Shear-Enhanced Crystallization in Isotactic Polypropylene. 1. Correspondence between in Situ Rheo-Optics and ex Situ Structure Determination

The effects of “short term shearing” on the subsequent crystallization of a polydisperse Ziegler−Natta isotactic polypropylene are observed using in situ optical measurements and ex situ microscopy. Imposition of brief intervals of shear (0.25−20 s, less than a thousandth of the quiescent crystallization time) can reduce the crystallization time by 2 orders of magnitude (e.g., at 141 °C with a wall shear stress of 0.06 MPa). With increasing shearing time, the crystallization time saturates and highly anisotropic growth ensues. This transition to oriented growth correlates with changes in the transient behavior during flow and the semicrystalline morphology observed ex situ. During flow, we observe the generation of long-lived, highly oriented structures (evident in the transient birefringence) under all conditions that induce subsequent growth of highly oriented crystallites. In turn, the development of oriented crystallites observed in situ after cessation of flow correlates with development of a “skin-core” morphology (highly oriented skin on a spherulitic core) observed ex situ. Interestingly, the long-lived structures generated during flow appear at shorter times with increasing temperature (at fixed shear stress), the opposite of the trend one would expect on the basis of the temperature dependence of quiescent crystallization

Shear-Enhanced Crystallization in Isotactic Polypropylene. 1. Correspondence between in Situ Rheo-Optics and ex Situ Structure Determination

The effects of “short term shearing” on the subsequent crystallization of a polydisperse Ziegler−Natta isotactic polypropylene are observed using in situ optical measurements and ex situ microscopy. Imposition of brief intervals of shear (0.25−20 s, less than a thousandth of the quiescent crystallization time) can reduce the crystallization time by 2 orders of magnitude (e.g., at 141 °C with a wall shear stress of 0.06 MPa). With increasing shearing time, the crystallization time saturates and highly anisotropic growth ensues. This transition to oriented growth correlates with changes in the transient behavior during flow and the semicrystalline morphology observed ex situ. During flow, we observe the generation of long-lived, highly oriented structures (evident in the transient birefringence) under all conditions that induce subsequent growth of highly oriented crystallites. In turn, the development of oriented crystallites observed in situ after cessation of flow correlates with development of a “skin-core” morphology (highly oriented skin on a spherulitic core) observed ex situ. Interestingly, the long-lived structures generated during flow appear at shorter times with increasing temperature (at fixed shear stress), the opposite of the trend one would expect on the basis of the temperature dependence of quiescent crystallization

Dynamics of Shear-Induced Alignment of a Lamellar Diblock: A Rheo-optical, Electron Microscopy, and X-ray Scattering Study

In-situ rheo-optical methods are used to guide electron microscopy (TEM) and X-ray scattering (SAXS) studies of structure development during flow-induced alignment in a lamellar block copolymer melt (nearly symmetric polystyrene−polyisoprene diblock, ODT ≃ 172 °C). The progress of shear-induced alignment is recorded in real-time using flow birefringence; at selected points during alignment samples are taken for ex-situ characterization by TEM and SAXS along all three axes (v, ∇v, ∇ × v) of the flow geometry. Three different trajectories are examined:  perpendicular alignment and two qualitatively different routes to parallel alignment in the high-frequency regime (ω > ω'_c). In general, the initial “fast” process not only enhances the projection of the orientation distribution that corresponds to the final state but also increases other projections of the distribution; the late-stage “slow” process eliminates these other projections and perfects a single alignment. For example, the highest frequency path to parallel alignment begins by transforming poorly organized regions into layers that are predominantly oriented along the parallel and transverse directions. The transition to the slow process is marked by the development of a characteristic texture in which tilt wall boundaries normal to the flow direction separate bands that form a repeating “chevron” pattern (layers tilted up and down about the ∇×v axis). The coarsening of this pattern dominates the slow process, during which the transverse projection is also eliminated

CCDC 1549064: Experimental Crystal Structure Determination

An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures.An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures

Catalyst-Free Synthesis of Borylated Lactones from Esters via Electrophilic Oxyboration

A catalyst-free oxyboration reaction of alkynes is developed. The resulting borylated iso­coumarins and 2-pyrones are isolated as boronic acids, pinacol­boronate esters, or potassium organo­trifluoro­borate salts, providing a variety of bench-stable organo­boron building blocks for downstream functionalization. This method has functional group compatibility, is scalable, and proceeds with readily available materials: <i>B</i>-chloro­catechol­borane and methyl esters. Mechanistic studies indicate that the <i>B</i>-chloro­catechol­borane acts as a carbophilic Lewis acid toward the alkyne, providing a mechanistically distinct pathway for oxy­boration that avoids B–O σ bond formation and enables this catalyst-free route

Mechanistic Studies of Formal Thioboration Reactions of Alkynes

Several formal heteroborylative cyclization reactions have been recently reported, but little physical–organic and mechanistic data are known. We now investigate the catalyst-free formal thioboration reaction of alkynes to gain mechanistic insight into <i>B</i>-chloro­catechol­borane (ClBcat) in its new role as an alkynophilic Lewis acid in electrophilic cyclization/dealkylation reactions. In kinetic studies, the reaction is second-order globally and first-order with respect to both the 2-alkynylthioanisole substrate and the ClBcat electrophile, with activation parameters of Δ<i>G</i>^‡ = 27.1 ± 0.1 kcal mol^–1 at 90 °C, Δ<i>H</i>^‡ = 13.8 ± 1.0 kcal mol^–1, and Δ<i>S</i>^‡ = −37 ± 3 cal mol^–1 K^–1, measured over the range 70–90 °C. Carbon kinetic isotope effects supported a rate-determining Ad_E3 mechanism wherein alkyne activation by neutral ClBcat is concerted with cyclative attack by nucleophilic sulfur. A Hammett study found a ρ⁺ of −1.7, suggesting cationic charge buildup during the cyclization and supporting rate-determining concerted cyclization. Studies of the reaction with tris­(pentafluoro­phenyl)­borane (B­(C₆F₅)₃), an activating agent capable of cyclization but not dealkylation, resulted in the isolation of a postcyclization zwitterionic intermediate. Kinetic studies via UV–vis spectroscopy with this boron reagent found second-order kinetics, supporting the likely relevancy of intermediates in this system to the ClBcat system. Computational studies comparing ClBcat with BCl₃ as an activating agent showed why BCl₃, in contrast to ClBcat, failed to mediate the complete the cyclization/demethylation reaction sequence by itself. Overall, the results support a mechanism in which the ClBcat reagent serves a bifunctional role by sequentially activating the alkyne, despite being less electrophilic than other known alkyne-activating reagents and then providing chloride for post-rate-determining demethylation/neutralization of the resulting zwitterionic intermediate