Synthesis of divalent monomersFC acylation of 1a was conducted with 2.2 equimolar 3-chloropropyonyl chloride using aluminum trichloride as a catalyst (yield: 18.3%). The solubility of the products in common organic solvents, such as CHCl3, toluene, and EtOAc, was poor, leading to a low yield after purification. The subsequent E1cB reaction with triethylamine (Et3N) yielded 3a in 45% yield. To improve the yield in the first step, 9,9-dimethylfluorene (1b) was examined because of its high solubility. As expected, the FC acylation of 1b afforded 2b in an almost quantitative yield (93.3%). The subsequent E1cB reaction resulted in 3b (83.1%). The structure of 3b was determined via 1H (Fig. 1A) and 13C (Fig. S7) spectra. Note that the purification in each step only involved washing, which enabled the scale-up of the synthesis; 3b (42.5 g) was obtained from 1b (35.9 g). Similarly, the FC reaction of 1b with 2-bromopropionyl bromide followed by elimination afforded 5b (total yield: 34.7%).Fig. 11H NMR spectra of monomer 3b (A) and polymer P1 (B) (400 MHz, 25 °C, CDCl3). The full-scale spectra are shown in Fig. S13Polyaddition of divalent VKs with dithiols and diaminesThe nucleophilic thiol-ene reaction involves typical click chemistry that proceeds quantitatively under ambient conditions without any side reactions [13, 14]. Therefore, the polyaddition reaction of 3a and 1,10-decandithiol (DT1) was carried out in the presence of a catalytic quantity of tributylphosphine (Bu3P) in tetrahydrofuran (THF) (Scheme 2, Table 1, Entry 1). DT1 was chosen as an aliphatic dithiol with a sufficiently long distance between the two mercapto groups to avoid neighboring effects. The 1H NMR signals of the vinyl protons, observed at 7.24, 6.49, and 5.96 ppm in the spectrum of 3a (Fig. S5), disappeared after polymerization, whereas the signals of the methylene protons formed by the thiol-ene reaction clearly appeared at 3.26 and 2.51 ppm in the 1H NMR spectrum of P0 (Fig. S13). Similarly, the polyaddition reaction of 3b and DT1 resulted in P1 (Entry 2), confirmed by the disappearance of the 1H NMR signals of the vinyl protons at 7.24, 6.48, and 5.98 ppm and the appearance of methylene signals at 3.33 and 2.95 ppm (Fig. 1B). The polyaddition reactions of 4,4′-thiobisbenzenethiol (DT2), an aromatic dithiol, and ethylene glycol bis(3-mercaptpropyonate) (DT3), an ester-containing dithiol, also yielded the corresponding polymers (Entries 3 and 4). Similarly, 5b afforded the desired polymers via polyaddition reaction with dithiols (Entries 5–7). These results indicated that 3b and 5b functioned as divalent monomers. The chemical structures of the obtained polymers were determined via 1H NMR spectroscopy (Figs. S13–18).Scheme 2Polyaddition of divalent VKs containing a fluorene skeleton with dithiols or diaminesTable 1 Synthesis and properties of poly(thioether)s obtained via the polyaddition of divalent VKs and dithiolsAn advantage of vinyl ketones over acrylates is their efficient reaction with amines. Thus, polymerization with divalent secondary amines via aza-Michael addition was investigated (Entries 8–10). The polyaddition reaction of 3b and 1,4-piperadine (DA1) afforded a product insoluble in toluene, THF, CHCl3, and acetone (Entry 8). To examine whether the insolubility was caused by cross-linking via side reactions such as enamine formation or poor solubility, a model reaction using DA1 and 2-acryloyl-9,9-dimethylfluorene (9), ‘half’ of 3b, was employed under similar conditions for polymerization (Scheme S1). The resulting product was an aza-Michael adduct 10, and side reactions, such as enamine formation, were not observed in the 1H and 13C NMR spectra (Figs. S25 and 26). On the other hand, the solubility of the product was poor, and precipitation was observed during the reaction in THF. These results suggest that the insolubility of the product in Entry 8 was caused not by crosslinking due to side reactions but by the poor solubility of the resulting polymers. The polyadditions of 3a (Entry 9) and 3b (Entry 10) with N,Nʹ-dimetyl-1,6-hexanediamine (DA2) resulted in polymers that were soluble in THF. The SEC curves of the obtained polymers exhibited a low molar mass (Mn,SEC), although the 1H NMR spectra (Figs. S19 and S20) indicated the quantitative consumption of vinyl groups and sharp NMR signals assignable to each expected polymer structure. It has been reported that polyamines interact with the stationary phase of SEC columns, making the exact evaluation of their molar mass difficult [15]. For such polymers, 1H DOSY NMR is known to be effective in evaluating the weight-average molar mass (Mw,DOSY) [16]. The Mw,DOSY values correlated with polystyrene standards in CHCl3 were 39800 and 8300 g·mol−1 for P8 and P9, respectively, which were higher than those obtained by SEC analysis (Table S1 and S2, and Fig. S27). Therefore, polymerization via the aza-Michael reaction was as effective as that via thia-Michael addition.Except for Entry 8, the obtained polymers were readily soluble in THF and CHCl3. Moreover, the polymer obtained from 5b showed excellent solubility in other solvents, such as toluene and acetone (Entries 5–7), indicating its high solubility. The 5% weight loss temperatures (Td5) of the polymers, except those in Entries 3, 9 and 10, were above 250 °C (Figs. S28–S36). The Td5 of P2 (Entry 3) was 105 °C, which was much lower than those of the other polymers. TG-DTA analysis was repeated several times with to obtain similar results. Therefore, the low Td5 was not a technical error. The TG curves (Fig. S30) revealed degradation in three steps, and a 14% weight loss corresponding to benzene liberation was observed in the first step below 150 °C (molecular weight; benzene: 78.11, P2: 552.77. Proportion of the molecular weight of benzene in the polymer: 14%). On the other hand, P3, synthesized via polyaddition with DT3 in the same way as P2, did not decompose at that temperature. Therefore, the 1,4-dithiaphenylene skeletons of the DT2 moieties might specifically lead to thermal degradation in this step, although further investigations using TG-GC/MS, for example, are needed to draw concrete conclusions. P3, P4, P6, and P8 presented Tg values of 43, 17, 30, and 30 °C, respectively (Figs. S39–44).Modification with methylol groups for polycondensationPolyesters containing α,β-unsaturated carbonyl skeletons, such as acrylates and VKs, have attracted attention as polymers carrying reactive sites on the backbone [17,18,19,20,21,22,23]. To prepare such polyesters, the Morita‒Baylis‒Hillman (MBH) reaction is often applied both in monomer synthesis and polymerization reactions [18, 24,25,26]. MBH reactions are useful for incorporating a methylol group at the α-position of the acrylic compound [27, 28]. In general, however, the MBH reaction of acrylates is slow and competitive with some side reactions, such as dimerization of the product [24]. For this reason, a large excess of a catalyst or reagent (an aldehyde) is required in the MBH reaction of diacrylate to promote the reaction and to avoid side reactions as much as possible. In contrast, the high electrophilicity of VKs appears to be suitable for the MBH reaction. Therefore, the MBH reaction of 3b with formaldehyde was investigated (Scheme 3). Surprisingly, the reaction was completed within 35 min, confirming the high reactivity of the VKs. After purification by washing with CHCl3, 6b was obtained with 50.9% yield, confirmed by 1H NMR spectrum (Fig. 2A).Scheme 3The MBH reaction and subsequent polymerization of 3bFig. 21H NMR spectra of monomer 6b (A, 400 MHz, 25 °C, dimethyl sulfoxide-d6) and polymer P10 (B, 400 MHz, 25 °C, CDCl3)6b, which has two methylol groups on the VK skeleton, was polymerized with isophthaloyl dichloride (7) (Table 2). A solution of 7 was added dropwise to a solution of 6b and Et3N. The reaction was performed in dichloromethane (CH2Cl2), THF, and DMF (Entries 1–3) and resulted in the expected polyesters, confirmed by 1H NMR spectrum (Fig. 2B). Notably, Et3N reportedly acts as a nucleophile to cause main-chain scission in similar polyesters; Lu et al. explained that the main-chain scission occurred by acyl substitution [17], whereas conjugate substitution was also a possible mechanism [22]. Moreover, Et3N may cause main-chain scission and may not be a suitable base for polycondensation reactions. Therefore, a similar polycondensation reaction was employed using EtNiPr2, a base with low nucleophilicity (Entry 4). The resulting molar mass was greater than that in Entry 1, although the difference was not significant. In addition, polycondensation reactions were carried out using an excess (4.0 equimolar to 7) of Et3N (Entry 5). The molar mass of the resulting product decreased slightly compared with that in Entry 2. Because the differences in the resulting molar masses in these entries were small, the influence of main-chain scission by Et3N seems insignificant in this polymerization system.Table 2 Polycondensations of 6b and 7The thermal properties of the polymers obtained were also investigated. Td5 was determined to be 91 °C by TG-DTA, and no exothermic peaks assignable to the curing point were observed (Fig. S37). The low thermal stability of the molecule despite the rigid aromatic skeleton was somewhat surprising; notably, monomer 6b also exhibited low thermal stability (Fig. S38). Thus, the low stability was attributed to the VK skeletons.
