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We report the design, synthesis, morphology, phase behavior, and mechanical properties of semicrystalline, polyolefin-based block copolymers. By using living, stereoselective insertion polymerization catalysts, syndiotactic polypropylene-blockpoly(ethylene-co-propylene)-block-syndiotactic polypropylene and isotactic polypropylene-block-regioirregular polypropyleneblock-isotactic polypropylene triblock copolymers were synthesized. The volume fraction and composition of the blocks, as well as the overall size of the macromolecules, were controlled by sequential synthesis of each block of the polymers. These triblock copolymers, with semicrystalline end-blocks and mid-segments with low glass-transition temperatures, show significant potential as thermoplastic elastomers. They have low Young's moduli, large strains at break, and better than 90% elastic recovery at strains of 100% or less. An isotactic polypropylene-block-regioirregular polypropylene-block-isotactic polypropylene-block-regioirregular polypropylene-block-isotactic polypropylene pentablock copolymer was synthesized that also shows exceptional elastomeric properties. Notably, microphase separation is not necessary in the semicrystalline isotactic polypropylenes to achieve good mechanical performance, unlike commercial styrenic thermoplastic elastomers.block copolymer ͉ polypropylene T he applications of a polymer are largely determined by its chemical, physical, and mechanical properties. These properties are in turn determined by polymer morphology, which is dictated by polymer structure and composition. Thermoplastic elastomers are a classic example where polymer architecture engenders unique properties. Block copolymers that contain at least two blocks that are hard at room temperature, separated by blocks with a glass transition temperature (T g ) below room temperature, typically exhibit elastomeric properties if the low T g block volume is large (1-5). The most well known elastomers of this type are the polystyrene (PS)-block (b)-polybutadiene-b-PS triblock copolymers, sold commercially by Kraton Polymers. Such materials normally possess a microphase-separated morphology in which 10-nm-scale domains of the hard blocks (e.g., PS) are embedded as spheres or cylinders within a continuous phase of the low T g soft blocks. The hard domains serve as thermally reversible crosslinks that at room temperature produce high levels of recoverable elasticity in the soft phase.Sequence control of a synthetic polymer is most easily accomplished by using a synthetic technique that allows the sequential addition of one or more monomers to the macromolecule in a chain growth process without spontaneous termination. These techniques are collectively called living polymerizations, and despite many successes over the last half century, their further development remains one of the most important frontiers in polymer science. In the case of the Kraton triblock copolymers, a living anionic polymerization process is used to synthesize the materials. Because approximately t...

Wormlike micelles are assemblies of amphiphilic molecules of intermediate mean curvature between spherical micelles and flat bilayer membranes, which often form in solutions of peptide amphiphiles (hydrophilic peptide modules conjugated to hydrophobic subunits). In an effort to better understand the factors controlling peptide amphiphile (PA) micellar shape, we synthetically linked a short peptide with an alpha-helix-forming tendency to a hexadecyl tail. These molecules initially dissolve as spherical micelles, which can persist for hours or days, followed by transformation to wormlike micelles, which occurs simultaneously with a transition in the secondary structure of the headgroup peptides to beta-sheet. This observation provides evidence that the extended micelle is the thermodynamically favored state sought by PA micelles in the process of forming beta-sheet structures among the head-groups, though they are not the structures formed during the initial kinetics of assembly.

General methods. All manipulations of air-and/or water-sensitive compounds were carried out under dry nitrogen using a Braun UniLab drybox or standard Schlenk techniques.1 H NMR spectra were recorded using either a Varian Mercury (300 MHz) or Varian Inova (500 MHz) spectrometer. 13 C NMR spectra were recorded on a Varian Inova (500 MHz) spectrometer equipped with a 1H/BB switchable with Z-pulse field gradient probe and referenced versus residual non-deuterated solvent shifts. Mass spectrometry analyses were performed by the School of Chemical Sciences Mass Spectrometry Laboratory at the University of Illinois at Urbana-Champaign. Elemental analyses were performed by Robertson Microlit Laboratories, Inc. Madison, New Jersey. The LDI mass spectrum was recorded using a Waters MALDI Micro MX system. The sample was prepared by using the dried droplet method with no matrix present. Ionization was by a 257 nm UV nitrogen laser and the accelerating potential was 17.2 keV. The spectrum was recorded using the reflectron in positive ion mode.Materials. Toluene and hexanes were purified over columns of alumina and copper (Q5). Methylene chloride and THF were purified over an alumina column and degassed by three freeze-pump-thaw cycles before use. Methylmagnesium bromide (3.0 M in Et 2 O) (Aldrich), 3,5-dimethylbenzoate (Alfa Aesar), 1-naphthoicacid methyl ester (TCI), zinc dust (Aldrich), diiodomethane (Aldrich), titanium(IV) chloride (Aldrich, 1.0 M in CH 2 Cl 2 ), 3',4'dimethylacetophenone (Avocado), 6-acetyl-1,2,3,4-tetrahydronaphthalene (Alfa Aesar), 2,6di-tert-butyl-4-methylphenol (BHT) (Aldrich), P 2 O 5 (Aldrich), α-methylstyrene (Aldrich), 2iso-propenylnaphthalene (TCI), acenaphthenequinone (Alfa Aesar), formic acid, acetic acid, p-toluenesulfonic acid, p-toluidine (Acros), CF 3 SO 3 H (Aldrich) and (dimethoxyethane)NiBr 2 ((DME)NiBr 2 , Aldrich) were used without further purification. . MeMgBr (3.0 M in Et 2 O, 19.7 mL, 59.2 mmol) was slowly added to a 0 °C solution of methyl 3,5-dimethylbenzoate (3.89 g, 23.7 mmol) in S3 40 mL THF, under N 2 . The resulting solution was allowed to warm to room temperature. After 4.5 hours, the reaction mixture was cooled to 0 °C and saturated NH 4 Cl(aq) was added. To dissolve the inorganic byproduct, 2 M HCl(aq) was carefully added to the resulting suspension. The mixture was extracted three times with diethyl ether and the organic layer was washed successively with water, saturated NaHCO 3 (aq) and brine and then dried over MgSO 4 . Volatiles were removed in vacuo to yield a yellow liquid. (2.82 g, 72.4% yield). 2-(3,5-Dimethylphenyl)propan-2-ol