Stereoselective Polymerization of a-Olefins

The mechanism for the stereoselective polymerization of a-olefins and other nonpolar alkenes is a Ti-complexation of monomer and transition metal (utilizing the latter s if-orbitals) followed by a four-center anionic coordination insertion process in which monomer is inserted into a metal-carbon bond as described in Fig. 8-10. Support for the initial Tt-com-plexation has come from ESR, NMR, and IR studies [Burfield, 1984], The insertion reaction has both cationic and anionic features. There is a concerted nucleophilic attack by the incipient carbanion polymer chain end on the a-carbon of the double bond together with an electrophilic attack by the cationic counterion on the alkene Ti-electrons. 

Metallocenes based on group 4 metals were first synthesized by Wilkinson et al. in 1953 however, the great utility of these compounds as catalyst precursors for the stereoselective polymerization of a-olefins was not realized until the mid-1980s when two key discoveries were made. The first of these was the discovery that alkyl substituents placed on the cyclopentadienyl framework of the metallocene can significantly influence the performance and behavior of the catalyst. The second was the discovery that enantioselectivity in the insertion step of an a-olefin polymerization could result if one made metallocene catalysts with the appropriate chirality and stereorigidity. ... 

The last decade has seen significant new advances achieved in the field of living olefin polymerization. Many efficient and selective catalysts are now available for the hving polymerization of ethylene in addition to living and stereoselective polymerization of a-olefins, resulting in the creation of a variety of new polymer architectures, such as block copolymers and end-functionalized macromolecules. The ability to synthesize such polymers... 

The Nobel Prizes in chemistry in 1963 for Ziegler and Natta for their stereoselective polymerization of a-olefins " and in 2005 for Chauvin, Grubbs, and Schrock for their achievements in the domain of ring-opening metathesis polymerization (ROMP) shows the importance of the research in the field of catalytic polymerization techniques. 

Studies on stereoselective polymerization of racemic olefins also support this view.338 Polymerization of 3,7-dimethyl-l-octene (the chiral center is in a position to the double bond) took place with 90% stereoselectivity yielding an equimolar mixture of homopolymers of the two enantiomers. No stereoselectivity was observed in the polymerization of 5-methyl-1-heptene (the chiral center is in y position to the double bond). The conclusion is that the ability of a catalytic center to distinguish between the two enantiomers of a monomer required for stereoselective polymerization must arise from its intrinsic asymmetry. The first-ever chiral polypropylene synthesized using a chiral zirconium complex with aluminox-ane cocatalyst is the latest evidence to testify the role of the catalyst center in isotactic polymerization.339... 

In the field of the stereospecific heterogeneous polymerization of a-olefins, the stereoselective (106,118) and stereoelective (103) polymerization of racemic monomers, having an asymmetric carbon atom in a. 

Natta first reported the stereoselective (isotactic and syndiotactic) polymerization of a-olefins using heterogeneous catalysts.In addition, he was the first to describe the solid state structures... 

A necessary (but not sufficient) prerequisite for models of catalysts for the stereospecific polymerization of 1-olefins polymerization, is the stereoselectivity of each monomer insertion step. The possible origin of stereoselectivity in this class of systems was investigated through simple molecular mechanics calculations [11, 14, 24, 32, 52, 78-80, 82-86]. 

The remarkable dependence of the polymerization stereoelectivity on the nature of the catalyst (107) and some aspects of the stereoselective polymerization of racemic a-olefins, which will be discussed later, seem to be more consistent with the second or the third hypothesis than with the first one. 

The temperature-dependent stereoregular MAO-promoted polymerization of a-olefins502 has been explained by /i-hydrogen interactions in the olefin insertion and formation of a six-membered — 0 —Cp—D — Al—O—A1 ring TS. The stereoselective isotactic product formation occurs as a result of the substituent orientation at the -carbon (R1 vs CH2CHR2R4 in the conformationally restricted 426 equation 255). 

Insertion reations of olefins into metal-carbon bonds are fundamental to catalytic oligomerization and polymerization (e.g., Ziegler-Natta systems). Furthermore, this reaction may provide a method for stereoselective formation of a carbon-carbon bond. 

It is now clear that the advent of single-site catalysts for the stereoselective polymerization of olefins has initiated a revolution in polymer synthesis. Although the vast majority of homogeneous polymerization catalysts are designed for olefin polymerization, the extension of this area of catalysis for the polymerization of other monomers, such as lactones and epoxides, is already in progress. 

A major consequence of this pathway, also known as migratory insertion, is that the growing chain sweeps from one side to another with every addition of monomer. This is a generalization, for some authors have explained the loss of stereoselection in metallocene polymerizations of propylene, for example, at low monomer concentrations to the action of a windshield wiper isomerization, in which the polymer chain and open coordination site switch places without the benefit of monomer insertion. At low insertion rates, this site inversion phenomenon may become competitive with insertion and thus render ineffective any substituent influences which differ between the two faces of the catalyst site. With appropriate ligand design, different or enantiomeric steric environments may be created for the two sides of the active site. This makes possible stereoselective polymerization of propylene and higher a-olefins, as will be seen below. 

As discovered by Natta, Pino and coworkers(16,17), isotactic polymerization of chiral -olefins is stereoselective (e.g., isotactic poly-(R,S)-3-methyl-1-pen-tene consists of enantiomeric macromolecules which can be partially resolved.) If one considers that the isotactic steric control arises from the enantiose-lectivity of the chiral catalytic sites toward the enantiotopic carbon of the monomer(7,11) then stereoselectivity simply means that the insertion is dia-stereoselective. In other words, the diastereotopic faces of the monomer must have a different reactivity in the insertion (Figure 3) Figure 4 shows the - G NM spectrum of isotactic poly-(R,S)-3-methyl-l-pen-tene obtained in the presence of 6TiCl3-Al( - CH3)3 (sample 8). The resonances at 13.2Zj, 13A57i 15 09 and 15.2 ppm are due to the 0113 s of 2 - - C-2,3-di-methyl-pentyl end groups formed in the initiation step(l8). 

These processes resulted in racemic mixtures. However, the resolution of this mixture is believed to have occurred by spontaneous crystallization. This process most likely occurred by chance. Minerals such as natural dissymmetric quartz crystals and metal ions may have played a crucial role of optical selection by selective chelation of only one stereoisomer. After all, stereoselective polymerization of olefins by metal surfaces (Ziegler-Natta catalysts) is a well-documented industrial process for the synthesis of isotactic polymers. We also know the importance of metal ion binding in many biochemical transformations. It is essential for the maintenance of the native structure of nucleic acids and numerous proteins and enzymes. Other physical forces through radioactive elements, 7-radiation, or from cosmic rays, may have also been involved in optical selection. For instance, recent experiments with strontium-90 indicate that D-tyrosine is destroyed more rapidly than the naturally occurring L-isomer. It is tempting to incorporate such factors into the origin of dissymmetry in life process (46). 

Stereoselective or stereoelective polymerization of racemic monomers. The stereoselective polymerization of racemic monomers was first investigated in the case of racemic a-olefins (184). When the asymmetric carbon atom was in the a-position with respect to the double bond, essentially stereoregular polymers were obtained, the single macromolecules having isotactic main chain and carbon atoms with mostly the seune configuration in the lateral chains. The stereoselectivity decreases when the asymmetric carbon atom of the monomer is in the 3-position with... 

Let us remember that according to the lUPAC report [138], a stereoselective polymerization is a process in which a polymer is formed from a RS mixture of chiral stereoisomer by preferential incorporation of one species into a growing polymer chain and a statistical copolymer of R and S antipodes is not obtained. The occurence of stereoselectivity can be experimentally proved when it is possible to separate the polymer into fractions having optical activity of opposite sign as it has been made e.g. for chiral poly-a-olefins by Pino s [139] group. Informations on stereoselectivity can be gained from structural and physical analysis, mainly by high field NMR. 

Catalysts developed in the titanium-aluminum alkyl family are highly reactive and stereoselective. Very small amounts of the catalyst are needed to achieve polymerization (one gram catalyst/300,000 grams polymer). Consequently, the catalyst entrained in the polymer is very small, and the catalyst removal step is eliminated in many new processes. Amoco has introduced a new gas-phase process called absolute gas-phase in which polymerization of olefins (ethylene, propylene) occurs in the total absence of inert solvents such as liquefied propylene in the reactor. Titanium residues resulting from the catalyst are less than 1 ppm, and aluminum residues are less than those from previous catalysts used in this application. 

---