Poly stereoblock

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]. 

Figure 3.6 Stereoisomerism of poly(ot-olefin)s - -CH(R) CH2- K-. Isoblock, stereoblock and isosyndioblock polymers...

During the last decade, a variety of new catalysts have been presented for the stereospecific polymerisation of a-olefins, based on non-bridged metallocene or stereorigid ansa-metallocene as the procatalyst and a methylaluminoxane activator [29,30,37,105-107,112-114,116-135], Apart from isotactic [118,119,124, 131,132] and syndiotactic [23,118,124,133] polypropylenes and other poly(a-olefin)s [121], hemiisotactic [112,121,124], isoblock [131,132,134], syndioiso-block (stereocopolymer) [127], stereoblock isotactic [135] and stereoblock isotactic atactic [116,128,129] polypropylenes have been obtained using these new catalysts. 

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... 

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... 

Poly(7V-isopropylacrylamide) stereoblock polymer (isotactic-atactic-isotactic) water 2 434... 

Poly(methyl methacrylate) which has a low heterotactic content compared with isotactic and syndiotactic one was first called a stereoblock polymer, but afterwards it was found that the pdymer often was a mixture of isotactic and ndiotactic polsmaers. The two tactic isomers in the mixture could be recog nized by thin layer chromatography on silica-gel, and recently separated by a competitive adsorption method. ... 

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. 

The stereocomplex formation of PLLA and PDLA is enhanced in the presence of stereoblock poly(lactic acid) [419], Stereocomplex formation was proposed to be a novel method of controlling the protein- and cell-adhesive properties of biodegradable matrices composed of PEG-PLA copolymers [420], A triangular growth mechanism was proposed for the PLLA/PDLA stereocomplex [95], and the nucleating mechanism is not altered by the PLLA/PDLA blending ratio [421],... 

Tsuji, H., Shimizu, K., Sakamoto, Y. and Okumura, A. (2011) Hetero-stereocomplex formation of stereoblock copolymer of substituted and non-substituted poly(lactide)s. Polymer, 52, 1318-1325. 

Fukushima, K.,Hirata, M. andKimura, Y. (2007) Synthesis and characterization of stereoblock poly(lactic acid)s with nonequivalent D/L sequence ratios. Macromolecules, 40, 3049-3055. 

Fukushima, K., Chang, Y.-H. and Kimura, Y. (2007) Enhanced stereocomplex formation of poly(L-lactic acid) and poly(D-lactic acid) in the presence of stereoblock poly(lactic acid). Macromolecular Bioscience, 7, 829-835. 

MUN Munk, T., Hietala, S., Kalliomaeki, K., Nuopponen, M., Tenhu, H., Tian, F., Rantanen, J., and Baldursdottir, S., Behaviour of stereoblock poly(A/-isopropyl acrylamide) in acetone-water mixtures, Polym. Bull, 67,677,2011. 

Anionic and Cationic Polymerizations o Radical Polymerization Advances o Coordination Polymerizations 0 Step-Growth Polymerization Advances 0 Synthesis of Tactic Polymers o Stereoblock Copolymers o Dispersion Polymerizations o Cellulosic Graft Copolymers o Diels-Alder Polymer Forming Reactions o A New Path To Phenolic Resins o Nitrogen Heterocycle Polymerizations o Optically Active Polymers o Poly (Phenylene Sulfide) o Poly (Aryl Ethers) o (Poly (Aryl Ether Sulfones) o Epoxy and Isocyanate Resin Replacement o Azlactone Functionalized Oligomers o Epoxy Resin-Isocyanate Reactions o Chelating Polymers o Oxazoline Functionalized Polymers o Poly (Alkyl Methacrylates) o Macromers... 

Ewen, J. A. Jones, R. L. Elder, M. J. Camurati, I. Pritzkow, H. Stereoblock isotactic-hemiisotactic poly(propylene)s and ethylene/propylene copolymers obtained with araa-cyclopenta[l,2-b 4, 3-b ]dithiophene catalysts. Macromol. Chem. Phys. 2004, 205, 302-307. 

Site epimerization is estimated to be fast relative to olefin uptake and insertion. Thus, repetitive enchainment of one olefin enantiomer will occur, producing an isotactic block of poly(a-olefin) until a syndiotactic enchainment error occurs. When this happens, olefin insertion will be switched to the opposite side, and another block of isotactic poly(a-olefin) will be produced. Thus, the resulting polymer is predominantly isotactic with isolated r dyads stereoblocks of the two olefin enantiomers surround each isolated r dyad (Figure 4.23). 

For example, see Zhang, Y. Keaton, R. J. Sita, L. R. Degenerative transfer living Ziegler-Natta polymerization Application to the synthesis of monomodal stereoblock polyolefins of narrow poly-dispersity and tunable block length. J. Am. Chem. Soc. 2003, 125, 9062-9069 and references therein. 

EIGURE 20.7 Dyad structures of hydrogenated poly((-t-)-enrfo,exo-5,6-dimethylnorbornene) (a) and stereoblock polymer composed of repeating monomer (M) units linked in a racemic (r) and meso (m) fashion (b). 

Stereoblock copolymers comprised of a sequence of high-trans poly(2,3-(CF3)2NBD) followed by a sequence of high-cis poly(2,3-(CF3)2NBD) are obtained when ROMP is carried out initially with 16a and the tert-butoxide ligands are subsequently replaced by hexafluoro-tm-butoxide before... 

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