Preparation of aromatic polyesters by direct polymerization in the presence of boronic anhydride under non-stoichiometric condition
Graphical abstract
Introduction
POB and PON attract a great deal of attention as high-performance polymers possessing many excellent properties such as mechanical property, thermal stability, chemical resistance and so on [1], [2], [3], [4], [5]. These aromatic polyesters are usually synthesized by the polymerization of the corresponding acetoxy aromatic acids with elimination of acetic acid. Although these polyesters are expected to be high-performance materials, they show neither fusibility nor solubility owing to their rigid structure, and the intractability makes them difficult to process by conventional procedures. In general, copolymerization is applied to give them the fusibility, and the aromatic copolyesters are well known as thermotropic liquid crystalline polymers. The whiskers of POB and PON had been prepared by using reaction-induced crystallization of oligomers, and they are expected as to be high-performance fillers [6], [7], [8]. This reaction-induced crystallization of oligomers during polymerization is a unique method to control the morphology of intractable polymers, because it cannot be limited by the intractability of polymers.
Direct synthesis of aromatic polyesters had also been developed as an elegant synthetic procedure, and numerous types of condensation reagents were developed so far [9]. These polymerizations are regarded as atom-economical syntheses and they will be widely accepted as environmental benign syntheses. It has been reported that the direct polymerization of HBA and HNA in the presence of anhydride of boronic acid such as 2,4,6-tri(biphenyl-4-yl)-1,3,5,2,4,6-trioxatriborinane (BPBA) shown in Scheme 1, gave the precipitates of POB and PON crystals having unique morphology [10], [11]. In these polymerizations, BPBA activates the carboxyl group to react with the phenolic hydroxyl group in liquid phase. Even though the oligomers were precipitated from the solution to form the crystals, the polymerization occurred in the precipitated crystals to yield high molecular weight polymers.
It is well known that stoichiometric balance between two reactive groups is of the greatest importance to synthesize high molecular weight polymers by the step-growth polymerization [12]. If the polymerization is carried out in homogeneous system under non-stoichiometric condition, it is usually terminated and the molecular weight becomes lowered according to the following equation (1).where, DPn: number-average degree of polymerization, N0: number of monomers, N1: number of monofunctional compounds, p: extent of reaction
Several unusual polymerizations which did not obey the equation (1) had been reported and high molecular weight polymers were obtained even under non-stoichiometric condition [13], [14], [15], [16], [17], [18], [19], [20]. These unique polymerizations can be categorized into two groups chemically and physically based on the mechanism, of which one is attributed to the drastic change in the reactivity of functional groups and another is attributed to the heterogeneous reaction field. In the latter case, heterogeneous polymerization in which the polymerization proceeds with precipitation yields polymers having higher molecular weights than the theoretically calculated values even though the reactivity of functional groups does not change during polymerization. The direct polymerizations of HBA and HNA in the presence of boronic anhydride are heterogeneous polymerizations as aforesaid, and they will be expected to afford high molecular weight polymers even under non-stoichiometric condition.
In this study, the polymerization of HBA and HNA in the presence of boronic anhydride was examined with the addition of monofunctional compounds such as an aromatic carboxylic acid and a phenol to clarify the influence of stoichiometric imbalance on molecular weight of POB and PON.
Section snippets
Materials
HBA and HNA were purchased from TCI Co. Ltd. and recrystallized from a mixture of water and methanol. 4-Biphenylboronic acid (BPB) was purchased from Sigma–Aldrich Co. Ltd. and recrystallized from toluene. It has been reported that BPB undergo dehydration upon simple heating to be converted into anhydrides, BPBA [21], [22], [23]. BPB was converted into BPBA during the purification by recrystallization as confirmed by IR spectroscopy. p-Hexyloxybenzoic acid (HOBA),
Polymerization under non-stoichiometric condition
The most suitable polymerization condition for the preparation of high molecular weight polymers was determined on the basis of the previous studies [10], [11]. The polymerizations of HBA or HNA were performed in a mixture of dibenzyltoluene at a concentration of 3.0% at 300 °C for 24 h in the presence of BPBA. The amount of BPBA was one third of the total amount of the carboxylic acid that was the equimolar amount of BPB moiety to the total amount of the carboxyl group. HOBA and HOPH, which
Conclusions
The polymerizations of HBA and HNA in the presence of BPBA afforded high molecular weight POB and PON in the form of the precipitate under non-stoichiometric condition. The calculated DPn at χ of 20 mol% was only 5 but, nevertheless, the DPn values of the POB precipitates prepared at χHOBA of 20 mol% and at χHOPH of 20 mol% were 54 and 40, corresponding to Mn of 6.7 × 103 and 5.0 × 103, respectively. In the case of PON, those prepared at χHOBA and χHOPH of 50 mol% were 61 and 26, corresponding
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