Chain end control mechanism

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... 

We have demonstrated that despite a chain-end control mechanism, the steric bulk of the substituent ortho to the phenoxy-0 controls the syndioselectivity of the... 

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. 

Site control versus chain-end control. Over the years two mechanisms have been put forward as being responsible for the stereo-control of the growing polymer chain firstly the site-control mechanism and secondly the chain-end control mechanism. In the site control mechanism the structure of the catalytic site determines the way the molecule of 1-alkene will insert (enantiomorphic site control). Obviously, the Cossee mechanism belongs to this class. As we have seen previously, propene is prochiral and a catalyst may attack either the re-face or the, v/-facc. If the catalyst itself is chiral as the one drawn in Figure 10.2, a diastereomeric complex forms and there may be a preference for the... 

Summarising, in the chain-end control mechanism the last monomer inserted determines how the next molecule of 1-alkene will insert. Several Italian schools [7] have supported the latter mechanism. What do we know so far Firstly, there are catalysts not containing a stereogenic centre that do give stereoregular polymers. Thus, this must be chain-end controlled. Secondly, whatever site-control we try to induce, the chain that we are making will always contain, by definition, an asymmetric centre. As we have mentioned above, the nature of the solid catalysts has an enormous influence on the product, and this underpins the Cossee site-control mechanism. Thus both are operative and both are important. Occasionally, chain-end control alone suffices to ensure enantiospecifity. 

Ewen was the first to report the synthesis of stereoregular propene polymers with soluble Group 4 metal complexes and alumoxane as the co-catalyst [13], He found that Cp2TiPh2 with alumoxane and propene gives isotactic polypropene. This catalyst does not contain an asymmetric site that would be able to control the stereoregularity. A stereo-block-polymer is obtained, see Figure 10.6. Formation of this sequence of regular blocks is taken as a proof for the chain-end control mechanism. 

Although uncommon, a chain-end control mechanism has been demonstrated to be operative for the formation of syndio-eiuiched poly(propylene carbonate)... 

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. 

Soluble Ziegler-Natta catalysts can exhibit unique stereochemical properties. Group IV metallocenes in combination with methylaluminoxanes produce isotactic polypropylene with two different isotactic microstructures. The usual enantio-morphic site control is characteristic of enantiomeric racemic titano- and zirco-nocene complexes (e.g., ethylene-bridged indenyl derivatives279,349). In contrast, achiral titanocenes (e.g., [Cp2TiPh2]) yield isotactic polypropylene with microstructure 49, which is consistent with a chain end control mechanism 279,349-351... 

Metal complexes which initiate rac-LA ROP with a high degree of stereocontrol are currently an area of major research interest and have the potential to produce a spectrum of different materials [19, 21], Much attention focuses on iso-selectivity as this can enable production of PLA of good thermal resistance (isotactic, stereoblock or even stereocomplex PLA). There are two mechanisms by which an initiator can exert iso-selectivity in rac-LA ROP (1) an enantiomorphic site control mechanism or (2) a chain end control mechanism. Enantiomorphic site control occurs using chiral initiators (Fig. 6) it is the chirality of the metal complex which... 

In the chain end control, the stereochemistry of the last inserted lactide unit influences the selectivity for a particular stereochemistry of lactide. Chain end control mechanisms can be invoked for the preparation of iso- or heterotactic PLA. For iso-selective polymerization of /y/c-LA. the two mechanisms can be differentiated on the basis of the stereoerrors or defects this is illustrated in Fig. 7. 

When the stereocontrol occurs by a chain end control mechanism, a stereochemical defect results in the stereochemistry of the defect being propagated along the chain until the next defect occurs (polymer 1 in Fig. 7). If a stereochemical defect occurs in a polymerization using an initiator that exhibits enantiomeric site control, the mistake will be rectified with the next incoming lactide unit (polymer 2 in Fig. 7). This is because it is the chirality of the metal centre which determines the PLA tacticity and not that of the last inserted lactide unit. 

The stereochemical control is achieved via a chain end control mechanism, enforced by the steric bulk of the ligand. The dinuclear complexes cannot control the stereochemistry, leading to atactic PLA, due to the lack of a single, hindered lactide coordination site. 

Considering the spatial cooperation between the ultimate unit and the approaching monomer molecule through complexation with the metal atom, the growing chain end control mechanism has been postulated to be valid for the coordination polymerisation of alanine iV-carboxyanhydride in the presence of metal alkyls [75]. 

In the mid-1980s the first metallocene/MAO catalysts for the isotactic polymerization of propene were described. Ewen found Cp2TiPh2/MAO to produces isotactic polypropene at low temperatures by chain end control mechanism (stereoblock structure). When using a mixture of racemic and meso [En(Ind)2] TiCl2 in combination with MAO, he obtained a mixture of isotactic and atactic polypropene, the isotactic polymer having a microstructure in accordance with... 

The structure of ligands in metallocene complexes determines activity, stereoselectivity, and molecular weight of 1-alkene polymerizations, by controlling the preferential conformation of the growing polymer chain which in turn controls the stereochemistry of monomer coordination ( enantiomorphic site control ). The difference between this and the chain-end control mechanism mentioned earlier is that stereo errors due to misinsertions can be repaired.101,106... 

The bis(guanidinate) alkoxide complexes [(Me3Si)2NC(NPr )2]2LnX (X = O Pr, 0 Bu Ln = Y, Nd, Sm, Lu) depicted in Scheme 22 are active catalysts/initiators for the ROP of / -butyrolactone (Scheme 53) under mild conditions to afford syn-diotactic poly(hydroxy-3-butyrate) through a chain-end control mechanism [32]. 

The mechanistic complexities of stereoselectivity is further evidenced by a recent report by Maudoux et a/. who describe a chiral aluminum salen catalyst that generates highly isotactic PLA from rac-lactide (Pm-0.90). In this example, the kinetics indicated a dominant chain-end control mechanism, which contrasts to other chiral aluminum salen catalysts where enantiomorphic site control is thought to predomi-nate. ° All the previously mentioned chiral aluminum salen alkoxide systems require multiple days at elevated temperatures to polymerize -200 equiv. of lactide. The low activity of chiral aluminum salen systems towards lactide polymerization is a major drawback of these systems. 

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). 

---