Exploring electronic versus steric effects in stereoselective ring-opening polymerization of lactide and β-butyrolactone with amino-alkoxy-bis(phenolate)-yttrium complexes

A series of methoxy‐amino‐bis(phenol)s (ONOO)H₂ possessing on the phenol rings R¹ ortho substituents with variable steric and electronic properties (R¹=CMe₂Ph, 1 ; CMe₂tBu, 3 ; CMe₂(4‐CF₃C₆H₄), 5 ; CPh₃, 9 ; Cl, 10 ) has been synthesized and further reacted with [Y{N(SiHMe₂)₂}₃](THF)₂ to give cleanly the corresponding yttrium compounds [Y(ONOO){N(SiHMe₂)₂}(thf)_n] ( Y‐x ); the solid‐state structures of Y‐3 and Y‐10 have been determined. These amido complexes have been used as initiators for the ring‐opening polymerization (ROP) of rac‐lactide (LA) and rac‐β‐butyrolactone (BBL) to provide heterotactically enriched poly(lactic acid)s (PLAs) and syndiotactically enriched poly(3‐hydroxybutyrate)s (PHBs), respectively, by means of a chain‐end control mechanism. Most of these polymerizations proceeded in a controlled fashion, giving polymers with narrow polydispersities and experimental molecular weights in good agreement with calculated values. The nature of the R¹ ortho substituents has a profound impact on the rates and, more spectacularly, on the stereocontrol of the polymerizations. The heterotactic stereocontrol in the ROP of rac‐LA appears to be governed essentially by steric considerations; the larger the substituent, the higher the heterotacticity: R¹=Cl (P_r=0.56)?CMe₃ (P_r=0.80)?CMe₂Ph (P_r=0.90)2(4 CF₃‐Ph) (P_r=0.93–0.94)≤CMe₂tBu (P_r=0.94–0.95)≤CPh₃ (P_r=0.95–0.96). On the other hand, the syndiotactic stereocontrol in the polymerization of rac‐BBL follows a quite different trend: R¹=Cl (P_r=0.42–0.45)?CMe₂tBu (P_r=0.62–0.70)3 (P_r=0.80)≤CMe₂(4 CF₃? Ph) (P_r=0.82–0.84)2Ph (P_r=0.89)3 (P_r=0.94), which suggests the involvement of electronic interactions. DFT computations on model intermediates confirmed a stabilizing C? H ??? π interaction between a methylene C? H of the ring‐opened BBL unit and the π system of one of the ortho‐aryl substituents of the ONOO ligand; by contrast, for model intermediates in the ROP of LA, no such C? H ??? π interaction involving the methyl group of lactate was observed.     

Design of Turn‐On Near‐Infrared Fluorescent Probes for Highly Sensitive and Selective Monitoring of Biopolymers

Simple, sensitive, and selective detection of specific biopolymers is critical in a broad range of biomedical and technological areas. We present a design of turn‐on near‐infrared (NIR) fluorescent probes with intrinsically high signal‐to‐background ratio. The fluorescent signal generation mechanism is based on the aggregation/de‐aggregation of phthalocyanine chromophores controlled by selective binding of small‐molecule “anchor” groups to a specific binding site of a target biopolymer. As a proof‐of‐concept, we demonstrate a design of a sensor for EGFR tyrosine kinase—an important target in cancer research. The universality of the fluorescent signal generation mechanism, as well as the dependence of the response selectivity on the choice of the small‐molecule “anchor” group, make it possible to use this approach to design reliable turn‐on NIR fluorescent sensors for detecting specific protein targets present in the low‐nanomolar concentration range.     

Group 3 and 4 single-site catalysts for stereospecific polymerization of styrene

The present review aims at a comprehensive survey of controlled stereospecific styrene polymerization by single-site catalysts to give syndiotactic and isotactic polystyrene. The early transition metals (groups 3 and 4) complexes that were successfully applied as catalyst precursors are reviewed. Catalytic performances and mechanistic insights, in particular in the stereoregulation phenomenon, are discussed.     

Retrieval of aerosol optical depth in vicinity of broken clouds from reflectance ratios: Sensitivity study

We conducted a sensitivity study to better understand the potential of a new method for retrieving aerosol optical depth (AOD) under partly cloudy conditions. This method exploits ratios of reflectances in the visible spectral range and provides an effective way to avoid three-dimensional (3D) cloud effects. The sensitivity study is performed for different observational conditions and random errors in input data. The results of the sensitivity study suggest that this ratio method has the ability to detect clear pixels even in close proximity to clouds. Such detection does not require a statistical analysis of the two-dimensional (2D) horizontal distribution of reflected solar radiation, which makes it suitable for operational retrievals. In comparison with previously suggested approaches, the ratio method has the capability to increase the “harvest” of clear pixels. Similar to the traditional independent pixel approximation (IPA), the ratio method has a low computational cost for retrieving AOD. In contrast to the IPA method, the ratio method provides much more accurate estimations of the AOD values under broken cloud conditions: pixel-based and domain-averaged estimations of errors in AOD are about 25% and 10%, respectively. Finally, both the ratio-based cloud screening and the accuracy of domain-averaged ratio-based AOD values do not suffer greatly when 5% random errors are introduced in the reflectances.     

Effect of surrounding point charges on the density functional calculations of NixOx clusters (x = 4-12)

Embedded Ni(x)O(x) clusters (x = 4-12) have been studied by the density-functional method using compensating point charges of variable magnitude to calculate the ionic charge, bulk modulus, and lattice binding energy. The computations were found to be strongly dependent on the value of the surrounding point charge array and an optimum value could be found by choosing the point charge to reproduce the experimentally observed Ni--O lattice parameter. This simple, empirical method yields a good match between computed and experimental data, and even small variation from the optimum point charge value produces significant deviation between computed and measured bulk physical parameters. The optimum point charge value depends on the cluster size, but in all cases is significantly less than +/-2.0, the formal oxidation state typically employed in cluster modeling of NiO bulk and surface properties. The electronic structure calculated with the optimized point charge magnitude is in general agreement with literature photoemission and XPS data and agrees with the presently accepted picture of the valence band as containing charge-transfer insulator characteristics. The orbital population near the Fermi level does not depend on the cluster size and is characterized by hybridized Ni 3d and O 2p orbitals with relative oxygen contribution of about 70%.     

Multilayer Ag/Y Mirrors for the Spectral Range of 9–11 nm

Technical Physics - Multilayer Ag/Y mirrors intended for the spectral range of 9–11 nm have been investigated. The parameters of mirrors, specifically, the thicknesses of transition layers,...     

Adaptive control for discrete-time Markov processes with unbounded costs: Average criterion

The paper deals with a class of discrete-time Markov control processes with Borel state and action spaces, and possibly unbounded one-stage costs. The processes are given by recurrent equations x _{t +1}=F(x _t ,a _t ,ξ _t ), t=1,2,… with i.i.d. ℜ ^k – valued random vectors ξ _t whose density ρ is unknown. Assuming observability of ξ _t , and taking advantage of the procedure of statistical estimation of ρ used in a previous work by authors, we construct an average cost optimal adaptive policy. Received March/Revised version October 1997