Stereoblock copolymers

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. 

Ethylene-a-olefin copolymers, stereoblock polypropylene, or EPR Enhanced inter-spherulitic and interlamellar strength Lustiger 1993... 

ABA block copolymer ABC block copolymer Multiblock copolymer Stereoblock copolymer... 

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. 

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. 

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... 

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. 

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. 

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. 

The coefficients 8.10 and 0.010 in the second equation are usually ascribed to the reactivity ratios rj and rj (Table 19). This catalyst produces poly-propene consisting mainly of syndiotactic stereoblocks, together with short disordered blocks resulting from head-to-head (hh) and tail-to-tail (tt) pro-pene enchainment and occasional isolated isotactic units, and if these features apply to copolymers prepared with vanadium catalysts, the reaction is in effect a terpolymerization. Locatelli et al. [322] derive the equation for monomer/polymer composition ratios... 

Numerous initiator systems have been used for the tactic polymerization of methyl methacrylate (7, 8, 14). Reaction conditions determine the structure of each polymer, and each structure might vary considerably from one polymer to another. Nearly pure isotactic and syndiotactic polymers are known, in addition to all transitions between these limits and also stereoblock copolymers. In the amorphous state, the differenes in the individual structures are clearly shown by their ultrared (J) and nuclear resonance spectra (2,11). 

Figure 9.21 (a) Stereorigid an a-metallocenes for the production of isotactic and atactic polymers. (b) Oscillating metallocenes for the production of stereoblock copolymers. (After Coates and Way mouth, 1995.)... 

As discussed earlier, ethylene propylene rubber (EPR or EPM) has been blended with PP and PE to improve the impact strength and to render the materials softer. Recently, metallocene catalysts or postmetallocene catalysts provide new pathways to generate elastic copolymers that can replace EPR. These pathways possess cheaper manufacturing cost and generate new materials with better compatibility to PP or PE. Such new materials included ethylene-propylene random copolymers with dominant ethylene component (33-34) or propylene-dominant component (35 1), propylene-ethylene block copolymer (42), ethylene-octene copolymer (43), poly(propylene-co-ethylene) (44), ethylene-hexene copolymer (45), ethylene-butene copolymer (46), low isotactic PP (47), and stereoblock PP (48). These materials are generally compatible with PP or PE, thus can be used to tailor the toughness (or the softness) of... 

Stereoblock copolymers Finally, a very special structure can be formed from one monomer in which the distinguishing feature is the tacticity of each block, i.e.,... 

Monomers Figure3.8 shows a small selection of cyclic monomers suitable for ROP [43]. Additionally, three different stereoisomers of lactide exist as a consequence of the presence of two stereocenters per monomer unit, namely meso-, L- and d-lactide, see Fig. 3.9. Further, racemic mixture of L- and D-lactide are commercially available. While ROP of either pure l- and D-lactide enables synthesis of highly crystalline poly(L-lactic acid) or poly(D-lactic acid), ROP of rac- or wcj< -lactide with adequate catalysts allows the synthesis of stereoblock copolymers, heterotactic and syndiotactic poly(lactic acid). Notably, stereoregular PLAs display much lower rates of degradation than the amorphous atactic polymer. 

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