Polymerization of Phenylacetylene-BasedMonodendrons with Alkoxy Peripheral Groups and Oxygen / Nitrogen Permeation Behavior of Their Membranes

1 Center for Transdisciplinary Research, Niigata University, Ikarashi 2-8050, Niigata 950-2181, Japan 2 Department of Chemistry and Chemical Engineering, Faculty of Engineering, Niigata University, Ikarashi 2-8050, Niigata 950-2181, Japan 3 Graduate School of Science and Technology, Niigata University, Ikarashi 2-8050, Niigata 950-2181, Japan 4 Venture Business Laboratory, Niigata University, Ikarashi 2-8050, Niigata 950-2181, Japan


Introduction
The chemical structure and/or morphology of polymers play an important role for the permselectivity of nonporous membranes.Recently, dendrimers have attracted much attention in organic and polymer chemistry due to their novel properties or functions, which are based on their specific shapes and precisely defined three-dimensional structures [1][2][3].Dendrimers are characterized by a regularlyand fractally-branched treelike architecture.This feature provides a nanoscale homogeneous space in the dendrimers particularly suited for rigid molecular architectures.Additionally, the center and periphery of dendrimer molecule can be modified by substituent groups, which may aid molecular recognition [4].

Synthesis of Monomers.
The general procedure of 3,5-dialkoxybenzyl chloride is as follows.A solution of 3,5-dialkoxybenzyl alcohol (5 mmol), DMF (0.5 mL), and thionyl chloride (10 mmol) in chloroform (25 mL) was stirred at room temperature for 2-12 h.The excess thionyl chloride and solvents were evaporated under reduced pressure, and then the residue was dissolved in diethyl ether.The ether solution was washed with water, then dried over anhydrous sodium sulfate, filtered, and evaporated.Then, 3,5-dialkoxybenzyl chloride was obtained without further purification.

2.3.
Polymerization.An appropriate amount of monomers (typically, 0.5-1.0g) was placed in a Schlenk tube equipped with a three-way stopcock, a rubber septum, and a Tefloncoated magnetic stirring bar.The tube was placed under vacuum, followed by a nitrogen backflush.Freshly distilled solvent was transferred to the tube, and the monomers were dissolved with stirring.The determined amount of [Rh(nbd)Cl] 2 and triethylamine dissolved in the solvent was added to the stirred monomers solution.The detailed polymerization conditions are tabulated in Table 1.The reaction solution was poured into methanol or methanol/benzene (3/2 v/v) to yield polymer precipitate.The precipitate was washed with the precipitant and then dried in vacuo to give a yellow polymer.

Membrane Preparation.
A 5 wt% (w/v) solution of a polymer in chloroform or toluene was cast on a Teflon sheet, and the solvent was evaporated at room temperature.The resulting solid membrane was detached from the sheet and dried in vacuo for 24 hr.Thickness (L) of the membranes was 70-160 μm.
2.6.Other Measurements.IR spectra were measured with a Hitachi IR 270-30 spectrometer.NMR ( 1 H, 13 C) spectra were measured with a Varian Unity 500SW (500 MHz) or a JEOL GSX-270 (270 MHz) spectrometer.Average molecular weights (M n and M w ) were evaluated by coupling of gel permeation chromatography and low angle laser light scattering (GPC-LALLS) at 40 • C on THF eluent using Tosoh Liquid Chromatograph instruments with SD-8000, CCPD, C0-8010, LS-8000, RI-8011, and PP8010.The optical spectra were measured with a JASCO Ubest V-550DS UV-vis spectrometer.The wide-angle X-ray scattering measurements were performed using a Rigaku Geigerflex with a graphitemonochromatized Cu Kα radiation, which was supplied at 40 kV and 20 mA.

Polymerization of Monodendron Monomers.
The monodendron monomers were polymerized by Rh catalyst, [Rh(nbd)Cl] 2 , in the presence of triethylamine cocatalyst.The polymerization mixtures were purified by precipitating into methanol or methanol/benzene (9/1-4/3 v/v) to yield the corresponding polydendrons as yellow powders.It is well known that the Rh catalyst can selectively promote the polymerization of monosubstituted acetylenes [14][15][16][17][18][19].Moreover, we have previously demonstrated that TMS1H, PMDS1H, and NMTS1H polymerize at the terminal acetylene group of the focal point [7].The same catalyst system promotes the polymerization of HG1PA and Hex1H to yield the corresponding polymers as shown in Table 1.HG0PA and DG0PA gave highly soluble polymers with a high degree of polymerization (DP > 10 3 ) in spite of the larger peripheral groups in comparison with MG0PA.The yield and the degree of polymerization of polydendrons (i.e., poly(HG1PA) and poly(Hex1H)) are similar to those of the corresponding zerogeneration polymers (i.e., poly(HG0PA) and poly(Hex1H)), since the first-generation dendrons are not too crowded to affect the polymerization ability.

Structure of Polydendrons.
The polymerization of the monodendrons at the terminal acetylene group of focal point was confirmed by means of IR and 1 H NMR; for example, in IR and 1 H NMR spectra of poly(Hex1H), the peaks assignable to the terminal acetylene group of the monodendron Hex1H, that is, 3308 cm −1 (the stretching vibration of the ≡C-H bond) in IR spectrum and δ 3.09 (s, 1H, ≡C-H) in 1 H NMR, completely disappeared.The visible absorption maxima (λ max ) of polydendrons significantly increased due to the construction of the dendritic structure, that is, poly(Hex0H) (400 nm) to poly(Hex1H) (455 nm), and poly(HG0PA) (400 nm) to poly(HG1PA) (441 nm).
A bathochromic shift has been reported for some orthoand meta-substituted poly(phenylacetylene)s in comparison with the simple or para-substituted poly(phenylacetylene) [20][21][22][23][24].This bathochromic shift indicates a developed π-conjugation in the main chain of polydendrons and suggests that steric hindrance and repulsion among bulky substituents at the m-position formed the extended main chain structure.This resulted in a highly twisted dihedral angle between the main chain and the attached phenyl rings and was counteracted by slightly or moderately twisted single bonds of the main chain.The visible absorption maxima (λ max ) of the first-generation polydendrons were significantly increased due to the effect of the bulky substituent at the m-position in comparison with the corresponding zerogeneration polymers poly(HG0PA) and poly(Hex1H).The wide-angle X-ray scattering (WAXS) of poly(HG0PA) and poly(HG1PA) was measured in film states (Table 2).The sharp peaks were observed at 2θ = 3 − 4 • , and these sharp crystalline peaks are attributed to the (100) reflection of the pseudohexagonal lattice of rod-like molecules [7,25].The interplanar d spacing of the polymers increased with increasing generation, while the density seldom decreased.These data suggest that the increase of d spacing is related to the increase of the column diameter with the generation.The column diameter of the polymers estimated from d spacing is only slightly smaller than that of molecular modeling.(The molecular modeling was performed on the assumption of a fully extended conformation for side-chain monodendrons and distorted cis-transoid main chain conformation (dihedral angle = 130 deg), as described in [7].)The column diameter difference between the values estimated from d spacing and from molecular modeling suggests that the flexible alkyl chain as the peripheral group was bent and/or the columns slightly overlapped each other in the part of the peripheral group.

Oxygen Permselectivity of Polydendron Membranes.
The polymers were soluble in common organic solvents, such as chloroform, toluene, and tetrahydrofuran.The polymers poly(HG0PA), poly(Hex0H), and poly(Hex1H) exhibited good film-forming abilities depending on their degree of polymerization and formed a self-supporting film colored orange due to π-conjugated main chain chromophore by the solvent-casting method.
Oxygen and nitrogen permeation were measured using air as feed gas at 298 K and 76 cmHg using poly(HG0PA), poly(Hex0H), and poly(Hex1H) membranes.Figure 1 shows the relationship between the oxygen permeability coefficient (P O2 ) and the oxygen separation factor (α = P O2 /P N2 ) for the polydendron membranes with the zero-generation polymer (poly(TMS0H), poly(PMDS0H), and poly(NMTS0H)) membranes.The oxygen permeability coefficient (P O2 ) of the membranes formed by first-generation polydendrons were smaller than that of the corresponding zerogeneration poly(phenylacetylene) derivative.It seems that this behavior was caused by the low mobility of the stiff and crowded dendritic structure.The zero-generation poly(phenylacetylene) derivatives show a balance between International Journal of Polymer Science 7  and poly(TMS0H) from [13], poly(PMDS0H), poly(NMTS0H), poly(TMS1H), and poly(PMDS1H) from [6].
the oxygen permeability coefficient and oxygen separation factor.However, the oxygen permselectivity of the polydendrons with a trimethylsilyl group, poly(TMS1H), shows good performance in comparison with corresponding zerogeneration poly(phenylacetylene) derivatives.In comparison, polydendrons with oligo(dimethylsiloxane) peripheral groups (i.e., poly(PMDS1H)) exhibited similar trend to that of the corresponding zero-generation poly(phenylacetylene) derivatives.On the other hand, polymers and polydendrons with alkoxy peripheral groups exhibited the lower oxygen permeability coefficient due to the alkyl domain that would have better gas barrier property than oligo(dimethylsiloxane) domain.But the oxygen permselectivity of poly(Hex1H) shows good performance in comparison with the trend between polyethylene and poly(Hex0H).These results suggest that the space-persistency due to the stiff and crowded dendritic structure play an important role in the high performance of the oxygen permselectivity, while the effect of the peripheral group was similar to that of the corresponding zero-generation poly(phenylacetylene) derivatives.

Conclusions
The phenylacetylene monomers consisting of monodendron with alkoxy peripheral groups were successfully polymerized with a Rh catalyst, [Rh(nbd)Cl] 2 , to yield corresponding polydendrons with high molecular weight, in spite of the bulkiness of the dendritic residue and the peripheral groups.The polydendrons were fabricated to self-supporting membranes, and the membrane formed by the polydendron of first generation, poly(Hex1H), showed an oxygen permselectivity higher than that of the corresponding zerogeneration poly(phenylacetylene) derivatives.