3,4-Enhanced Polymerization of Isoprene Catalyzed by Side-Arm Tridentate Iminopyridine Iron Complex with High Activity: Optimization via Response Surface Methodology.

3,4-Enhanced polymerization of isoprene catalyzed by late transition metal with high activity remains one of the great challenges in synthetic rubber chemistry. Herein, a library of [N, N, X] tridentate iminopyridine iron chloride pre-catalysts (Fe 1-4) with the side arm were synthesized and confirmed by the element analysis and HRMS. All the iron compounds served as highly efficient pre-catalysts for 3,4-enhanced (up to 62%) isoprene polymerization when 500 equivalent MAOs were utilized as co-catalysts, delivering the corresponding high-performance polyisoprenes. Furthermore, optimization via single factor and response surface method, it was observed that the highest activity was obtained by complex Fe 2 with 4.0889 × 107 g·mol(Fe)-1·h-1 under the following conditions: Al/Fe = 683; IP/Fe = 7095; t = 0.52 min.

Synthesis, Characterization and Catalytic Property Studies for Isoprene Polymerization of Iron Complexes Bearing Unionized Pyridine-Oxime Ligands.

Iron complexes of the types [Fe(HL)2Cl2] (Fe1: HL1 = pyridine-2-aldoxime; Fe2: HL2 = 6-methylpyridine-2-aldoxime; Fe3: HL3 = phenyl-2-pyridylketoxime; Fe4: HL4 = picolinaldehyde O-methyl oxime) were prepared and characterized by elemental analysis and IR spectroscopy. The crystal structure of Fe2, determined by single-crystal X-ray diffraction, featured a distorted octahedral coordination of the iron center binding with two ligands of HL2. The X-ray structure and infrared spectral data indicated that pyridine-oxime ligands act as unionized bidentate ligand by coordinating with Npyridine and Noxime. The catalytic performance for isoprene polymerization, catalyzed by these pyridine-oxime-ligated iron complexes, was examined. For a binary catalytic system combined with MAO, complexes Fe1, Fe3 and Fe4 were found to be highly active (up to 6.5 × 106 g/mol·h) in cis-1,4-alt-3,4 enchained polymerization, with average molecular weights in the range of 60-653 kg/mol and narrow PDI values of 1.7-3.5, even with very low amounts of MAO (Al/Fe = 5). Upon activation with [Ph3C][B(C6F5)4]/AlR3 for the ternary catalytic system, theses complexes showed extremely high activities, as well about 98% yield after 2 min, to afford cis-1,4-alt-3,4-polyisoprene with a molecular weight of 140-420 kg/mol.

A 2.5 V, 2.56 ppm/°C Curvature-Compensated Bandgap Reference for High-Precision Monitoring Applications.

This work presents a high-precision high-order curvature-compensated bandgap voltage reference (BGR) for battery monitoring applications. The collector currents of bipolar junction transistor (BJT) pairs with different ratios and temperature characteristics can cause greater nonlinearities in ΔVEB. The proposed circuit additionally introduces high-order curvature compensation in the generation of ΔVEB, such that it presents high-order temperature effects complementary to VEB. Fabricated using a 0.18 µm BCD process, the proposed BGR generates a 2.5 V reference voltage with a minimum temperature coefficient of 2.65 ppm/°C in the range of -40 to 125 °C. The minimum line sensitivity is 0.023%/V when supply voltage varies from 4.5 to 5.5 V. The BGR circuit area is 382 × 270 µm2, and the BMIC area is 2.8 × 2.8 mm2.

An unsymmetrical binuclear iminopyridine-iron complex and its catalytic isoprene polymerization.

A series of chloride-bridged unsymmetrical mixed Fe(ii)-HS/Fe(ii)-LS binuclear structures has been prepared and characterized. Upon activation with MAO, highly efficient catalytic polymerization of isoprene was achieved, delivering an ultra-high molecular weight (catalyst loading = 2.5 ppm, Mn = 1.8 × 106 g mol-1, Mw/Mn = 1.4).

Isoprene Polymerization: Catalytic Performance of Iminopyridine Vanadium(III) Chloride versus Vanadium(III) Chloride.

A series of vanadium complexes bearing iminopyridine bidentate ligands with various electronic and steric properties: V1 [CH2Ph], V2 [CMe2CH2CMe3], V3 [Ph] and V4 [2,6-iPr2Ph] were prepared and characterized by IR spectroscopy and microanalytical analysis. The catalytic capacity of all the complexes has been investigated for isoprene polymerization and was controlled by tuning the ligand structure with different N-alkyl and N-aryl groups. Activated by methylaluminoxane (MAO), the aryl-substituted complex V3 [Ph] exhibited high cis-1,4 selectivity (75%), and the resultant polymers had high molecular weights (Mn = 6.6 × 104) and narrow molecular weight distributions (PDI = 2.3). This catalyst showed high activity up to 734.4 kg polymer (mol V)-1 h-1 with excellent thermostability even stable at 70 °C. Compared to the traditional VCl3/MAO catalytic system, iminopyridine-supported V(III) catalysts displayed higher catalytic activities and changed the selectivity of monomer enchainment from trans-1,4 to cis-1,4.

Synthesis and characterization of aminopyridine iron(ii) chloride catalysts for isoprene polymerization: sterically controlled monomer enchainment.

In this study, a series of 2-R-6-(1-(alkylamino)methyl)pyridine-iron complexes [alkyl: (CPh3) Fe1H; (CHPh2) Fe2H; (CHPh2) Fe3Me; (CHMePh) Fe4H; (CH2Ph) Fe5H; (CHMe2) Fe6H; (C6H11) Fe7H; (CH2(4-OMe)Ph) Fe8H; (CH2(4-CF3)Ph) Fe9H; (CH2(2,4,6-Me3)Ph) Fe10H; (CH2Ph) Fe11Me] were synthesized and well characterized by ATR-IR spectroscopy, HRMS spectroscopy and elemental analysis. In addition, Fe3Me, Fe4H, Fe7H and Fe11Me were characterized by X-ray diffraction analysis: Fe3Me and Fe11Me adopted distorted tetrahedral geometries in the solid state while Fe4H and Fe7H were found in dimeric or polymeric forms respectively in which chlorides acted as bridging ligands. The catalytic capacities of these iron complexes were investigated for isoprene polymerization. Upon activation with a MAO cocatalyst, the catalytic activities of complexes varied as a function of the steric and electronic influences of substituents. In general, the catalysts bearing the least steric groups and electron-withdrawing groups exhibited relatively high activities. An outstanding activity of 190.6 × 104 g·mol-1·h-1 was obtained by Fe5H [CH2Ph]. Moreover, changes in the steric hindrance around the metal center showed a notable effect on the selectivity of monomer enchainment. In particular, most of the polymers obtained by these complexes bearing flexible frameworks were in favor of 3,4-enchainment.

Influences of Fluorine Substituents on Iminopyridine Fe(II)- and Co(II)-Catalyzed Isoprene Polymerization.

A series of iminopyridine complexes of Fe(II) and Co(II) complexes bearing fluorinated aryl substituents were synthesized for the polymerization of isoprene. The structures of complexes 3a, 2b and 3b were determined by X-ray diffraction analysis. Complex 3a contained two iminopyridine ligands coordinated to the iron metal center forming an octahedral geometry, whereas 2b adopted a chloro-bridged dimer, and 3b featured with two patterns of cobalt centers bridged via chlorine atoms. Complexes 2b and 3b represented rare examples of chlorine bridged bimetallic Co(II) complexes. The fluorine substituents effects, particularly on catalytic activity and polymer properties such as molecular weight and regio-/stereo-selectivity were investigated when these complexes were employed for isoprene polymerization. Among the Fe(II)/methylaluminoxane (MAO) systems, the 4-CF3 substituted iminopyridine Fe(II) complex 1a was found as a highly active isoprene polymerization catalyst exhibiting the highest activity of 106 g·(mol of Fe)-1·h-1. The resultant polymer displayed lower molecular weight (Mn = 3.5 × 104 g/mol) and moderate polydispersity index (PDI = 2.1). Furthermore, the ratio of cis-1,4-/3,4 was not affected by the F substituents. In the series of Co(II)/AlEt2Cl binary systems, complexes containing electron-withdrawing N-aryl substituents (R = 4-CF3, 2,6-2F) afforded higher molecular weights polyisoprene than that was obtained by the complex containing electron-donating N-alkyl substituents (R = octyl). However, ternary components system, complex/MAO/[Ph3C][B(C6F5)4] resulted in low molecular weight polyisoprene (Mn < 2000) with high trans-1,4-unit (>95%).