Isotactic-hemiisotactic polymer

SCHEME 2.5 Isotactic-hemiisotactic polymers should arise from a two-site catalyst that has one highly enantioselective coordination site (in gray) and one coordination site that is moderately enantioselective for the same monomer enantioface (in brackets). R is substantially larger than methyl the bridge substituents have been omitted for clarity P represents the growing polymer chain and M = cationic Zr" " or Hr ", generally. 

FIGURE 2.24 Typical isotactic polymers generally adhere to a linear relationship between the [mmmm] pentad fraction and melting temperature (gray circles), whereas isotactic-hemiisotactic polymers can exhibit faint melting endotherms at considerably higher temperatures (black squares) than predicted by this relationship. 

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. 

Modifications of Cs-symmetric metallocenes may lead to Ci-symmetric metallocenes (Fig. 8). If a methyl group is introduced at position 3 of the cyclopentadienyl ring, stereospecificity is disturbed at one of the reaction sites so that every second insertion is random a hemiisotactic polymer is produced (276, 277). If steric hindrance is greater (e.g., if a /-butyl group replaces the methyl group), stereoselectivity is inverted, and the metallocene catalyzes the production of isotactic polymers (178-180). 

A cyclopentadienyl, indenyl-ethane ligand undergoes insertion alternately in an isotactic and an atactic fashion to give a hemiisotactic polymer, as is to be expected. 

FIGURE 2.22 The [m] dyad fraction and the polymer melting temperature of isotactic-hemiisotactic polypropylenes can be controlled by manipulation of the catalyst steric framework. 

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. 

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]. 

An analogous method was used to obtain a new class of macromolecular stereoisomers The hemitactic polymers (99-101). This term refers to a head-to-tail vinyl polymer in which the tertiary carbon atoms constitute two distinct series one, which includes monomer units 1,3,5, 7,..., possesses strict steric regularity, whereas the other, with monomer units 2, 4, 6, 8,. .., is completely at random. In such polymers only one in every two tertiary atoms is influenced by an ordering rule 58 and 59 show the schematic structure of the hemiisotactic and hemisyndiotactic polymers where the white circles indicate the positions of disordered s ubstituents. The hemiisotactic polypropylene was obtained by Farina, Di Silvestro, Sozzani and Savar6 (99, 101) by nonste-reoselective reduction,of. isotactic frans-l,4-poly-2-methylpentadiene. 

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]. 

Substituents in the distal position of the cyclopentadienyl ring in these cyclopentadienyl-fluorenyl catalysts has a profound effect on the polymer product produced. A small substituent like methyl produces a novel material, hemiisotactic polypropylene, in which methine carbons of specific conformation alternate with methine carbons of random conformation. A larger substituent hke r-butyl makes isotactic polypropylene (Figure 6). 

Metallocenes with diastereotopic sites for monomer coordination show quite an interesting polymerization behavior introduction of a methyl group in position 3 of the cyclopentadienyl ring in (21) disturbs the stereospecificity at this site, giving rise to hemiisotactic polypropylene [53], while a f-butyl group at the same position inverts the preferred mode of coordination (22) thus an isotactic polymer is generated [54]. Metallocene (8) has one nonspecific and one stereo-specific site, too at low temperature, hemiisotactic polypropylene is produced while at high temperatures site isomerization without insertion facilitates the formation of isoblock polypropylene. 

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... 

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