Stereoblocks

The successive repeat units in strucutres [VI]-[VIII] are of two different kinds. If they were labeled Mj and M2, we would find that, as far as microstructure is concerned, isotactic polymers are formally the same as homopolymers, syndiotactic polymers are formally the same as alternating copolymers, and atactic polymers are formally the same as random copolymers. The analog of block copolymers, stereoblock polymers, also exist. Instead of using Mj and M2 to differentiate between the two kinds of repeat units, we shall use the letters D and L as we did in Chap. I. 

Two random copolymers of this type are of importance, ethylene-propylene copolymers and ethylene-but-l-ene copolymers. The use and properties of polypropylene containing a small quantity of ethylene in stereoblocks within the molecule has already been discussed. Although referred to commercially as ethylene-propylene copolymers these materials are essentially slightly modified polypropylene. The random ethylene-propylene polymers are rubbery and are discussed further in Section 11.9. 

Optical properties of the blends are somewhat dependent on the molecular weight of the polystyrene, presence of additives such as lubricant in the polystyrene, ratio of polystyrene to SBS, processing conditions and mixing effectiveness of the extruder. It is stated that the optical properties of the sheets are similar whether linear or radial type stereoblock polymers are used. 

It is known the case of i-PP, for which the copolymerization with small amounts of ethylene tends to stabilize the y form [84] for instance, by melt crystallization of a copolymer with 6% by mol of ethylene more than 80% of the crystalline phase is in the y form [85], It is also known that the obtainment of the y form by melt crystallization, is also favored for samples of low molecular mass [86, 87] and for stereoblock fractions [88]. This seems to suggest that, whenever the preferential crystallization of the y-form is observed, there is the concomitant occurrence of a reduction in the polymer of the length of the chain stretches with polypropylene head to tail constitution and isotactic configuration. 

Stereoblock copolymers with blocks of the same monomeric unit but different configurations are out of the scope of this review. 

PP synthesized using TiCl4/Et3Al is mostly isotactic, but two minor fractions are also produced. One is a soluble, atactic PP, whilst the other fraction is a partially crystalline, elastomeric stereoblock of iso- and a-tactic PP sequences.98 Elastomeric PP may also be prepared using the ansa-titanoccnc complex, (26), (although this catalyst does undergo rapid deactivation).99 Stereoblock formation was attributed to an equilibrium mixture of slowly interconverting isospecific and aspecific catalyst sites. Other stereoblock PP materials have been prepared via chain transfer between two catalysts of different stereoselectivities.101,102... 

Elastomeric PP has also been synthesized using Ti, Zr and Hf ansa-metallocenes, (27). An alternative explanation for stereoblock formation was proposed, in which epimerization between isospecific and aspecific sites is rapid, affording predominantly atactic PP with short isotactic-rich sequences.103-105... 

Waymouth and coworkers reported a unique system where the unbridged bis(2-phenylindenyl)zirconium-based catalysts (129) gave elastomeric, isotactic-atactic stereoblock polypropylene, controlled by rotation of the 2-phenylindenyl as shown in Scheme VII [173]. 

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... 

According to analogous molecular mechanics analyses,38 this stereoselectivity mechanism would also operate for catalytic systems with oscillating stereocontrol, leading to atactic-isotactic stereoblock polymers,39,40 like those based on two unbridged 2-phenyl-indenyl ligands.40... 

Isotactic polystyrenes (IPS), 10 180 23 365 Isotactic propylene polymers, 17 703, 704 Isotactic-syndiotactic stereoblock PP, 16 110... 

The interaction may not be quite as strong as in the case of 2,1 insertion discussed above, but there will always be a tendency of the growing chain to arrive at an isotactic stereochemistry when 1,2 insertion occurs. One example of chain-end control leading to isotactic polymer was reported by Ewen [13] using Cp2TiPh2/alumoxane as the catalyst. The stereoregularity increased with lower temperatures at -45 °C the isotactic index as measured on pentads amounted to 52 %. The polymer contains stereoblocks of isotactic polymer. At 25 °C the polymerisation gives almost random 1,2 insertion and an atactic polymer is formed. 

Above we mentioned the results reported by Ewen [13] who found that Cp2TiPh2/alumoxane gives a polypropene with isotactic stereoblocks. Naturally, this achiral catalyst can only give chain-end control as it lacks the necessary chiral centre for site control. In the 13C NMR the stereoblocks can be clearly observed as they lead to the typical 1 1 ratio of mmmr and mmrm absorptions in addition to the main peak of mmmm pentads. These are two simple examples showing how the analysis of the 13C NMR spectra can be used for the determination of the most likely mechanism of control of the stereochemistry. Obviously, further details can be obtained from the statistical analysis of the spectra and very neat examples are known [18],... 

Polymer, the molecules of which consist of stereoblocks, and possibly some nonstereoregular blocks. 

In this case the blocks are stereoblocks but the block polymer is not a block copolymer because all the units derive from a single monomer. 

In the folowing example of a regular polypropene chain, the stereoblocks are denoted by I I. Here, the sequence of identical relative configurations of adjacent units that... 

Each of these types of copolymers offers different physical properties for a particular copolymer combination. It is interesting to note that block copolymers may be produced from one monomer only if the arrangement around the chiral carbon atom changes sequentially. These copolymers are called stereoblock copolymers. 

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... 

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