유기상-수상 경계면에서 Cu-Dibutylethylenediamine 복합체 촉매를 이용한 Poly(2,6-dimethyl-1,4-phenylene oxide)의 합성

정영민, 안화승

인하대학교 화학공학과

Organic-Water Interfacial Synthesis of Poly(2,6-dimethyl-1,4-phenylene oxide) using Cu-Dibutylethylenediamine Complex Catalysts

Young Min Chung, Wha Seung Ahn

Department of Chemical Engineering, Inha University

INTRODUCTION

Biphasic catalytic reaction scheme in which a surface-active complexing agent or ligand is used to draw the catalyst to the organic-water interface where it effects the desired reaction can offer the following advantages over the corresponding homogeneous or heterogeneous reactions : the avoidance of the use of a toxic or environmentally-troublesome solvent, easy of catalyst recovery and substrate recycle, and the attainment of high reactivity, selectivity under mild reaction conditions [1].

In this report, we applied organic-water interfacial technique to the synthesis of poly(2,6-dimethyl-1,4-phenylene oxide) (PPO), a commercially important engineering plastics and showed some important features in biphsic synthesis including comparison with homogeneous reaction schemes.

EXPERIMENTAL

The reaction was carried out batchwise at 1atm in a 500ml three-necked flask reactor using chloroform-water as the biphasic mixture, a cuprous chloride- dibutylethylenediamine complex as the surface-active coupling catalyst, ammonia as the base to deprotonate the 2,6-dimethylphenol substrate, sodium chloride as a solubility enhancer for cuprous chloride through chloride complexation, and dodecyl sulfate, sodium salt(DSS) as the emulsifier with vigorous mechanical stirring.

A representative run was carried out as follows : The reactor was charged with 200ml of a 1:1 (by volume) chloroform-water mixture and the desired amount of the reaction ingredients, except for the catalyst. The reactor was placed in a water bath and purged with oxygen for about 10 minutes and then allowed to equilibrate to the set temperature, which was typically 25℃. The catalyst, cuprous chloride, was then added to initiate the reaction. After the reaction was initiated by starting the mechanical stirrer, the oxygen uptake was monitored using a constant-pressure manometric unit.

After completion of the reaction, the reaction mixture was emptied into a separating funnel. Upon standing overnight, the organic and aqueous phase was separated out with an emulsion layer formed between the organic and aqueous phase. The organic and aqueous phase were separated from the emulsion suspension between the two by pipetting and by draining. Each of three portions was dropped slowly to excess volume of methanol solution containing approximately 1% hydrochloric acid with vigorous stirring. The precipitated polymer was filtered out, washed with methanol, and dried under vacuum at 65℃.

Identification and characterization studies were performed on the purified product using FT-IR spectroscopes, differential scanning calorimetry (DSC). Intrinsic viscosities were measured in Ubbelohde viscometers in chloroform solution at 25℃, using the customary extrapolation of several values of the reduced viscosity to zero concentration. The distribution of the copper complex in the reaction mixture was determined by analyzing the metal concentrations in the aqueous phase, organic phase, and interfacial emulsion layer using ICP (or inductively-coupled plasma atomic emission) spectroscopy and the copper complex was identified with UV spectroscopy.

RESULTS AND DISCUSSION

The FT-IR spectra of the purified reaction product and the PPO standard showed good match and in DSC analysis, a glass transition temperature was observed at 236℃ which was somewhat higher than the literature value of 210℃ due possibly to differences in molecular weight distribution. Oxidative coupling of 2,6-dimethylphenol in homogeneous reaction proceeded to form either PPO or diphenoquinones depending on the ligand-catalyst ratio. Whilst higher ligand-catalyst ratio was required to promote the C-O coupling dominantly [2],  the C-C coupling was not found even at lower ligand-catalyst ratio of 1:1 in biphsic reaction. It may be due to the fact that the surface-active ligand enables the catalyst to be anchored at the organic-water interface at which the desired coupling reaction is favorable.

The ligand screening results are shown in Table.1. Dibutylethylenediamine (DBEDA) was most effective among the ligands tested. For all the ligands tested, except for tetraethylethylenediamine, the reaction rate decreased in the absence of ammonia.

Whereas the ability of surface-active ligand to draw the catalyst to the O-W interface increases at higher ligand-catalyst ratios, the excess ligand may prevent the reactant from approaching to the catalyst's coordination sites. Thus, optimum ligand-catalyst ratio exists which corresponds to an optimum balance between these two factors. The optimal ligand-catalyst ratio was 3:1 with ammonia as deprotonation agent, but it was shifted to 5:1 in the absence of ammonia. This may be rationalized in terms of the fact that without ammonia, DBEDA should serve both  as ligand and as deprotonation agent simultaneously.

The reaction rate was also affected by the size of hydrophobic group in anionic emulsifier. Since all the anionic emulsifiers used in this study have the same hydrophilic group of OSO3-, the relation between the hydrophobic group length and reaction rate could be studied separately. The maximum reaction rate was obtained when dodecyl sulfate, sodium salt(DSS) (C12) was used, but in the case of octyl sulfate, sodium salt(C8) or octadecyl sulfate, sodium salt(C18), the reaction rate was significantly retarded. It is known that the emulsifier properties are dependent on the length of hydrophobic group [3]. DSS brings about a greater dispersion action which produced a finer emulsion that could provide a larger interfacial area and a shorter diffusion pathlength for the reactants, catalyst complex, and polymer chains.  Sonification effect was, on the other hand, negligible.

The comparison of oxygen uptake rate between biphasic and one-phase homogeneous reaction using either DBEDA or pyridine as ligand was shown in Table.2. It was shown that faster reaction rate could be attained in biphasic mode when DBEDA was used in conjunction with ammonia. However, the biphasic reaction with pyridine had little advantage over the homogeneous reaction scheme. It was also suggested that if ammonia existed as deprotonation agent in one-phase reaction, it can compete with ligands for catalyst's coordination sites. In addition, DBEDA had shown higher surface-activity than pyridine. The results of ICP analysis in Table.3 and Table.4 which identified distribution of copper in the reaction mixture and, UV spectra which showed that Cu-NH3 complex was formed in aqueous phase, indicated that surface-activity of ligand used in biphasic reaction had significant effect on the reaction rate. Ammonia in biphasic reaction may form Cu-NH3 complex and draws copper to aqueous phase which leads to decreases in copper concentration at interfacial area. However, this effect can be overcome by the ligand such as DBEDA which has good surface-activity or amphiphilic character, since the reaction is now preferentially taking place at interface. As a result, higher reaction rate in comparison with homogeneous reaction scheme is achieved. Concurrently, the surface-active ligand facilitates the effective recovery of catalyst. When surface-activity of ligand is poor, i.e. pyridine, however, since the extent of reaction occurring at the interface was substantially reduced, the advantage of biphasic reaction decreases.

Table 1. Results of ligand screening in the oxidative coupling of PPO

Ligand	O2 uptake rate (M/h)
	with NH3	without NH3
Triethyl phosphite	0.382	negligible
Tributyl phosphite	0.363	negligible
N,N'-Dimethylethylenediamine	0.751	0.187
Tetramethylethylenediamine	1.366	0.107
Dibutylethylediamine	4.73	2.107
Tetraethylethylendiamine	1.1	1.431
Dibenzylethylenediamine	1.517	0.25
Pyridine	0.611	0.013

Table 2. Comparison of the reaction rate in biphasic and one-phase reaction

Ligand	O2 uptake rate (M/h)
	Biphasic mode		One-phase mode
	with NH3	without NH3	with NH3	without NH3
Dibutylethylenediamine	4.73	2.107	0.318	3.93
Pyridine	0.611	0.013	0.088	0.88

Table 3. Distribution of copper in the reaction mixture

Ligand : Dibutylethylenediamine

Condition	Phase	Copper distribution (%)
Without NH3	Organic phase	12.1
	Interfacial area	83.9
	Aqueous phase	4
With NH3	Organic phase	2.2
	Interfacial area	71.4
	Aqueous phase	26.4

Table 4. Distribution of copper in the reaction mixture

Ligand : Pyridine

Condition	Phase	Copper distribution (%)
Without NH3	Organic phase	67.6
	Interfacial area	31.8
	Aqueous phase	0.6
With NH3	Organic phase	0.5
	Interfacial area	49
	Aqueous phase	50.5

REFERENCES

1. P.K.Lim and Y.Zhong : ACS Symposium Series No.626, 168 (1996)

2. G.F.Endres, A.S.Hay, J.W.Eustance : J.Org.Chem., 28, 1300 (1963)

3. M.R.Porter : "Handbook of Surfactants", Chapman&Hall, London (1994)