Isotactic polypropene

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. 

Using an intrinsically chiral titanium compound (rac ethylene-bis-indenyl titanium dichloride, Figure 10.7), first described by Brintzinger [14], Ewen [13] obtained polypropene that was in part isotactic. Subsequently Kaminsky and Brintzinger have shown that highly isotactic polypropene can be obtained using the racemic zirconium analogue of the ethylene-bis(indenyl) compound [15],... 

It is immaterial whether (1) or (2) is taken as the configurational repeating unit and ster corep eating unit of isotactic polypropene (see Definition 1.7) this is so because the two infinite chains, one built up of identical configurational units (1) and the other built up of... 

The chain conformation of isotactic polypropene in the crystalline state is . .. TGTGTGTG. .. 

The right-handed sense of a helix traces out a clockwise rotation moving away from the observer the left-handed sense of a helix traces out an anticlockwise rotation moving away from the observer, e.g., the. ..TG TG TG. .. helix of isotactic polypropene is left-handed. 

C NMR of isotactic polypropene shows the main error is pairs of racemic dyads instead of isolated racemic dyads (Table 8-3) [Heatley et al., 1969 Resconi et al., 2000 Wolfsgruber et al., 1975]. A stereoerror in the addition of a monomer molecule is immediately corrected when stereocontrol is by the chiral active site. If stereocontrol was due to the propagating chain end, an error would continue to propagate in an isotactic manner to yield a polymer, referred to as an isotactic stereoblock, containing long isotactic all-R and all-5 stereoblocks on each side of the error. 

C NMR of ethylene-propene copolymers of low ethylene content produced by initiators that yield isotactic polypropene shows that the isotactic propene units on each side of an ethylene unit have the same configuration (i.e., all-R or all-5) [Zambelli et al., 1971, 1978, 1979]. For stereocontrol by the propagating chain end, the amount of polymer in which the polypropene blocks on either side of an ethylene unit have the same configuration would equal that in which the blocks have the opposite configuration. 

There are striking differences in physical properties between the atactic and isotactic forms. The atactic material is soft, elastic, somewhat sticky, and rather soluble in solvents such as 1,1,2,2-tetrachloroethane. Isotactic polypropene is a hard, clear, strong crystalline polymer that melts at 175°. It is practically insoluble in all organic solvents at room temperature, but will dissolve to the extent of a few percent in hot 1,1,2,2-tetrachloroethane. That the difference between the atactic and isotactic polymers arises from differences in the configurations of the methyl groups on the chains is shown in a... 

Figure 29-9 Proton-decoupled 13C spectra of different polypropene samples taken in CHCI2CHCI2 solution at 150° at 15.9 MHz. The upper spectrum is of a highly isotactic polypropene, which shows only the faintest indication of lack of stereoregularity. The middle spectrum is of atactic polypropene, which shows a variety of chemical shifts for the CH3 groups as expected from the different steric interactions generated by random configurations of the methyl groups. The lower spectrum is of a sample of so-called stereoblock polymer, which is very largely isotactic. The 13C spectrum of syndiotactic polypropene looks exactly like that of the isotactic polymer, except that the CH3— peak is about 1 ppm upfield of the position of the isotactic CH3 peak and the CH2 peak is about 1 ppm downfield of the isotactic CH2 peak.

Although both linear polyethene and isotactic polypropene are crystalline polymers, ethene-propene copolymers prepared with the aid of Ziegler catalysts are excellent elastomers. Apparently, a more or less random introduction of methyl groups along a polyethene chain reduces the crystallinity sufficiently drastically to lead to an amorphous polymer. The ethene-propene copolymer is an inexpensive elastomer, but having no double bonds, is not capable of vulcanization. Polymerization of ethene and propene in the presence of a small amount of dicyclopentadiene or 1,4-hexadiene gives an unsaturated heteropolymer, which can be vulcanized with sulfur in the usual way. 

Exercise 29-10 Suppose one had a sample of completely isotactic polypropene prepared from nonoptically active substances with the structure H-j-CH(CH3)— CH2 55-C(CH3)=CH2. 

Recent developments in catalyst design have offered more sophisticated catalyst structures with even better copolymerisation ability. Me2Si(Me-Benz(e)Ind)2ZrCl2 is an example of such a catalyst [57, 58]. This catalyst, as mentioned earlier, was originally designed for polymerisation of isotactic polypropene, but it also shows a very high comonomer response in ethene polymerisation [51]. 

FIG. 19.3 Boon s data as a function of the dimensionless parameter v[=(Tm - Tx)/(Tm - Tg)], the relative undercooling (Tx — crystallisation temperature). In this master-form the graph also fits with the data of Von Falkai (1960) for isotactic polypropene and with those of Van Antwerpen (1971,1972) on PETP. 

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... 

Besides the bis(indenyl) ansa compounds, C2 symmetric bridged bis(cyclo-pentadienyl) metallocenes of zirconium and hafnium were found to be able to produce isotactic polypropene (Table 8) [103]. The key for high isotaeticity are substituents in positions 2,4,3 and 5 generating a surrounding of the transition metal similar to the one in bis(indenyl) metallocenes. 

Table 12. Comparison of low molecular weight atactic polypropene from solvent extraction of a polymer produced using a conventional TiCU/MgCb catalyst with two high molecular weight atactic polypropenes of different molecular weight prepared by [Me2Si(Flu)2)ZrCl2MAO. For comparison typical data of an isotactic polypropene prepared by a conventional catalyst are also given [150]...

The properties and melting point of isotactic polypropenes prepared by metallocene catalysts are determined by the amount of irregularities (stereo- and regioerorrors) randomly distributed along the polymer chain. Thus the term stereospecificity does not refer to extractable aPP as for conventional PP always... 

Table 13. Comparison of isotactic polypropenes pepared by different metallocene/MAO catalysts-[En(IndH4)2ZrCl2 (I), [Me2Si(4,5BenzInd)2]ZrCl2 (II), [Me2Si(4,6iPrInd)2]ZrCl2 (III)- at 70°C in a 11 bulk polymerization at Al/Zr = 15 000 to conventional isotactic PP prepared by a TiCl4/MgCl2 catalyst (IV) [156]...

Table 16. Isotactic polypropene waxes prepared by [Me2Si(2Me-4tBu-Cp)2]ZrCl2/MAO (I), [Me2Si(IndH )2]ZrCl2/MAO (II) compared to waxes produced by conventional catalysts via molecular weight regulation by hydrogen (III) or visbreaking of PP random copolymers (IV) or homopolymers (V) [155]...

Metallocenes may also be fixed on silica or alumina by direct reaction of the two components. Marks has shown that reaction of dimethyl metallocenes with alumina results in the formation of an active catalyst [174]. Others have investigated the direct reaction of metallocene dichlorides with silica (and alumina) and faced the problem of metallocene decomposition [175-177]. Nevertheless an active species is formed which produces isotactic polypropene but with rather low activities. 

Figure 22-11 Stereochemistry of successive propane insertion steps into M—R bonds to give isotactic polypropene (a) and syndiotactic polypropene (b). In the absence of a stereocontrol mechanism atactic polypropene is formed. Note that in (a) the prochiral monomers coordinate to the metal with the same ff-face, so that an isotactic polymer is formed. This stereochemistry is favored by C2-symmetric ligands of type (22-XXVIII). In reaction (b) the second monomer coordinates with the opposite w-face to the first this stereochemistry is enforced by metallocenes of Cs-symmetry (22-XXX, R = H).

In heterogeneous catalysts this stereoselectivity to isotactic polypropene is achieved by control of the way the monomer is coordinated by the chiral /3-carbon of the polymer chain ( chain end control ). Thus conformation A in (22-XXVII) is sterically favored over intermediate B.101... 

Examples of important metallocene catalysts are the C2 symmetric compounds (22-XXVIII) and (22-XXIX), which are highly selective for isotactic polypropene,107... 

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