Chain end stereocontrol

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. 

In the simpler cases, the discrimination between the two faces of the prochi-ral monomer may be dictated by the configuration of the asymmetric tertiary C atom of the last inserted monomer unit (chain-end. stereocontrol) or by the chirality of the catalytic site (chiral site stereocontrol). The distribution of steric defects along the polymer chain may be indicative of which kind... 

Scheme 1.1 Typical steric defects in a (mainly) isotactic poly-l-alkene chain (adapted Fisher projections) for (a) chain-end-stereocontrol (b) enantiomorphic site stereocontrol.

The chain-end stereocontrol for olefin polymerizations leads generally to lower stereoselectivities (differences in activation energy for insertion of the two enantiofaces generally lower than 2 kcal/mol) than the chiral site stereo-control.18131132 For this reason, the corresponding catalytic systems have not reached industrial relevance for propene homopolymerization. However, some of them are widely used for propene copolymerization with ethene. 

Possible mechanisms for chain-end stereocontrol for catalytic systems presenting primary and secondary 1-alkene (mainly propene) insertion will be described in Sections 4.1.1 and 4.1.2, respectively. 

The problem of the origin of chain-end stereocontrol for secondary 1-alkene insertions has been relatively little investigated up to now. The only reported... 

According to an early model.13 1 there are two adjacent accessible positions at the catalytic site, each favoring the coordination of the prochiral monomer with one of its two faces if the growing polymer chain alternates between the two positions at each insertion step, syndiotactic propagation is ensured. Due to the successive finding of a chain-end stereocontrol, this model has to be rejected. 

The mechanisms of stereoselectivity which have been proposed for chain-end stereocontrolled polymerizations involving secondary monomer insertion also present a general pattern of similarity. In fact, molecular modeling studies suggest that for olefin polymerizations (both syndiospecific and isospecific, Section 4.1.2) as well as for styrene polymerization (syndiospecific, Section 4.2), the chirality of the growing chain would determine the chirality of a fluxional site, which in turn would discriminates between the two monomer enantiofaces. 

Both possibilities, i.e. enantiomorphic site stereocontrol (in the case of an optically inactive catalyst it consists of a racemic mixture of enantiomorphic sites) and chain end stereocontrol, have been verified, depending on the kind of catalyst. These two essential types of stereocontrol mechanism operating in propylene polymerisation with various stereospecific Ziegler-Natta catalysts are presented in Table 3.3. 

In view of the data concerning propylene polymerisation in the presence of homogeneous vanadium-based Ziegler-Natta catalysts, the syndiospecificity of the polymerisation is believed [387,395] to arise from steric repulsions between the last inserted monomer unit of the growing chain and the methyl group of coordinated propylene molecule, i.e. chain end stereocontrol is postulated to play the essential role in the stereoregulation. 

Chain end stereocontrol, which gives rise to single-inversion pentads, mmrm and mmmr, as main error signals in 13C NMR spectra, is characteristic of propylene polymerisation with the Cp2TiPli2— [A Me) ] catalyst at lowered temperature (leading to stereoblock polypropylene) in this case an error pentad distribution is observed close to mmmr mmrr mmrm mrrm= 1 0 1 0 (Figure 3.35a) [1,30]. 

Figure 3.47 Typical steric defects (pentad distribution of stereoerrors) in a (mainly) syndiotactic poly(a-olefin) chain (a) isolated m diad, characteristic of chain end stereocontrol (b) pair of m diads, characteristic of enantiomorphic site stereocontrol during the propagation...

A much lower stereospecificity of the ZrBz4—[Al(Me)0]x catalyst for styrene polymerisation (ca 58% of the hot acetone-insoluble polymer fraction [56]) compared to that for the respective Ti-based catalyst should be noted. This can be explained in terms of the larger radius of zirconium than titanium, thus resulting in the impossibility of sufficiently effective chain end stereocontrol. 

Note the chain end stereocontrol mechanism operating in cyclopentene polymerisation, which is similar to norbornene polymerisation but differs from the case of norbornene in the fact that in cyclopentene polymerisation the chiral centres of the growing polymer chain are located in the a and y position [15]. 

It is well accepted that two mechanisms of stereocontrol (the chiral induction responsible for selecting the monomer enantioface) are operative in stereoselective a-olefm polymerizations. In the simpler cases, the discrimination between the two faces of the prochiral monomer may be dictated either by the configuration of the asymmetric tertiary C atom of the last inserted monomer unit or by the chirality of the catalytic site. These two different mechanisms of stereocontrol are named chain-end stereocontrol and enantiomorphic-site or site stereocontrol. In the case of chain-end stereocontrol, the selection between the two enantiofaces of the incoming monomer is operated by the chiral environment provided by the last inserted tertiary C atom of the growing chain, whereas in the case of site stereocontrol this selection is operated by the chirality of the catalytic site. The origin of stereocontrol in olefin polymerization has been reviewed extensively.162,172-178... 

Chain-end stereocontrol with error propagation XVII... 

Mechanisms of Stereocontrol. Stereochemistry of the olefin insertion step can be controlled by both the steric environment of the active site (enantiomorphic-site control) as well as the growing polymer chain (chain end control). In chain end stereocontrol, stereospecificity arises from the chiral )3-carbon atom of the last enchained monomer imit, which in turn influences the stereochemistry of monomer addition. Chain-end control is usually less effective than site control and has been observed for some achiral metallocenes at low polymerization temperatures. Partially iPP resulting from chain end stereocontrol has been obtained with Cp2TiPh2/MAO (56,272). The syndiospecific polymerization of 1-butene using the Cp 2MCl2/MAO (M = Zr, Hf) catalyst systems has been described (273). Predominantly sPP has been obtained under chain end control, using Brookhart s diimine nickel catalysts (274-277). 

Minieri, G Corradini, R Guerra, G ZambeUi, A. Cavallo, L. A theoretical study of syndiospecific styrene polymerization with Cp-based and Cp-free titanium catalysts. 2. Mechanism of chain-end stereocontrol. Macromolecules 2001, 34,5379-5385. 

Scheme 2 (a) Chain-end stereocontrol mechanism, (b) Enantiomorphic site stereocontrol mechanism, (c) Tactic polymers via chain-end control mechanism, (d) Isotactic polymers via enantiomorphic site control mechanism. L pM-OR is an enantiomerically pure metal alkoxide complex that prefers R-monomer L n is an enantiomerically pure, chiral ligand. 

In principle, rac-lactide, a racemic mixmre of d- and L-lactide, may be polymerized in a stereoselective fashion. Depending on the stereoselection as the ROP proceeds, the resulting polymer may thus exhibit different stereoregularities these directly influence the thermal and mechanical properties of the produced PLAs. In this regard, isotactic PLA stereoblocks and PLA stereocomplexes, which are of interest for their thermal and mechanical properties, may be produced via the ROP of rac-lactide initiated by an achiral derivative, provided the polymerization proceeds via a chain-end stereocontrolled mechanism i.e., the last inserted lactide unit stereo-controls the insertion of the incoming monomer. This strategy has been first validated using salen-based aluminum complexes such as 16 (Scheme 16, top) to produce PLLA-PDLA isotactic stereoblocks [95, 96]. Alternatively, the chiral racemic salen aluminum complex 17 was found to be suitable for the parallel stereoselective synthesis of isotactic poly(D-lactide) and poly(L-lactide) from rac-... 

---