Stereoregulation mechanism

It has been reported [497] for cyclopolymerisations with metallocene-based catalysts that the enantioface selectivity of the insertion of the first olefinic bond in the a, co-diolefin determines the relative stereochemistry between the rings (the occurrence of m or r diads), i.e. the tacticity of the cyclopolymer the diastereoselectivity of the subsequent cyclisation involving the remaining olefinic bond determines, on the other hand, the relative stereochemistry in the rings (the occurrence of M or R diads), i.e. the cis-trans geometrical isomerism of the rings. 

Heterogeneous Ziegler Natta catalysts were found [446,495,496] to polymerise a, ffl-diolefins, yielding polymers with predominantly occurring c/.v-fl-mcth-ylene-3-cyclopentyl) units. The cis-trans diastereoselectivity, which reflects the catalyst structure, can hardly be controlled with heterogeneous catalysts, but it is strongly influenced by substituents at the cyclopentadienyl ligands in 

The cyclopolymerisation of 1,5-hexadiene leads to polymers of substantially higher molecular weights than the polymerisation of 1-hexene with the same catalysts [498], undoubtedly owing to some steric hindrance of the atom transfer that usually terminates the growth of a polymer chain [30], 

The symmetry properties of cycloaliphatic polymers are such that polymers with certain microstructures, e.g. tram-isotactic poly (methylene-1,3-cyclopen-tane), are chiral therefore, the cyclopolymerisation of a, w-diolefins presents new opportunities in enantioselective polymerisation. The cyclopolymerisation of 1,5-hexadiene carried out with isospecific as well as trans selective catalysts of C2 symmetry, such as methylaluminoxane-activated resolved (li )-(Thind CH2)2Zr l,l -bi-2-naphtholate, yielded optically active tram-isotactic poly(-methylene-1,3-cyclopentane). The cyclopolymerisation with the (15) enantiomer of the catalyst gave an enantiomeric polymer [505], On the basis of analysis of 13C NMR spectra, the degree of enantioface selectivity for this cyclopolymerisation was estimated to be of 91% [503,505]. 

Under an exclusively enantiomorphic site control mechanism, the symmetry of the catalyst would mandate homofacial insertion and cyclisation to yield cis 

---

Further work on these systems would obviously be very welcome, considering the implications of such stereoregulated mechanisms to biological systems, and in view of the rather unexpected findings reported lately66,67 which add new interest to this field. 

Recently, Hogen Each et al. Q,4. 5) obtained the stereo-specific oligomerization of 2-VP, by using organollthlum derivatives as Initiators In THF solution. They proposed a stereoregulating mechanism mainly based on the analysis of the configuration of low molecular weight products formed. 

Our study concerns the polymerization of 2-VP, Initiated by organomagneslum derivatives of the type R Mg R, In hydrocarbon solvents. In order to obtain a detailed knowledge of the stereoregulating mechanism, we focused this study on the determination of the nature and of the structure of active centers. 

Obviously, among the 4 configurational stereoisomers, none seems strongly favored. Thus, from the structure of the living species only, we cannot propose a stereoregulating mechanism and we searched for an explanation by the mode of complexatlon of monomer molecules to the active centers. 

The study of the stereoregularity of the polymers prepared, provides also Information about the stereoregulating mechanism. The probability of formation of the different types of sequences, was determined on the basis of the resonance of the quaternary carbon of pVP (12). The NMR spectrum performed at 15 MHz allows one to determine the concentration of triads. The values summarized In Table 4 do not agree with those expected for bernoullllan statistics. Hence, more than the last unit of the living chain Is Involved In the process. In order to obtain more precise Information about the process, It is necessary to measure the probability of formation of pentads. Such measurements are possible with spectra performed at 63 MHz (Figure 18). In spite... 

Analysis of multads distribution by 13c 11 , propagation kinetics and study of electron density on active centers model molecules, suggest the stereoregulating mechanism. 

The strategy we adopted for attacking our problem, i.e., the complete understanding of the stereoregulation mechanism in the stereospecific polymerization reaction, has been successfully applied to the stereospecific polymerization of acetaldehyde and propylene oxide. The same strategy should be applicable also to other types of catalysts and monomers, even if the difficulty encountered in the experimental performance is greater. The fruitful harvest must await future investigation. 

At the end of considerations of the stereoregulation mechanism in the isospecific polymerisation of a-olefins with homogeneous catalysts obtained from chiral stereorigid metallocenes of class III with C2 molecular symmetry, let us address isospecific propylene polymerisation catalysts, which are methy-laluminoxane-activated non-bridged metallocenes (Table 3.2). 

It has been suggested recently [410] that, also in the case of heterogeneous isospecific Ziegler-Natta catalysts, the stereoregulation mechanism assuming a growing chain orientation is more reasonable than that predicted by the enantiomorphic site model [411,412],... 

The influence of electronic factors on the stereospecific behaviour of metallocene-based catalysts in a-olefin polymerisation is an important aspect of the stereoregulation mechanism. Two phenomena, lanthanide contraction and relativistic effects, that appear in group 4 transition metal series result in... 

Kuran, W., Stereoregulation Mechanism in Coordination Polymerization of Hydrocarbon Monomers. Part I Polymerization of ot-Olefins , Polimery, 39, 570-578 (1997). 

Which are the essential factors determining the stereoregulation mechanism in the polymerisation of ot-olefins with coordination catalysts ... 

Let us recall that the origin of stereospecificity in the syndiospecific polymerisation of styrene lies in a chain end stereochemical control mechanism [52,70]. Key features of the stereoregulation mechanism are stereorigid rf coordination of the growing chain end and diastereoselective rj2 coordination of the styrene... 

Considering the above stereochemical model for syndiospecific styrene polymerisation, one may conclude reasonably that tf coordination of the monomer at the active site could hardly be possible, and r 2 coordination would always be involved in the syndiospecific polymerisation of this monomer [87]. One should note that preliminary concepts concerning the stereoregulation mechanism of syndiospecific styrene polymerisation assumed the styrene monomer to undergo only t]4 coordination at the titanium centre, the propagating chain being anchored via a benzylic bond as an t]3 ligand at the titanium [44,55,70]. 

The mechanism of stereoregulation in the stereoselective polymerisation of propylene oxide with zinc dialkoxide and related zinc dialkoxide-ethylzinc alkoxide complexes has been satisfactorily explained by the enantiomorphic catalyst sites model prepared by Tsuruta et al. [52,75], According to this model, the presence of chiral sites with a central octahedral zinc atom, bearing the polymer chain and coordinating the monomer, was assumed to be the origin of the stereoregulation mechanism. 

A variety of stereoregulating mechanisms have been invoked to explain the stereochemistry of anionic polymerization of alkyl methacrylates. As discussed by Pino and Suter [189], although syndiotactic diads are thermodynamically slightly more favored over isotactic diads, the free energy differences are so small that the formation of stereoregular... 

Several methods have been proposed to explain the stereoregulation mechanism in metallocene catalysis. Most now agree that the active spedes is cationic, the insertion of the monomer is between the transition metal and growing chains, and that the steric structure of the catalyst determines the orientation of the incoming monomer. Without giving detailed analyses of all the models, the essential features can be summarized as follows. 

Reaction stereoregulation mechanisms have been reduced to mathematical, kinetic models in several cases. A one-parameter Bernoullian statistical model accounts for the NMR methyl pentad intensities of both syndiotactic and isotactic polypropylenes obtained at subambient temperatures with homogeneous, achiral catalyst precursors. The polypropylenes have. ..rrrrrmrrrrr... and. ..mmmmmrmmmmm... microstructures respectively. The m and r deffects are consistent with the chain-end configurations being responsible for stereoregulation. Two additional models of the chain-end control type are the first- and the... 

Cationic group 3 metal catalysts of the type [Ln(T -C5Me4R )R], which are isostructural with the trivalent titanium system [Ti(r -C5Me4R )R]+, clearly indicate that the structure of the active species as well as the stereoregulating mechanism are related, supporting the original proposal by Zambelli et al. [32],... 

---