Article Cite This: Macromolecules XXXX, XXX, XXX−XXX
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A Facile Synthetic Route to Ether Diols Derived from 1,1Cyclopentylenylbisphenol for Robust Cardo-Type Polyurethanes Chien-Hsin Wu,†,‡ Yu-Ru Lin,†,‡ Shih-Chieh Yeh,†,‡ Ya-Chin Huang,†,‡ Kuo-Hua Sun,†,‡ Yeng-Fong Shih,∥ Wen-Chiung Su,⊥ Chi-An Dai,§ Shenghong A. Dai,*,# and Ru-Jong Jeng*,†,‡ †
Institute of Polymer Science and Engineering, ‡Advanced Research Center for Green Materials Science and Technology, and Department of Chemical Engineering, National Taiwan University, Taipei 10617, Taiwan ∥ Department of Applied Chemistry, Chaoyang University of Technology, Wufeng District, Taichung 41349, Taiwan ⊥ National Chung-Shan Institute of Science & Technology, Longtan District, Taoyuan City 32546, Taiwan # Department of Chemical Engineering, National Chung Hsing University, Taichung 402, Taiwan
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ABSTRACT: An efficient scheme for the synthesis of 1,1cyclopentylenylbisphenol (bisphenol CP) has been developed starting from dicyclopentadiene, a C5 byproduct from the petroleum cracking process. The synthetic steps leading to bisphenol CP consist of mostly isomerization and addition reactions, which are higher in their atom-economy efficiencies than those based upon condensation reactions. In addition, alkoxylation by cyclic carbonates converted bisphenol CP into ethoxylated and propoxylated ether diols. The transformation of hydroxyl functional groups from bisphenols to alkoxylated alcohols increased the reactivity of their hydroxyl groups toward isocyanates, as evidenced by achieving >3 times higher molecular weights of the segmented polyurethanes (PUs) in GPC analysis using alkoxylated diols as chain extenders instead of bisphenols. In addition, the incorporation of five-membered cardo-type groups onto the PU side chains through alkoxyl diols of bisphenol CP also significantly enhances the phase mixing of the resulting hard and soft segments, leading to a series of robust PUs.
1. INTRODUCTION Bisphenols and their diol derivatives are the important polymeric intermediates for making contemporary polymers. Industrial commodity polymers such as polycarbonates (PCs), epoxy resins, polysulfones, and polyurethanes (PUs) have all utilized bisphenols such as bisphenol A (BPA), bisphenol F (BPF), and bisphenol S (BPS) as their key building blocks.1−5 The feasibility of converting these bisphenols into useful intermediates still has been sought actively to extend their utilities into polymers with special performances. 4-[1-(4-Hydroxyphenyl)cyclopentylenyl]phenol, bisphenol CP, bearing cyclopentylenyl five-membered ring in its molecule, is one of our targeted bisphenols due to its possessing cardo-type intermediates to expand our research in this special field.6 Current production of bisphenol CP requires strong acid-catalyzed condensation reaction of cyclopentanone with phenol,7,8 as shown in Scheme 1. Strong acids and excess phenol used in the condensation processes generate environmentally unfriendly acid wastes while giving only low-yield bisphenol CP. Furthermore, the production bisphenol CP has been hinged upon cyclopentanone as the raw material, which is synthesized through oxidation of ketone− alcohol oil of cyclohexane (KA oils) and low-yield process including base cyclization of the resulting adipic acid.9 It is the © XXXX American Chemical Society
purpose of the present paper to report a new alternative route based on cyclopentadiene as the raw material in the syntheses of bisphenol CP and its alkoxylated diols for further PU evaluations. In our new route, the synthesis of bisphenol CP utilized the dimer of cyclopentadiene as the starting material. Dicyclopentadiene (DCPD) is one of the C5 byproducts generated in the steam-cracking process of naphtha,10 which is an abundant and inexpensive raw material. Furthermore, our synthetic steps leading to bisphenol CP consist of mostly isomerization and addition reactions, which are higher in their atom-economy efficiencies than those based upon condensation reactions.11 In our study, it was also found that alkoxylations of bisphenols could afford primary and secondary alkoxylated ether diols with increased basicity that imparted higher reactivity toward isocyanates. High molecular weight PUs could then be prepared through a traditional two-step process by using the novel diols possessing a cardo moiety in their structures as the chain extenders. By incorporation of cardo five-membered ring structures into the PUs (Scheme S1), we have found that the Received: October 11, 2018 Revised: January 8, 2019
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DOI: 10.1021/acs.macromol.8b02186 Macromolecules XXXX, XXX, XXX−XXX
Article
Macromolecules
Scheme 1. Preparations of 4-[1-(4-Hydroxyphenyl)cyclopentyl]phenol (C0) and Its Key Intermediates with Primary Hydroxyl (C1) or Secondary Hydroxyl (C2) Functional Groupsa
a Reagents and conditions: (a) phenol, phosphoric acid, room temperature, 2 h, 80−85%; (b) Pd(II), toluene, 90 °C, 91−95%; (c) phenol, hydrochloric acid, room temperature, 90 °C, 70−75%; (d) ethylene carbonate, sodium carbonate, 170 °C, 75−80%; (e) propylene carbonate, sodium carbonate, 170 °C, 80−85%.
Figure 1. IR spectra of C0, C1, and C2 cardo-type monomers and the resulting PUs.
resulting polymers were imparted with certain distinctive mechanical and morphological properties as compared to those of the PUs based on common bisphenols. These cardo-type PUs exhibited a large modulus variation at glass transition, and an extended rubbery plateau in dynamic mechanical analyses, which might be fit for shape memory materials.12−15
in a two-step sequence from DCPD and phenol as the raw materials. We employed essentially the same strategies and conditions developed previously by Dai and co-workers.16 In the first step, phenol was directly alkylated with cyclopentadiene in the presence of phosphoric acid at room temperature. A yield of about 80% of one-to-one alkylated phenols was obtained, but the products consisting of two isomers, 4-(cyclopenten-2-yl)-phenol (1) and 2-(cyclopenten2-yl)-phenol, in 88 to 12 ratio were isolated. After the separation by fractional distillation, isomer (1) was then subjected to isomerization using dichlorobis(benzonitrile)palladium(II) as the catalyst to shift the olefin bond into the conjugated position to the aromatic ring. The new isomer, 4-
2. RESULTS AND DISCUSSION Preparation of Bisphenol CP (C0) and Its Alkoxy Ether Diols (C1 and C2) (Scheme 1). Bisphenol CP (C0) and Derivatives Prepared from the Alkylation of Phenol with 1,3-Cyclopentadiene. In our efforts to prepare bisphenol CP, two interim isomeric phenols, (1) and (2), were prepared B
DOI: 10.1021/acs.macromol.8b02186 Macromolecules XXXX, XXX, XXX−XXX
Article
Macromolecules Table 1. Chemical Structure of Bisphenolic Intermediates in This Study
a
Molecular weight was determined by FAB MASS. bMelting point was measured by open capillary method. cBond angle between two phenolic groups was calculated using the density functional theory (DFT) and the B3LYP functional method in conjunction with the 6-31G(d,p) basis set.
For the propoxylation process of C0 by using PPC, the reaction conditions are about the same as those of ethoxylation, but the propoxylated products are slightly more complex. This is due to the asymmetrical structure of the PPC compound, resulting in formation of two isomeric propoxylated products for each phenol group. However, the formation of secondary propoxylated ether alcohols does appear to be always the dominant products over its isomeric primary alcohols. This is because the phenolic ion has a higher tendency to attack the less hindered position of PPC, resulting in the formation of major product (C2), as shown in Scheme 1. By comparison of the peak area by integrations obtained in the 1H NMR spectrum between 1.14 and 1.07 ppm, one could easily estimate the isomeric ratio between primary alcohol and secondary alcohol to be 12% to 88%, respectively. Therefore, the doublet peaks observed at 3.60 and 3.90 ppm are also indicative of the existence of two regioisomers from each propoxylation. To further the confirmation of our structural assignments, two more bisphenolic compounds, bisphenol F (F0) and bisphenol A (A0), were similarly alkoxylated under similar processing condition (F1 for ethoxylated F0, F2 for propoxylated F0; A1 for ethoxylated A0, and A2 for propoxylated A0). The key physical and spectral data are all compiled in Table 1 and Figures S4−S7. Interestingly, the molecular simulation of these key intermediates suggests that the bond angles between two phenolic groups are noticeably larger (∼115°) for the compounds without cardo-containing structures, F0, F1, and F2. This result reveals that BPF-based polymer intermediates exhibited better coplanarity than the compounds with cardocontaining structures (A0, A1, A2, C0, C1, and C2); the bond angles are somewhat smaller, ∼109°. It is important to note
(cyclopenten-2-yl)-phenol, (2), was isolated almost in quantitative yield. In the subsequent step, alkylation of (2) under excess of phenol under acidic media yielded bisphenol CP (C0). This alkylation was accomplished by a simple protonation of (2) with concentrated HCl at 90 °C by trapping of the interim carbocation with phenol as shown in Scheme 1. After recrystallization of the crude product using toluene as the solvent, pure bisphenol CP with a melting point (Mp) of 150 °C was obtained in 70% isolated yield. The bisphenol CP structure was confirmed by 1H NMR (Figure S1) and IR (Figure 1). Ether Diols C1 and C2 Prepared from the Alkoxylation of Bisphenol CP. Because phenolic hydroxyl groups are slightly acidic in nature17 and far less reactive toward isocyanates than primary or secondary hydroxyl groups of alkyl alcohols,18 alkoxylation of C0 to produce short-alkoxylated derivatives was performed to produce diols for use in PU applications.19,20 These alkoxylation reactions were done by reacting C0 with ethylene carbonate (EC) or propylene carbonate (PPC) at 160−180 °C for producing the respective ethoxylated or propoxylated diols, and the spectra of recrystallized products, C1 and C2, are shown in Figures S2 and S3, respectively. In the syntheses of ethoxylated diol (C1), C0 and EC were heated in a solution of dimethylacetamide (DMAc) in the presence of Na2CO3 as catalyst at ∼170 °C for 2° alcohol > phenol,21 the corresponding Mw appears to reflect in the same order for the PUs based on 1° alcohol-, 2° alcohol-, and phenol-type chain extenders. Phase Separation of PUs. Typically, PUs are composed of soft segments (SS), usually polyether or polyester, and hard segments (HS), which typically consist of diisocyanates and chain extenders. Because of thermodynamic incompatibility, the HS phase-separate into hard domains, while SS aggregate into soft domains. As a result, the thermomechanics of PUs greatly depend on the phase separation morphology between hard and soft segments, which can be adjusted by changing the HS/SS ratios and chemical structures of monomers.22−27 The phase separation was investigated by the DSC second heating of the PU samples, as shown in Figure 2. For the PUs chain-extended with F0, F1, and F2, relatively low glass transition temperatures, Tgs (−41.9, −39.0, and −7.5 °C for F0-PU, F1-PU, and F2-PU, respectively), were attributed to the thermal transition of PCL-based soft segment.28 In
that bond angles do not change after ethoxylation or propoxylation of the bisphenolic compounds (Table 1). PU Syntheses by Using Bisphenolic Intermediates as Chain Extenders. In this study, there are three comparative series of PUs, synthesized based on bisphenol F’s (FPUs), bisphenol A’s (APUs), and cardo five-membered rings containing bisphenol CP’s (CPUs). Because the properties of PUs are highly dependent on the formulation ratios and types of the building blocks such as compositions of hard segment (HS, isocyanates and chain extenders) and soft segment (SS, polyester), the HS contents of PUs in this study were all fixed at 50 wt % for the present comparative study. In each of the three series, three hard segments including bisphenols and their corresponding exthoylated and propoxylated ether diols were incorporated into PUs as chain extenders, as shown in the formulations in Table 2. The preparation of PUs was conducted by a traditional twostep process. First, MDI and polycaprolactone diol (PCL3000) were mixed together for 0.5 h for obtaining the isocyanatecapped prepolymers, followed by the addition of various diol extenders (C0, C1, C2, A0, A1, A2, F0, F1, and F2) as chain extenders. After vigorous stirring for 3 h, PU thin films were prepared in a Teflon plate by vacuum removal of the solvent. The syntheses of PUs were characterized by IR spectra, as shown in Figure 1. The major absorption peaks at 1720 cm−1 (CO stretching of urethane), 1537 cm−1 (N−H deformation of urethane), and 1236 cm−1 (C−O−C adsorption of urethane) along with the absence of peak at 2270 cm−1 (isocyanate adsorption) in the spectrum for the C0-PU sample were indicative of the 100% conversion of isocyanates into formation of urethane groups. In the IR spectra of C1-PU and C2-PU, similar peaks at about 1725 cm−1 (CO stretching of urethane) and ∼1535 cm−1 (N−H deformation of urethane) were observed. However, the peaks at about 1050 cm−1 (C(O)−O−C adsorption of urethane) would vibrate at slightly lower frequencies due to the fact that C1-PU and C2-PU were chain extended from primary and secondary alcohol, respectively. The molecular weights of PUs were measured by using GPC, as shown in Table 2. PUs prepared from phenolic monomers such as F0-PU, A0-PU, and C0-PU all exhibited weight-average molecular weight (Mw) of
