Stereoblock polymerizations

Chien JCW, Llinas GH, Rausch MD, Lin YG, Winter HH, Atwood JL, Bott SG (1992) Metallocene catalysts for olefin polymerizations. XXIV. Stereoblock propylene polymerization catalyzed by rac-anri -ethylidene(l-T 5-tetramethylcyclopentadienyl)(l-r 5-indenyl) dimethyltitanium A two-state propagation. J Polym Sci A 30 2601-2617... 

As stated above, we postulated that fast, reversible chain transfer between two different catalysts would be an excellent way to make block copolymers catalytically. While CCTP is well established, the use of main-group metals to exchange polymer chains between two different catalysts has much less precedent. Chien and coworkers reported propylene polymerizations with a dual catalyst system comprising either of two isospecific metallocenes 5 and 6 with an aspecific metallocene 7 [20], They reported that the combinations gave polypropylene (PP) alloys composed of isotactic polypropylene (iPP), atactic polypropylene (aPP), and a small fraction (7-10%) claimed by 13C NMR to have a stereoblock structure. Chien later reported a product made from mixtures of isospecific and syndiospecific polypropylene precatalysts 5 and 8 [21] (detailed analysis using WAXS, NMR, SEC/FT-IR, and AFM were said to be done and details to be published in Makromolecular Chemistry... 

Spassky and coworkers discovered a remarkable stereocontrol of an enantiomerically pure A1 complex (7 )-161a for the ROP of rac-lactide resulting in a tapered stereoblock PLA microstructure with high melting point =187 °C) (Fig. 26) [160]. Structurally analogous, racemic salen-Al complex 162 resulted in highly isotactic PLA [161]. Feijen s enantiopure chiral complex (RJ )-163 (Fig. 26) exhibited an excellent reverse stereocontrol by preferential polymerization of L-lactide over D-lactide monomer (Kss/Krr = 14) that resulted in PLA with... 

A series of pseudo-C or pseudo-C2 symmetric complexes 168-171 (Fig. 27) exhibited isotactic predominance P = 0.50-0.75) however, the isotacticity is compromised in solvent-free bulk polymerization at 130 °C [129]. Fluorous tertiary alcohol ligands with electron-withdrawing CF3 group are weakly basic and thus expected to reduce the possibility of catalyst deactivation by bridged species formation. Al complexes 172 and 173 offered highly isotactic-enriched stereoblock PLA (Pm = 0.87) from ROP of rac-lactide [168]. 

Unbridged metallocenes rarely achieve highly stereoselective polymerizations because free rotation of the r 5-ligands results in achiral environments at the active sites. An exception occurs when there is an appreciable barrier to free rotation of the r 5-ligands. Fluxional (con-formationally dynamic) metallocenes are initiators that can exist in different conformations during propagation. Stereoblock copolymers are possible when the conformations differ in stereoselectivity and each conformation has a sufficient lifetime for monomer insertion to occur prior to conversion to the other conformation(s). Isotactic-atactic stereoblock polymers would result if one conformation were isoselective and the other, aselective. An isotactic-atactic stereoblock polymer has potential utility as a thermoplastic elastomer in which the isotactic crystalline blocks act as physical crosslinks. 

Polymerization with oscillating metallocenes is complicated because solvent fractionation of the polymer product shows separate fractions—highly atactic, mostly isotactic, and isotactic-atactic stereoblock. The mechanism of this phenomenon is not clear. It may result from the initiators not being perfectly single-site initiators. There is some evidence that a metallocene initiator may consist of more than one species, and that each species produces a different stereochemical result (Sec. 8-5g-l, 8-5h-l). 

The atactic amorphous portion (9-16% of the total) contained in the obtained polypropylene has been separated by treating the raw polymer with n-heptane at room temperature. When operating in such a way, we have not separated the stereoblock fraction (extractable in boiling n-hep-tane) from the isotactic (not extractable in boiling n-heptane) fraction of polymer. The results reported in this paper are generally referred to the crystalline fraction, named non-atactic, which contains also some stereoblock polymers (at the considered polymerization temperatures, the latter generally correspond only to 5-7 % of the total) (9)... 

Kinetic analysis (1) indicates that the concentration of active centres is very low, probably in the iiM-mM range. This being so, spectrometric methods are unlikely to provide direct evidence of the covalent nature of the active centres or their detailed structures. Such methods are valid for investigating the structure of the initiator, but the nature of the active centre must be deduced from indirect evidence kinetic analysis, for instance, has shown that in the syndiotactic-like or stereoblock polymerizations initiated by n-butylmagnesium compounds monomer is coordinated to the active centre (8). 

Preliminary results on the kinetics of the polymerization and the efficiency of initiation of the isotactic polymerizations initiated by t-BuMgBr in toluene solution are consistent with the Bateup mechanism proposed for the stereoblock and syndio-tactic-like polymerizations initiated by n-BuMgBr in THF-rich solution — a mechanism which involves initiation and propagation through monomer — active centre complexes (5,8). 

COLEMAN and Fox (18) have pointed out that the non Bernoullian sequence distribution observed in some of these systems can be formed without the hypothesis of penultimate effects. All that is required is that two or more types of active species be present which do not rapidly interconvert. Each can add monomer at its own rate and with its own characteristic regulating effect. No penultimate effect is necessary but the sequence distribution will be non-Bernoullian. This type of mechanism is particularly attractive in the explanation of stereoblock polymer formation in the lithium alkyl systems in toluene with small amounts of ether present. The presence of at least two species of active centres has been inferred from an examination of polymer fractions obtained from butyllithium initiated polymerizations (19) in toluene. The change in molecular weight distribution with time suggests the presence of two... 

Commercial polymers have been produced from methyl, ethyl, isopropyl, n-butyl, isotubtyl, t-butyl, stcaryl, benzyl and trimethylsilyl vinyl ethers. The polylmethyl vinyl ether) called PVM or Resyn is produced by the polymerization of the monomer by boron trifluoride in propane at —40°C in the presence of traces of an alkyl phenyl sulfide. The polymer may have isotactic, syndiotactic or stereoblock configurations depending on the solvent and catalyst used. 

The tacticity of PLA influences the physical properties of the polymer, including the degree of crystallinity which impacts both thermo-mechanical performance and degradation properties. Heterotactic PLA is amorphous, whereas isotactic PLA (poly(AA-lactide) or poly (55-lac tide)) is crystalline with a melting point of 170-180°C [26]. The co-crystallization of poly (RR-lactide) and poly(55-lactide) results in the formation of a stereocomplex of PLA, which actually shows an elevated, and highly desirable, melting point at 220-230°C. Another interesting possibility is the formation of stereoblock PLA, by polymerization of rac-lactide, which can show enhanced properties compared to isotactic PLA and is more easily prepared than stereocomplex PLA [21]. 

Coates and co-workers reported polymerizations using both the enantiomerically pure (7) and the racemic (8) Ao-propoxide analogues [21], Complex 8 polymerized rac-lactide iso-selectively producing stereoblock PLA (P = 0.96). The isoselectivity was achieved due to the high selectivity of 7 for RR-lactide and corresponding S -enantiomer for S S -lactide. Further studies of the aluminium-salen system have led to the proposal that several factors influence the polymerization... 

Fig. 6.15. Formation of stereoblocks in a chain-end controlled isoatactic polymerization and schematic 13C NMR spectrum of methyl region.

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... 

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