, depending on the temperature and other reaction conditions. 

The synthesis of syndiotactic alternating styrene/CO co-polymers with bidentate nitrogen-ligated Pd complexes has already been reported, but thus far, there have never been any reports of a-olefm and CO-based congeners. Syndioselective alternating co-polymerization is believed to proceed via a chain-end control mechanism. 

Fig. 7 Polymer 1, chain end control polymer 2, enantiomeric site control, where i and s are the relative stereochemistry of a pairwise addition of lactide units, i isotactic enchainment, s syndiotactic enchainment...

We now turn to the actual polymerization process and we will try to present a series of pictures that clarifies how chain-end control can be used to obtain either syndiotactic or isotactic polymers. Subsequently we will see how a chiral site can influence the production of syndiotactic or isotactic polymers. Finally, after the separate stories of chain-end control and site control, the reader will be confused by introducing the following elements (1) pure chain-end control can truly occur when the catalyst site does not contain chirality (2) but since we are making chiral chain ends in all instances, pure site control does not exist. In a polymerization governed by site control there will potentially always be the influence of chain-end control. This does not change our story fundamentally all we want to show is that stereoregular polymers can indeed be made, and which factors play a role but their relative importance remains hard to predict. 

Fig. 6.16. Chain-end control leading to syndiotactic polymer. The polymer chain is arranged in such a way that all carbons of the chain lie in the plane of the figure. The double bond of the propene to be inserted also lies in the plane, with its substituents in a plane perpendicular to the plane of the figure. The methyl groups above or below the plane of the figure are drawn in the...

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]. 

This brings us to double stereoselection and reinforcement of the mechanisms. If the site (a)symmetry were to control the orientation of the chain, and if, then, the orientation of the incoming propene is controlled by both the chain and the site, the highest stereoselection is obtained when the two influences reinforce one another. For 1,2-insertion this can be done most effectively for isotactic polymerization, since chain-end control naturally leads to isotactic polymer and this we can reinforce by site control with ligands of the bis(indenyl)ethane type. The chain-end influence of short chains is smaller than that of longer polymer chain and therefore short chain ends lead to lower selectivities. It may also be irrferred that making syndiotactic polymer via a 1,2-insertion mechanism on Ti or Zr complexes is indeed more difficult than making an isotactic polymer, because the two mechanisms now play a counterproductive role. 

Chiral catalyst 171 was used to effect kinetic resolution of the racemic lactide in the polymerization of the racemic lactide [216]. At low conversion high enantiomeric enrichment in the polymer was observed (Scheme 6.169). The stereochemistry of the catalyst overrides the tendency for syndiotactic placements that are typically favored by chain-end control. At higher conversions, the ee in the polymer decreases. 

In 1962. Natta and Zambelli reported a heterogeneous. vanadium-based catalyst mixture which produced partially syndiotactic polypropylene at low polymerization temperatures. " The regiochemistry of the insertion was determined to be a 2.1-insertion of propylene, and a chain-end control mechanism determined the s mdiospecificity of monomer insertion. This catalyst system suffered from both low activity and low stereoselectivity. Highly active single-site olefin polymerization catalysts have now been discovered that make syndiotactic polypropylene with nearly perfect stereochemistry. Catalysts of two different symmetry classes have been used to make the polymer, with Cs-symmetric catalysts typically outperforming their Q -symmetric counterparts due to different mechanisms of stereocontrol (Figure 10). 

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. ... 

If the enantiomorphic site control is operative (top-half view), stereoerrors do not propagate, and the corresponding iso- and syndiotactic polymers are characterized by the presence of rrand mm triads, respectively. If chain-end control is operative (bottom-half view), stereoerrors propagate, and the corresponding iso- and syndiotactic polymers are characterized by the presence of Isolated r and m dlads, respectively. Reprinted from ref 115. Copyright 1992 American Chemical Society. 

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