Propylene monomer coordination

Here, the constants KM and ks denote the equilibrium constant for the propylene monomer coordination and the rate constant for the insertion of the coordinated monomer, and L represents a ligand bound to vanadium. Then, the energy diagram of chain propagation reaction with propylene may be visualized as in Fig. 13. The... 

Figure 7.14a illustrates the insertion of a propylene monomer into an edge vacancy in a crystal adjacent to an alkylated titanium atom. In Fig. 7.14b a cross-sectional view of the same site shows how the preferential orientation of the coordinated monomer is dictated by constraints imposed by the protuberances on the crystal surface. 

Polypropylene (PP) is a semicrystalline commodity thermoplastic produced by coordination addition polymerization of propylene monomer [197]. Most frequently, stereospecific Ziegler-Natta catalysts are used in industrial processes to produce highly stereospecific crystalline isotactic (iPP) and syndiotactic (sPP) polymer with a small portion of amorphous atactic PP as a side product. Polymerization of non-symmetrical propylene monomer yields three possible sequences however, the steric effect related to the methyl side group highly favors the head-to-tail sequence. The occurence of head-to-head and tail-to-tail sequences produces defects along the PP chain [198]. Presence of such defects affects the overall degree of crystallinity of PP. 

Eq. (21) has been interpreted in terms of a mechanism 84) where the rate-determining step of the propagation of a living polymer chain is the insertion of the coordinated propylene monomer into a vanadium-polymer bond, V -P. 

The relations (23) and (24) seem to indicate that a strong interaction between vanadium and the propylene monomer is unfavorable for the insertion of the coordinated propylene into a living polymer chain. 

For the discussed model sites, the two situations presenting the outward and inward propylene coordination are enantioselective and non-enantioselective respectively. Hence, these model sites are isospecific only under the assumption that propylene always coordinates at a given coordination position (i.e. that the chain skips back to the starting position after each monomer insertion and prior or simultaneously to the coordination of the new propylene molecule) [345]. 

Scheme 14 /3-H transfer to a coordinated propylene monomer after a secondary propylene insertion.

The first kinetic model for propagation in homogeneous systems was proposed by Ewen [47], assuming that the propagation took place as shown in Fig. 9.18. This scheme, shown for Cp2Ti(IV) polymerization of propylene, is representative of the kinetics for dl of the polymerizations with Group IVB metallocenes. In the scheme, species 1 and 4 represent coordinatively unsaturated Ti(IV) complexes that are-formally 16-electron pseudo-tetrahedral species, species 2 represents the interacting catalyst/cocatalyst combination, while intermediate 3 is shown with the monomer coordinated... 

Propylene monomer, like ethylene, is obtained from petroleum sources. Free-radical polymerizations of propylene and other a-olefins are completely controlled by chain transferring. It is therefore polymerized by anionic coordination polymerization. At present, mainly isotactic polypropylene is being used in large commercial quantities. There is some utilization of atactic polypropylene as well. Syndiotactic polypropylene, on the other hand, still remains mainly a laboratory curiosity. 

Another example of ionic graft copolymerization is a reaction carried out on pendant olefinic groups using Ziegler-Natta catalysts in a coordinated anionic-type polymerization. The procedure consists of two steps. In the first, diethylaluminum hydride is added across the double bonds. In the second the product is treated with a transition metal halide. This yields an active catalyst for polymerizations of a-olefms. By this method polyethylene and polypropylene can be grafted to butadiene styrene copolymers. Propylene monomer polymerization results in formations of isotactic polymeric branches ... 

Stereo- and Regiochemistry of Monomer Insertion. For substituted O -olefins a number of issues concerning monomer coordination/insertion must be considered. The way in which the monomer inserts itself into the pol5uner chain determines the microstructure of the polymer and subsequently the properties. Does the methyl group on the propylene end up towards the chain or away What is the position of the methyl group on the propylene molecule with respect to coordination site of the metal and the polymer chain, cis versus trans The way in which the propylene molecule may be inserted in the polymer chain by various mechanisms can give rise to complex phenomena of structural and steric isomerism. 

As mentioned previously, a two-stage reaction mechanism for the pol5mier-ization of propylene was proposed which consists of a coordination stage of the olefin then followed by an insertion step of the monomer into the Ti-pol5mier bond. Although this picture (Fig. 15) reveals the basic mechanism of monomer coordination and then insertion, it does not provide any insight into the mechanism for stereocontrol of the propylene monomer. This is addressed by chirality considerations. 

As discussed in Section 1.1.1.2, metallocene catalysts insert propylene from two sites in an alternating manner (exceptions will be discussed in Section 1.2.2.2). Each insertion of the prochiral propylene monomer into the growing polymer chain creates a new chiral center. The configuration of the new chiral center will be determined by which enantioface of propylene coordinated to the metal prior to insertion. The two enantiofaces, re and si, are shown coordinated to a metallocene in Figure 1.3. 

FIGURE 1.8 Lowest energy structures for propylene coordination to a chiral, C2-symmetric metallocene-isobutyl complex. For both enantiomers, the two sites shown for monomer coordination and chain growth are equivalent. 

L = phenoxy-imine ligand). Two subscript characters attached to k stand for regiochemistry of the last-inserted propylene unit (left) and that of a coordinating propylene monomer (right). For example, ifcps is the rate constant of a secondary (2,1-) insertion to a primary chain end. 

At low propylene monomer concentrations, the back-skip of the polymer chain (B C) is faster than monomer coordination. This leads to high isotacticities at low propylene concentrations and elevated temperatures. At higher propylene monomer concentrations, however, coordination of monomer at the less hindered site D is favored over back-skip of the polymer chain from site B to site C. Subsequent coordination of the monomer at site D followed by migratory insertion leads to the formation of a stereoerror (D E). Insertion from E F proceeds in a stereoselective way, similar to the process A B, but instead leads to the formation of an rr triad owing to the previous nonselective insertion (D E). At low propylene monomer concentrations, the back-skip of the polymer chain to the less encumbered site (F C) is favored over monomer coordination (F D). At site C, the catalyst follows the isotactic cycle A B C. 

---