Hemiisotactic polymers

In the absence of degeneracy the number of possible stereosequences is equal to 2" (n being the number of stereogenic centers present in the sequence). In vinyl polymers, because of the equivalence of the two directions of the chain, many sequences differ only in the way they are observed (e.g., rrmr and rmrr) and must, therefore, be considered only once. The number of independent sequences, calculated by the Frisch, Mallows, and Bovey formula 10, 44) is shown in Table 2. In hemiisotactic polymers, where strict selection criteria exist, this number is drastically reduced (100). 

Sequences Nonvinyl Polymers" Vinyl Polymers Hemiisotactic Polymers... 

Sequence Chain End Control Enantiomeric Site Control Two-Parameter Nonsymmetric Chains Hemiisotactic Polymers ... 

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 11. Asymmetric (Ci) catalyst precursors for the synthesis of hemiisotactic polymers.

The correct pentad distribution for a perfectly hemiisotactic polymer is the following [mmmm] [mmmr [rmmr [mmrf [rrrf [mrrr]. [mrrm] = 3 2 1 4 3 2 1. 

Finally a hemiisotactic polymer (d) is obtained with the system Me2C(3-Me-cyclopentadienyl)(9-fluorenyl)ZrCl2 (4) / MAO. 

Figure 2.19 depicts several reported hemiisoselective fluorenyl-containing metallocenes. The parent catalyst, h-1, appears to be the most hemiisoselective with [m] = 49.6% at Tp = 20 C. Any deviation in catalyst stracture tends to effect less hemiisotactic polymers. For example, ethyl substitution (h-S) instead of methyl substitution (h-l) results in a measurable increase in syndiose-lectivity. The indenyl variant h-4 commits a considerable fraction of stereoerrors ([mmrm] + [rrmr] + [mrmr] = 7.5%at7 p = 10°C). " In addition, the hafnium complex h-l-Hf is more isoselective than the parent zirconium complex h-1 under identical conditions ([mmmm] = 24.2% versus [mmmm] = 15.1% at Tp = 60 °C). ... 

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. 

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. 

---