• Bulletin of the Korean Chemical Society
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• 제32권6호
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• pp.1884-1890
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• 2011

A series of Ni(II) and Pd(II) complexes bearing N4-type tetradentate ligands, [Ni($X^1X^2$-6-$Me_2bpb$) 1] and [Pd($X^1X^2$-6-$Me_2bpb$) 2]; 6-$Me_2bpb$ = N,N'-(o-phenylene)bis(6-methylpyridine-2-carboxamidate), $X^1$ = Cl, H, or $CH_3$, $X^2$ = $NO_2$, Cl, F, H, $CH_3$, or $OCH_3$) were designed, synthesized, and characterized to investigate electronic and steric effects of ligand on the norbornene polymerization catalysts. Using modified methylaluminoxanes as an activator, the complexes exhibited high catalytic activities for the polymerization of norbornene and the nickel complexes exhibited better catalytic activity the palladium complexes. Ni complex 1a with $NO_2$ group on the benzene ring showed the highest catalytic activity of $4.9{\times}10^6$ g of PNBEs/$mol_{Ni}{\cdot}h$ and molecular weight of $15.28{\times}10^5$ g/mol with PDI < 2.30. Complexes with electron-withdrawing groups are more thermally stable (> 100 $^{\circ}C$), and tend to afford higher polymerization productivities than the ones having electron-donating groups. Amorphous polynorbornenes were obtained with good solubility in halogenated aromatic solvents. A vinyl addition mechanism has been proposed for the catalytic polymerization.

• Bulletin of the Korean Chemical Society
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• 제34권12호
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• pp.3811-3816
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• 2013

Nucleophilic substitution reactions of ethyl methyl (2), ethyl propyl (4) and diisopropyl (7) chlorothiophosphates with substituted anilines and deuterated anilines are investigated kinetically in acetonitrile at $55.0^{\circ}C$. A concerted mechanism is proposed based on the selectivity parameters. The deuterium kinetic isotope effects (DKIEs; $k_H/k_D$) are secondary inverse ($k_H/k_D=0.66-0.99$) with 2, primary normal and secondary inverse ($k_H/k_D=0.78-1.19$) with 4, and primary normal ($k_H/k_D=1.06-1.21$) with 7. The primary normal and secondary inverse DKIEs are rationalized by frontside attack involving hydrogen bonded, four-center-type transition state, and backside attack involving in-line-type transition state, respectively. The anilinolyses of ten chlorothiophosphates are examined based on the reactivity, steric effect of the two ligands, thio effect, reaction mechanism, DKIE and activation parameter.

• 한국전산유체공학회지
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• 제15권4호
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• pp.40-45
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• 2010

The flow rate is higher in ACEO micro-pumps with step electrodes than in micro-pumps with planar asymmetric electrodes. In the present study, numerical simulations were made of a ACEO micro-pump with step electrodes to investigate the effects of electrode design parameters on the pumping flow rate. The electrical charge at the electrodes, the fluid flow, and potential were solved, taking into account the finite size of ions, that is, the steric effect. This effect is recognized to be capable of quantifying the electrical charge more accurately in the electrical double layer subject to high voltages. Geometrical parameters such as heights, widths, and gaps of three-dimensional electrodes were optimized to enhance the pumping flow rate. Moreover, the effect of amplitude and frequency of AC was studied.

• 한국세라믹학회지
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• 제35권2호
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• pp.180-184
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• 1998

It was examined that the rheological behaviors of fly ash 70wt%-clay 30wt% slip in which nonplastic fly ash was a major component. We have systematically investigated the effects of deflocculant(Tetrasodium pyrophosphate ; Na4P2O7 nH2O Sodium silicate; Na2SiO3) and coagulant(CaSO4) on the rheological behavior of ash-clay slip. Ash-clay slip have been characterized on the basis of the time dependent rheology which was done out by the gel-curve test. Dispersion mechanism of ash-clay slip is the steric stabilization by the Na2SiO3 coating of cenospheres surface. Coagulated slip seems to have the new network structure and shows the gellation behaviors which makes it possible to direct coagulated casting(DCC).

• Bulletin of the Korean Chemical Society
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• 제15권12호
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• pp.1080-1084
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• 1994

Asymmetric inducing effects of substituents attached at nitrogen, the 5-position and boron in oxazaborolidine rings on asymmetric borane reduction of ketones were investigated. Thus, the effect of N-substituents examined with the oxazaborolidines prepared from (lR,2S)-N-alkyl norephedrine derivatives showed the remarkable decrease of enantioselectivities of the product alcohols by the variation of the steric size of alkyl groups on nitrogen from Me${\leftrightarro}$n-Bu(${\simeq}$Bn)${\leftrightarro}$ neopentyl${\leftrightarro}$i-Pr, such as 83${\%}$ ee with 5b, 22${\%}$ ee with 5c, 23${\%}$ ee with 5f, 16${\%}$ ee with 5e, and 3${\%}$ ee with 5d for the reduction of acetophenone. The presence of diphenyl groups at the 5-position enhanced the enantioselectivities dramatically. The effect of B-alkyl substituents in the oxazaborolidines derived from (lR,2S)-ephedrine showed that the enantioselectivities of product alcohols decreased gradually when the substituents were changed from hydrogen to steric bulky groups such as methyl, n-butyl, thexyl and phenyl.

• Bulletin of the Korean Chemical Society
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• 제17권7호
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• pp.653-655
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• 1996

A comparative molecular field analysis (CoMFA) was carried out for the correlation of the transition state structures and the reaction rates for the SN2 reaction of 4-nitrophenyl benzoate and its sulfur analogs with anionic nucleophiles. The CoMFA analysis showed that both steric and electrostatic effects are important, and the steric contribution increased when nucleophiles are alkoxides or arylsulfides. In this study, we have demonstrated that the CoMFA analysis can be expanded beyond the scope of dealing with reactants and products. The reactant complex and transition state conformations generated along the reaction path can be more appropriately used for the correlation of structures and reaction rates.

• Bulletin of the Korean Chemical Society
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• 제33권6호
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• pp.1879-1884
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• 2012

The kinetic studies on the reactions of dipropyl chlorophosphate (3O) with substituted anilines ($XC_6H_4NH_2$) and deuterated anilines ($XC_6H_4ND_2$) have been carried out in acetonitrile at $55.0^{\circ}C$. The obtained deuterium kinetic isotope effects (DKIEs; $k_H/k_D$) are primary normal ($k_H/k_D$ = 1.09-1.01) with the strongly basic anilines while secondary inverse ($k_H/k_D$ = 0.74-0.82) with the weakly basic anilines. The steric effects of the two ligands on the rates are extensively discussed for the anilinolyses of the ($R_1O$)($R_2O$)P(=O or S)Cl-type chlorophosphates and chlorothiophosphates. A concerted mechanism is proposed with a frontside nucleophilic attack involving a hydrogen-bonded four-center-type transition state for the strongly basic anilines and with a backside attack transition state for the weakly basic anilines on the basis of the DKIEs, primary normal and secondary inverse with the strongly and weakly basic anilines, respectively.

• 분석과학
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• 제25권6호
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• pp.476-482
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• 2012

황사입자들은 수 나노미터에서 수 마이크론 사이의 크기를 가지는 것으로 알려져 있다. 황사가 환경 및 인체 건강에 미치는 영향은 황사 입자의 크기에 의존한다. 입자가 작을수록 멀리까지 이동하며, 인체의 호흡기관 깊숙이 침투한다. 침강장-흐름 분획법(sedimentation field-flow fractionation, SdFFF)은 채널 내 포물선형태의 흐름(parabolic flow profile)과 외부에서 가해지는 원심력의 상호작용을 이용하여 나노 및 마이크론 크기의 입자들의 분리를 제공한다. 본 연구에서는 황사입자의 크기별 분리와 특성분석을 위한 steric 모드 침강장-흐름 분획법(Sd/StFFF)의 응용 가능성을 테스트하였다. 이를 위하여 다양한 Sd/StFFF 파라미터들을(유속, stop-flow time, 원심력의 세기, 등) 최적화 하였다. Sd/StFFF 보정곡선의 $R^2$값은 0.9983으로 높은 직선성을 보였으며, 실험결과는 Sd/StFFF가 마이크론 입자의 크기별 분리에 우수함을 보여주었다. 광학현미경(optical microscopy, OM)을 이용하여 황사입자들의 크기와 모양을 조사하였다. 황사가 진할 때에는 약할 때보다 입자크기가 증가함을 보여주었다. 또한 비가 올 때에는 건조할 때보다 입자크기가 감소하였는데, 이는 입자 표면에 흡착되어 있는 성분들이 빗물에 의해 제거되었기 때문인 것으로 보여진다. 본 연구의 결과는 Sd/StFFF가 황사와 같이 환경입자의 크기특성분석에 유용함을 보여준다.

• 폴리머
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• 제27권4호
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• pp.358-363
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• 2003

하이드록시프로필 셀룰로오스와 암모니움 퍼설페이트를 각각 입자 안정제와 개시제로 사용하여 에틸 알코올/물 혼합 분산매에서 아크릴아미드의 분산 중합을 다양한 중합 조건하에서 수행하여 생성되는 poly(acrylamide) (PAM) 라텍스의 평균 입자경, 라텍스 고분자의 점도 평균 분자량, 수용화 정도 등을 관찰하였다. 일반적으로 PAM 라텍스의 평균 입자경은 개시제의 농도, 분산매 중의 물의 농도, 중합 온도가 증가함에 따라 증가하였으나 단량체의 농도, 입자 안정제의 농도는 감소함에 따라 증가하였다. 라텍스 상태로 얻어진 PAM 고분자의 점도 평균 분자량은 단량체의 농도, 입자 안정제의 농도, 분산매 중 물의 농도가 증가함에 따라, 또는 개시제의 농도, 중합 온도가 감소함에 따라 증가하였다. 본 연구에서 얻어진 PAM 라텍스는 0.5~2.4 $\mu\textrm{m}$의 평균 입자경과 20000~335000g/㏖의 점도 평균 분자량을 나타내었으며 이들은 물중에 분산하였을 때 순간적으로 수용화되었다.

• Bulletin of the Korean Chemical Society
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• 제34권8호
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• pp.2325-2329
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• 2013

Second-order rate constants $k_N$ have been measured for reactions of benzyl 2-pyridyl thionocarbonate (4b) and t-butyl 2-pyridyl thionocarbonate (5b) with a series of cyclic secondary amines in MeCN at $25.0{\pm}0.1^{\circ}C$. The $k_N$ values for the reactions of 4b and 5b have been compared with those reported previously for the corresponding reactions of benzyl 2-pyridyl carbonate (4a) and t-butyl 2-pyridyl carbonate (5a) to investigate the effect of changing the electrophilic center from C=O to C=S on reactivity and reaction mechanism. The thiono compound 4b is more reactive than its oxygen analogue 4a. The Br${\o}$nsted-type plots for the reactions of 4a and 4b are linear with ${\beta}_{nuc}=0.57$ and 0.37, respectively. The reactions of 4a were previously reported to proceed through a concerted mechanism, while those of 4b in this study have been concluded to proceed through a stepwise mechanism with formation of an intermediate being the rate-determining step on the basis of the ${\beta}_{nuc}$ value of 0.37. Enhanced polarizability upon changing the C=O in 4a by C=S has been suggested to be responsible for the reactivity order and the contrasting reaction mechanisms. In contrast, the reactivity of 5a and 5b is similar, but they are much less reactive than 4a and 4b. Furthermore, the reactions of 5a and 5b have been concluded to proceed through the same mechanism (i.e., a concerted mechanism) on the basis of linear Bronsted-type plots with ${\beta}_{nuc}=0.45$ or 0.47. It has been concluded that the strong steric hindrance exerted by the t-Bu in 5a and 5b causes a decrease in their reactivity and forces the reactions to proceed through a concerted mechanism.