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Probing the Interaction of Diester Internal Donors Formulations
Organic electron donors are essential components of Ziegler-Natta (ZN) catalysts to produce isotactic polypropylene. In particular, aromatic or aliphatic diesters are widely used as ‘Internal Donors’ (ID) in MgCl[sub.2]/ID/TiCl[sub.4] precatalyst formulations. Diesters are reactive with AlEt[sub.3] (by far the most common ZN precatalyst activator) and are partly removed from the solid phase in the early stages of the polymerization process; this is detrimental for catalyst functioning, and a surrogate donor (‘External Donor’ (ED), usually an alkoxysilane) is added to the system to restore performance. Recent studies, however, demonstrated that even in cases where most of the diester is extracted by AlEt[sub.3], the active sites retain a ‘memory’ of it in several aspects of the catalytic behavior (such as, e.g., the average productivity and the polydispersity index of the polymer produced). Considering that the residual diester is always in molar excess with respect to the active Ti, one may speculate that long-lasting interactions between the latter and diester molecules can occur. In turn, this should imply that the reactivity of AlEt[sub.3] is different with binary MgCl[sub.2]/ID or ternary MgCl[sub.2]/ID/TiCl[sub.4] mixtures. In this work, the latter hypothesis was explored for a library of diester IDs with large structural diversity. In line with the anticipation, the fractional amount of ID extracted by AlEt[sub.3] was generally lower for ternary mixtures, although to an extent exquisitely dependent on diester structure.
Journal Article
The Effect of Alkyl Substituents on the Formation and Structure of Homochiral sub.2 Species—Towards the Factors Controlling the Stereoselectivity of Dialkylgallium Alkoxides in the Ring-Opening Polymerization of rac-Lactide
Building on our previous studies, which have demonstrated that homochiral propagating species—(R*,R*)-[Me[sub.2]Ga(µ-OCH(Me)CO[sub.2]R)][sub.2]—were crucial for the heteroselectivity of [Me[sub.2]Ga(µ-OCH(Me)CO[sub.2]Me)][sub.2] in the ring-opening polymerization (ROP) of racemic lactide (rac-LA), we have investigated the effect of alkyl groups on the structure and catalytic properties of dialkylgallium alkoxides in the ROP of rac-LA. Therefore, we have isolated and characterized the rac-[R[sub.2]Ga(µ-OCH(Me)CO[sub.2]Me][sub.2] (R = Et (1), iPr (2) and rac-[R[sub.2]Ga(µ-OCH(Me)C[sub.5]H[sub.4]N][sub.2] (R = Et (3), iPr (4)) complexes, to demonstrate the effect of alkyl groups on the chiral recognition induced the formation of the respective homochiaral species—(R*,R*)-[R[sub.2]Ga(µ-OCH(Me)CO[sub.2]Me)][sub.2] and (R*,R*)-[R[sub.2]Ga(µ-OCH(Me)C[sub.5]H[sub.4]N][sub.2]. Moreover, we have investigated the structure of (S,S)-[R[sub.2]Ga(µ-OCH(Me)CO[sub.2]Me][sub.2] (R = Et ((S,S)-1, R = iPr ((S,S)-2,) and their catalytic activity in the ROP of rac-LA. With an increase in the bulkiness of alkyl substituents on gallium the following can be observed: (a) the tendency for the formation of homochiral complexes decreased, (b) the symmetry of homochiral (S,S)-[R[sub.2]Ga(µ-OCH(Me)CO[sub.2]Me][sub.2] (M = Me, Et (S,S)-1), iPr (S,S)-2) changed, and both have resulted in (c) lower or no heteroselectivtity across these complexes in the ROP of rac-LA. Importantly, the results have confirmed the crucial role of the chiral-induced formation of homochiral asymmetric dimers on the heteroselectivity of dialkylgallium alkoxides in the ROP of rac-LA.
Journal Article
Green polymer chemistry and composites : pollution prevention and waste reduction
\"This new book examines the latest developments in the important and growing field of producing conventional polymers from sustainable sources. With recent advancements in synthesis technologies and the discovery of new functional monomers, research shows that green polymers with better properties can be produced from renewable resources. This volume describes these advances in synthesis, processing, and technology and provides not only state-of-the-art information but also acts to stimulate research in this direction. Green Polymer Chemistry and Composites: Pollution Prevention and Waste Reduction illustrates how chemical industries play an essential role to sustain the world economies and looks at forthcoming technologies and scientific developments in novel products, less toxicological materials, and industrial procedures with high efficiency and renewable energy products. Green chemistry seeks for the design of innovative chemical products with higher efficiency and lowest hazardous substances for the health and the environment. This book offers an excellent resource for researchers, upper-level graduate students, brand owners, environment and sustainability managers, business development and innovation professionals, chemical engineers, plastics manufacturers, biochemists, and suppliers to the industry to debate sustainable, economic solutions for polymer synthesis\"-- Provided by publisher.
Book
Modern Trends in Polymerization-Induced Self-Assembly
Polymerization-induced self-assembly (PISA) is a powerful and versatile technique for producing colloidal dispersions of block copolymer particles with desired morphologies. Currently, PISA can be carried out in various media, over a wide range of temperatures, and using different mechanisms. This method enables the production of biodegradable objects and particles with various functionalities and stimuli sensitivity. Consequently, PISA offers a broad spectrum of potential commercial applications. The aim of this review is to provide an overview of the current state of rational synthesis of block copolymer particles with diverse morphologies using various PISA techniques and mechanisms. The discussion begins with an examination of the main thermodynamic, kinetic, and structural aspects of block copolymer micellization, followed by an exploration of the key principles of PISA in the formation of gradient and block copolymers. The review also delves into the main mechanisms of PISA implementation and the principles governing particle morphology. Finally, the potential future developments in PISA are considered.
Journal Article
caprolactone ring-opening polymerization catalyzed by imidazolium-based ionic liquid under mild reaction conditions
Polycaprolactone is a biodegradable, biocompatible, and versatile polymer commonly used in the pharmaceutical and biomedical industry and the development of new catalysts that allow for the synthesis under milder reaction conditions and in shorter reaction times is an appealing alternative. The iron-containing imidazolium-based ionic liquid 1-n-butyl-3-methylimidazolium heptachlorodiferrate was able to efficiently catalyze the ring-opening polymerization of [epsilon]-caprolactone under mild reaction conditions. Polymerization yields higher than 80% were obtained after 4 h of reaction at temperatures up to 85 °C, using low ionic liquid:[epsilon]-caprolactone molar ratios (1:720 - 1:1500), in the absence of solvent and without an intentionally added alcohol as an initiator. Semi-crystalline polycaprolactones, with molecular weights up to 14 kDa and narrow molecular weight distributions were synthesized. The chemical structure of the polymer was confirmed by Nuclear Magnetic Resonance (.sup.1H NMR) and Fourier Transform Infrared (FTIR) spectroscopy, and its crystalline content was estimated from the enthalpy of melting of the differential scanning calorimetry (DSC) thermogram. Finally, a caprolactone-activated ROP mechanism mediated by the ionic liquid was suggested.
Journal Article
Microwave absorption properties of gamma-Fe.sub.2O.sub.3/.sub.x-SO.sub.3H/polypyrrole core/shell/shell microspheres
Highly regulated [gamma]-Fe.sub.2O.sub.3/(SiO.sub.2).sub.x-SO.sub.3H/polypyrrole (FS.sub.xHP) composite core/shell/shell microspheres are successfully prepared through a SiO.sub.2 sol-gel process and an in situ polymerization of pyrrole monomers. [gamma]-Fe.sub.2O.sub.3 functions as the \"core,\" whereas SiO.sub.2 and polypyrrole (PPy) act as the \"shells\" of the composite. In contrast to many previously reported FS.sub.2H-P and PPy-based composites, the FS.sub.xHP composites display substantially improved microwave absorption properties, which can reach nearly - 43.1 dB (15.1 GHz) with a broader effective absorption bandwidth (RL < - 10 dB) of 6.1 GHz. The excellent microwave-absorbing properties of the FS.sub.xHP composites are closely associated with the layer of SiO.sub.2 shells and can be attributed to the proper impedance matching and synergistic interaction of the special core/shell/shell structures, dielectric loss (SiO.sub.2 and PPy layers) and magnetic loss ([gamma]-Fe.sub.2O.sub.3). These results show that the core/shell/shell structure may be an efficient and promising way to improve microwave absorption properties.
Journal Article