Syndiotactic-hemiisotactic polymer

SCHEME 2.6 Syndiotactic-hemiisotactic polymers result from a two-site catalyst that has one highly enan-tioselective coordination site (in gray) and one coordination site that is moderately enantioselective for the opposite monomer enantioface (in hrackets). The bridge substituents have been omitted for clarity P represents the growing polymer chain and M = cationic Zr or Hf ", generally. 

The statistical treatment of a hemiisotactic polymer can be made on the basis of a single parameter a the corresponding formulas are reported in Table 4, last column. For extreme values of a the polymer is no longer hemiisotactic but syndiotactic (for a = 0) or isotactic (for a = 1). The particular distribution existing in the hemiisotactic polymer is not reproducible with either the Bernoulli or the Markov processes expressed in m/r terms. 

In addition to polypropylenes in which the entire polymer chain consists of isotactic, syndiotactic, hemiisotactic, or atactic chains, polypropylenes in which the chain consists of alternating blocks of two microstructures have been prepared. Perhaps the most interesting and useful of these stereoblock polymers consists of the combination of a crystalline block, such as a unit of isotactic or syndiotactic polypropylene, and an amorphous block, such as atactic polypropylene. Polymers containing this combination of microstructures often behave as a thermoplastic elastomer and have properties... 

FIGURE 2.27 The [m] dyad fraction and the polymer melting temperature of syndiotactic-hemiisotactic polypropylenes can be controlled by manipulation of the catalyst steric framework. Flu = fluorenyl Oct = octamethyloctahydrodibenzofluorenyL... 

Figure 2.27 plots the observed polymer melting temperature as a function of the [m] dyad composition. The syndiotactic-hemiisotactic polypropylenes with [m] > 30% exhibit no strong endotherm through DSC and are essentially amorphous. Below [m] 25%, normal DSC traces are obtained and melting temperature is proportional to the syndiotacticity as quantified by 1 — [m]. For comparison, a data point for highly syndiotactic polypropylene from 5-90 (R = has been included. 

The fluorenyl ligand has proven to be an essential component in many homogeneous, stereoselective olefin polymerization catalysts. Since the first fluorenyl-containing metallocene was employed in 1988 for syndioselective propylene polymerization, other tacticities have been accessed isotactic, hemiisotactic, isotactic-hemiisotactic, and syndiotactic-hemiisotactic. Indeed, fluorenyl-containing metallocenes are remarkably versatile, valuable for mechanistic inquiry, and functional in the synthesis of polymers with novel structures and properties. 

Further support for this mechanism was provided by Ewen in the form of a catalyst which polymerizes propylene to hemiisotactic polypropylene. The metallocene shown in Scheme IV has two different coordination sites, one which is isospecific and one which is aspecific.51 When used for propylene polymerization, the alternation between iso- and aspecific sites results in a hemiisotactic polymer (Scheme IV). The polymer was readily characterized due to the pioneering work of Farina, who independently prepared this material previously. The rational synthesis of isotactic, syndiotactic, and hemiisotactic polyolefins represents a crowning achievement in the application of transition metal catalysts in stereocontrolled reactions. 

A model assuming that Cp substituents distal to the bridge experience steric non-bonded contacts with the monomer methyl group, perhaps mediated by the chain end, accounts for the specificity of the chiral metallocenes that produce isotactic, atactic, syndiotactic, hemiisotactic, and random or block cotactic polypropylenes. The tacticities as well as the microstructures of these polymers are accomodated by these simple concepts, the geometry of the metallocene ligands, and by generally accepted fundamental aspects of the polymerization and stereochemical control mechanisms. 

It should be noted that other polymer structures can be postulated—those where one substituent is atactic while the other is either isotactic or syndiotactic or those where one substituent is isotactic while the other is syndiotactic. However, these possibilities are rarely observed since the factors that lead to ordering or disordering of one substituent during polymerization generally have the same effect on the other substituent. An exception is the formation of hemiisotactic polypropene where isotactic placements alternate with atactic placements [Coates, 2000]. 

The polymer made by Ewen using 39 was readily characterized since Farina had previously prepared this hemiisotactic material by an alternate route. Microstructural analysis using C NMR spectroscopy revealed the virtual absence of methyl shifts due to isolated r or m dyads mrmr, rmrr, and mrmm pentads are absent) and the appropriate ratio of the remaining pentads " which is consistent with the hemiisotactic microstructure. Razavi has suggested the polymer might actually have an isotactic—syndiotactic stereoblock microstructure.Such a polymer should have a [mmrr] [mrrr] ratio of... 

It is satisfying to note that the syndiotactic ( F = 0.75) and isotactic ( F = 1.00) polymers represent local maxima on the stereoregularity curve. In the hemiisotactic continuum, the polymer with the least stereoregularity occurs when [m] = 0.375 = 0.609375). This mathematical analysis of... 

In a perfectly random, atactic polymer, the position of pendant groups varies randomly along the chain. Such a random distribution is said to be BemouUian or to be a zero-order Markov chain. There are also intermediate structures, such as hemiisotactic and isotactic stereoblock polymers, which have randomly occurring, short rims of isotactic or syndiotactic structures. A detailed explanation of tacticity is given by Koenig [34]. 

Using high-field NMR it is possible to detect the presence of these defects in isotactic and syndiotactic polypropylene prepared with special catalyst systems [20]. The 150-MHz C-NMR spectra (Fig. 7.14) are shown of the methyl region of four model polypropylene samples A-D dissolved in 1,1,2,2-tetrachloroethane-r/2 at 70°C. Polymer A was obtained in the presence of the catalyst system bis(cyclopentadienyl)-TiCh/methylalumoxane at low temperature (m 0.80) (see also Table 7.10). Polymer B was prepared with the catalyst system rac-ethylenebis(4,5,6,7-tetrahydro-l-in-denyl)ZrCl2 (m 0.70). Polymer C was prepared in the presence of (methyl)2C(cy-clopentadienyl)(9-fluorenyl)ZrCl2 (r 0.80). Polymer D is a hemiisotactic polypropylene obtained from the catalyst system (methyl)2C(cyclopentadienyl)(9-fluorenyl)-ZrCh/methylalumoxane. 

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