Polymers, stereoblock properties

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. 

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]. 

Of all the catalyst types, the sterically fluxional bis(2-arylindenyl) complexes749,752-768 produce elastomeric PP with the best combination of properties, that is, relatively high melting points and very low crystallinity, due to their stereoblock nature,769-772 but unfortunately their activity and molecular mass capability are too low at industrial polymerization temperatures (60-80 °C). The mechanism of stereoblock formation originally proposed by the inventors74 has recently been questioned and an active role of the counterion has been proposed to better account for the heptad distribution in the 13C NMR spectra of the elastomeric, stereoblock pp 773>773a These polymers are PP reactor blends, since they can be fractionated with solvents into low- and high-tacticity components.162,767,769,770,774,775... 

The simple homoleptic complex Sn(Oct)2 catalyzes lactide polymerization both in solution and in the melt at temperatures >130°C and is the most widely used catalyst for lactide polymerization industrially. As previously mentioned, commercially available lactide with >90% L-LA polymerized with Sn(Oct)2 will generate -90% isotactic PLA. While this material has commercial applications, its thermal and mechanical properties are not suitable for many applications where polyolefins are typically used. One way to improve the polymer properties is by increasing the isotacticity or forming isotactic stereoblock PLA through stereoselective polymerization of L- and D-LA mixtures. However, this cannot be achieved with Sn(Oct)2 and other simple initiators. ... 

Collins has reported the synthesis of a related class of metallocenes (43), some of which form elastomeric, stereoblock polypropylene when activated by The elastomeric properties of the polymer formed using 43 (M = Hf, X = SiMe2 7)xn = 25 °C) were far superior to those formed by the other metallocenes in the study. The polymers made using 42 and 43 have similar microstructures, as well as physical and mechanical properties. However, after detailed microstructural analysis of the polymer the authors proposed an alternate mechanism of stereocontrol to Chien s site epimerization model (Scheme... 

The microstructure and stereoblock distribution peculiar of polypropenes produced with this class of catalysts imparts thermoplastic elastomeric properties to the polymers. Thermoplastic elastomers or elastoplasts (TPEs) owe their elastomeric properties of resiliency and high tensile strength to physical cross-linking (formation of hard domains in a soft matrix) due to the presence of short, crystallizable... 

MAJOR APPLICATIONS The polymers referred to in this chapter include those families of homopolymers of propylene which are known to have elastomeric recovery properties at reasonable molecular weight and for which properties have been attributed to a crystallizable-noncrystallizable (e.g., isotactic-atactic) stereoblock structure, or to a major component with a stereoblock structure, whether or not the compositions are homogeneous by solvent fractionation tests. Copol)rmers and blends are not deliberately included in the data presented, but are described in some of the references. (See also some of the closely related elastomeric polymers presented in the entry on Polypropylene, atactic in this handbook.) The criterion of multiple crystallizable blocks per polymer chain may be met in significant fractions of low-tacticity, low-stereoregularity polymers of very high molecular weight. 

PLA is a fascinating material and there are big differences in the thermomechanical properties of the polymer depending on the tacticity formed. For example, atactic and heterotactic PLA are amorphous and have no defined melting point. Polymerisation of either the pure l- or d- stereoforms produce isotactic PLA with a Tg of ca. 50 °C and a T , of 170-180 °C. However, a 50 50 mixture of PLLA and PDLA has a melting point of ca. 220-230 °C, this enhancement is due to the formation of a stereocomplex between opposite chiral chains. It is also possible, and indeed preferred, to produce the stereocomplexed polymer from the isoselective polymerisation of rac-LA forming stereoblock PLA. There are many examples of zirconium, and indeed group-4 metals as a whole, in the literature for the polymerisation of rac-LA, the next section will focus on the stereoselective examples of ROP. 

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... 

If stereocomplex formation were to occur only between I-PMMA and S-PMMA, then the mixing of isotactic-atactic-isotactic (lAtl)- and syndiotactic-atactic-syndiotactic (SAtS)-PMMA stereoblock polymers in complexing solvents should result in stereocomplexes imbedded in an atactic matrix (Figure 2). Here the stereocomplexes would act as physical cross-links providing a network structure from a single chemical composition. From an application viewpoint, such a material should possess interesting properties compared to conventional PMMA. 

Hirata, M., Kimura, Y., 2008. Thermomechanical properties of stereoblock poly(lactic acid)s with different PLLA/PDLA block compositions. Polymer 49, 2656—2661. 

The catalytic route closest to being completely managed and understood is stereoblock-isotactic polymerization in the presence of bis(2-Ar-indenyl) group 4 metallocenes with very bulky aryl substituents. The simple and clean mechanism of stereocontrol entails an oscillation of the active cation between the two enantiomorphous rac-like conformations. Unfortunately, the resulting polymers have fairly high crystallinity and thermoplastic properties which are of rather limited practical interest, compared with those in which the alternation of crystallizable and amorphous blocks results in materials behaving as TPEs. 

Whatever the synthetic strategy, it is highly advisable that the modem tools of high-field NMR microstmctural analysis are used for stereoblock polypropylene evaluation. Until recently, with few exceptions, polymer microstructures have been determined at an inadequate level of detail as a result, many stmcture/property studies are flawed at the origin. ... 

There is a clear trend in current polyolefin research toward the synthesis of novel and well-controlled polymer architectures, leading to materials with special properties and higher added value. Stereoblock polypropylenes can still represent a promising area for development it is up to those who invest in it to take full advantage of the modern experimental and theoretical toolkits, and to transform the traditional, substantially blind approach into a more conscious (and by no means less exciting) scientific adventure. 

Natta, G. Properties of isotactic, atactic and stereoblock homopolymers, random and block copolymers of a-olefins. J. Polym. Sci. 1959, 36, 531-549. 

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