Introduction

To address the problem of environmental pollution, particularly microplastic pollution, polymer chemists are working on developing novel polymer synthesis methods. Of particular importance is the use of biobased materials for sustainability because of the high biodegradability and attractive thermal properties of the produced polymers, a combination that has unfortunately been difficult to achieve. In recent decades, the most reliably biodegradable polymers have been aliphatic polyesters [1,2,3,4,5], but these polymers also have drawbacks with respect to their thermal properties [6,7,8,9]. Even though unsaturated polyesters have improved thermal and mechanical properties [10,11,12], investigations on the biodegradation of unsaturated polyesters are still limited [10, 13,14,15]. Approaches employing chemical modification are also possible using internal double bonds as reactive centers. However, the synthetic procedure generally involves the polycondensation of unsaturated dicarboxylic acids and diols under severe dehydration or transesterification conditions (>200 °C). Moreover, we reported the dehydration polycondensation of fumaric and maleic acids with several diols, during which the undesirable polycondensation isomerization of internal double bonds and cross-linking occurred [16,17,18].

Over the last decade, click chemistry has progressed, and several polymers have been synthesized via successive click reactions, i.e., click polymerization [19,20,21,22]. We also reported click polymerizations using monomers containing ester linkages via copper-catalyzed alkyne-azide and thiol-ene click reactions, affording polyesters containing triazole rings and thioether moieties. As extensions to this work, we synthesized poly(ester-thioether)s using Michael addition-type thiol-ene click reactions of two diacrylates esterified by 1,4:3,6-dianhydroglucitol and 1,4:3,6-dianhydromannitol with several dithiols at ambient temperature and investigated the biodegradability as well as the thermal properties of the synthesized poly(ester-thioether)s. As a result, while some polymers had limited biodegradability, their thermal properties were overall very poor [23]. Therefore, in the present study, we attempted to synthesize biodegradable unsaturated poly(ester-thioether)s with relatively high glass transition temperature (Tg) value via thiol-yne Michael addition. The synthesis of unsaturated poly(ester-thioether)s using a thiol-yne Michael addition has been previously reported, but its monomer synthesis consists of complicated and unsustainable procedures, and the polymer biodegradability has not yet been investigated [24].

In our previous study, the introduction of an unsaturated bond into the main chain of a polyester influenced not only its thermal properties but also its biodegradability. In 2002, we reported an interesting finding that the biodegradability of unsaturated polyesters is affected by the E/Z ratio of the polymer, where polyester with an E/Z ratio of 70/30 showed the fastest degradation of the series of unsaturated polyesters [18]. In 2013, it was reported that the geometry of a thiol-yne Michael addition is more controlled in the Z configuration for more polar reaction solvents [24,25,26,27]. We hypothesized that the unsaturated polyester geometry could be selectively changed by changing the solvent used in the thiol-yne Michael addition polymerization, thereby controlling the thermal properties and biodegradability of the polymer.

In this study, we first prepared a dialkynyl monomer using 1,4-butanediol (BD) as a starting material and performed thiol-yne click polymerizations with several dithiols. Subsequently, we performed similar polymerizations using meso-erythritol (mET) and isomannide (IM)-based dialkynyl monomers, as shown in Scheme 1. We expected that using these biobased starting materials with several dithiols would increase both the thermal properties and biodegradability of the polymer.

Scheme 1
scheme 1

Preparation of dialkynyl monomers catalyzed by Sc(OTf)3

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Experimental section

Materials

mET (>99%), propiolic acid (PA) (>98%), 1,4-butanedithiol (BDT) (>95.0%), ethylene bis(thioglycolate) (EBTG) (>97%), D,L-dithiothreitol (DTT) (>98.0%), 3,6-dioxa-1,8-octanedithiol (DODT) (97.0%), IM (>98.0%), mercaptoacetic acid (MA) (>95%), butanediol diacrylate (BDA), and scandium(III) trifluoromethanesulfonate [Sc(OTf)3] (>98.0%) were obtained from Tokyo Chemical Industry Co., Ltd. (Tokyo, Japan) and used as provided. BD and aniline were purchased from Wako Co. (Osaka, Japan) and used as delivered. Standard buffer solution (pH 6.86 ± 0.02) was purchased from NACALAI TESQUE, Inc. (Kyoto, Japan) and used as delivered. Lipase, which was immobilized on Immobead 150 from Thermomyces lanuginosus, was purchased from Sigma‒Aldrich, Inc. (Missouri, USA) and used as delivered. Standard chemicals and other solvents were purchased from commercial suppliers and used without purification.

Analytical measurements

1H NMR spectra (at 400 MHz) were measured at 27 °C in a Bruker Analytik DPX400 spectrometer. The internal standard was tetramethylsilane (0 ppm). The molecular weights of some monomers were obtained from electron spray ionization mass spectrometry (ESI-MS) measurements using a Synapt G2 HDMS+Acquity manufactured by Waters Co., Ltd. The samples were dissolved in a mixture of dichloromethane and methanol, and the measurements were performed in positive ion mode using sodium trifluoroacetic acid (TFANa) as the cationic agent. The number average molecular weight (Mn) and polydispersity index (Mw/Mn) of the poly(ester-thioether)s were measured by Shodex KD803 and KD804 size exclusion chromatography (SEC) columns on an SEC system consisting of a PU-2080 pump system, an RI-2031 differential refractometer, and a co-2065 Plus Intelligent Column Oven (all from JASCO; Tosoh Corp.). The eluent was N,N-dimethylformamide with lithium bromide (0.05 wt%), the flow rate was 0.5 ml/min, the temperature was 40 °C, and calibration was performed using poly(methyl methacrylate)s as standards. Differential scanning calorimetry (DSC) was performed on a DSC7020 instrument (HITACHI). The initial heating was from 30 to 100 °C at a rate of 10 °C/min. The temperature was then reduced to −50 °C, and a second heating step was performed from −50 to 150 °C at 10 °C/min. The Tg was determined by the second heating. All measurements were carried out under a nitrogen gas flow.

Preparation of monomers

1,4-Butanediol dipropiolate (BDP)

BD (1.3 ml, 15 mmol), PA (3.7 ml, 60 mmol) and Sc(OTf)3 (74 mg, 1 mol% for BD) were added to an eggplant flask under N2 in a glovebox and stirred. Afterward, the solution was kept at 60 °C for 21 h. The reaction mixture was then dissolved in a trace amount of methanol and precipitated twice in cold water. The monomer was dried in vacuo at 50 °C to obtain BDP (69% yield) as a light brown powder. 1H NMR (400 MHz, DMSO-d6, δ, ppm): 1.67 (t, 4H, -CH2-CH2-, 6.0 Hz), 4.17 (t, 4H, H≡C-CH2-, 6.0 Hz), 4.55 (s, 2H, H≡C-) (Fig. S1). 13C-NMR (100 MHz, DMSO-d6, δ, ppm): 24.82 (-O-CH2-CH2-), 66.01 (-O-CH2-CH2-), 75.28 (H≡C-), 79.40 (H≡C-CH2-), 152.72 (-(C=O)-) (Fig. S2).

meso-Erythritol dipropiolate (mETP)

mET (1.8 g, 15 mmol), PA (1.9 ml, 30 mmol) and Sc(OTf)3 (74 mg, 1 mol% for mET) were added to an eggplant flask under N2 in a glovebox and stirred. Afterward, the solution was kept at 70 °C under reduced pressure conditions for 24 h. The reaction mixture was then dissolved in ethyl acetate and passed through a silica gel column to remove byproducts. The ethyl acetate was then concentrated by vacuum distillation and recrystallized by adding heated chloroform. After recrystallization, the product was washed by suction filtration and dried in vacuo at 50 °C to obtain mETP (41% yield) as a white powder. 1H NMR (400 MHz, DMSO-d6, δ, ppm): 3.60 (s, 2H, -CH2-CHOH-), 4.14 (dd, 2H, H≡C-CH2-, 12.3 Hz), 4.33 (dd, 2H, H≡C-CH2-, 12.3 Hz), 4.57 (s, H≡C-), 5.34 (d, -OH, 5.8 Hz) (Fig. S3). 13C-NMR (100 MHz, DMSO-d6, δ, ppm): 68.19 (-O-CH2-), 69.05 (-CHOH-), 75.34 (H≡C-), 79.44 (H≡C-CH2-), 152.86 (-(C=O)-) (Fig. S4). In the ESI-MS spectrum, the molecular weight was 249.04 (theoretical molecular weight: 249.18 for C10H10O6 + Na) (Fig. S5).

Isomannide dipropiolate (IMDP)

IM (2.4 g, 10 mmol), PA (2.2 ml, 40 mmol), and Sc(OTf)3 (49 mg, 1 mol% for IM) were added to an eggplant flask under N2 in a glovebox and stirred. Afterward, the solution was kept at 70 °C under reduced pressure conditions for 12 h. The reaction mixture was then dissolved in methanol and precipitated twice in cold water. The monomer was dried in vacuo at 50 °C to obtain IMDP (38% yield) as a light brown powder. 1H NMR (400 MHz, DMSO-d6, δ, ppm): 3.73 and 3.98 (dd, 4H, sugar H-1 and H-6, 9.7 Hz), 4.64 (s, 2H, H≡C-), 4.67 (dd, 2H, sugar H-3 and H-4, 4.1 Hz), 5.10-5.17 (m, 2H, sugar H-2 and H-5) (Fig. S6). 13C-NMR (100 MHz, DMSO-d6, δ, ppm): 70.16 (sugars H-1 and H-6), 74.73 (H≡C-), 75.62 (sugars H-2 and H-5), 80.24 and 80.28 (H≡C-CH2-, sugars H-3, and H-4), 151.90 (-(C=O)-) (Fig. S7). ESI-MS spectrum revealed a molecular weight of 273.04 (theoretical molecular weight: 273.21 for C10H10O6 + Na) (Fig. S8).

D-2,5-Di-O-(2-mercaptoacetate)-1,4:3,6-dianhydromannitol (MAIM) synthesis

The synthesis was conducted using the same method detailed in a previous study [23].

Unsaturated poly(ester-thioether) synthesis via a thiol-yne Michael polyaddition

The thiol-yne Michael polyaddition of BDP, mETP or IMDP with dithiols was carried out in a flame-dried eggplant flask. The practical procedure for the use of dithiol BDT and chloroform as the reaction solvent is given here as an example. BDP (3.2 × 102 mg, 1.5 mmol) and BDT (1.7 × 102 μl, 1.5 mmol) were dissolved in dry chloroform (2.9 ml) in a flame-dried 20 ml eggplant flask under N2 in a glovebox and stirred at −20 °C for 15 min. Next, 29 µl of Et3N was added to chloroform to make a 1.0 ml solution, which was stirred thoroughly. An Et3N solution (100 μl) was added to a mixture of BDP, BDT and chloroform (3.0 ml of chloroform, 2.9 µl of Et3N, and 1 mol% dithiol) and stirred for 5 min, after which the solution was kept at room temperature for 72 h. The reaction mixture was then diluted with reaction solvent and precipitated twice in hexane. The polymer was dried in vacuo at 40 °C to yield poly(BDP-alt-BDT) as a brown polymer. When DMF was used as the reaction solvent, the products were precipitated by methanol instead of hexane. The poly(ester-thioester)s were structurally characterized by NMR and SEC and thermally characterized by DSC.

poly(BDP-alt-BDT) [E/Z = 83/17]

1H NMR (400 MHz, CDCl3, δ, ppm): 1.75 (brs, 4H, -S-CH2-CH2-), 1.83 (brs, 4H, -O-CH2-CH2-), 2.76–2.90 (m, 4H, -S-CH2-CH2-CH2-CH2-S-), 4.16 (brs, 4H, -O-CH2-CH2-), 5.74 (d, 2H, -S-CH = CH-(trans), 14.1 Hz), 5.86 (d, 2H, -S-CH=CH- (cis), 10 Hz), 7.06 (d, 2H, -S-CH=CH- (cis), 10.1 Hz), 7.66 (d, 2H, -S-CH=CH- (trans), 15.1 Hz) (Fig. 1).

Fig. 1
figure 1

1H NMR spectrum of poly(BDP-alt-BDT) (CDCl3, 400 MHz, TMS)

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poly(BDP-alt-EBTG) [E/Z = 49/51]

1H NMR (400 MHz, DMSO-d6, δ, ppm): 1.75 (brs, 4H, -O-CH2-CH2-), 3.46 (d, 4H, -S-CH2- (trans), 7.4 Hz), 3.61 (d, 4H, -S-CH2- (cis), 8.5 Hz), 4.13–4.22 (brm, 4H, -O-CH2-CH2-CH2-CH2-O-), 4.37 and 4.40 (2s, 4H, -COO-CH2-), 5.83 (dd, 2H, -S-CH=CH-(trans), 15.0 Hz), 5.93 (dd, 2H, -S-CH=CH-(cis), 10.0 Hz), 7.21 (d, 2H, -S-CH=CH- (cis), 10.1 Hz), 7.65 (d, 2H, -S-CH=CH- (trans), 15.2 Hz) (Fig. S9).

poly(BDP-alt-DTT) [E/Z = 49/51]

1H NMR (400 MHz, DMSO-d6, δ, ppm): 1.65 (brs, 4H, -O-CH2-CH2-), 2.78–3.11 (m, 4H, -S-CH2-), 3.61 (t, 4H, -CH2-CHOH-, 6.4 Hz), 4.08 (d, 4H, -O-CH2-CH2-, 4.4 Hz), 5.10, 5.15. 5.22, 5.27 (4d, 2H, -OH, 6.4 Hz), 5.84 (d, 2H, -S-CH=CH-(cis), 10.1 Hz), 5.86 (d, 2H, -S-CH=CH-(trans), 15.2 Hz), 7.47 (dd, 2H, -S-CH=CH- (cis), 10.1 Hz), 7.80 (d, 2H, -S-CH=CH- (trans), 15.1 Hz) (Fig. S10).

poly(BDP-alt-DODT) [E/Z = 82/18]

1H NMR (400 MHz, CDCl3, δ, ppm): 1.75 (brs, 4H, -O-CH2-CH2-), 2.95 (t, 4H, -S-CH2-CH2- (cis), 6.5 Hz), 3.02 (t, 4H, -S-CH2-CH2- (trans), 6.5 Hz), 3.64 (t, 4H, -O-CH2-CH2-O-, 4.7 Hz), 3.70–3.77 (m, 4H, -S-CH2-CH2-), 4.16 (brs, 4H, -O-CH2-CH2-CH2-CH2-O-), 5.79 (d, 2H, -S-CH=CH-(trans), 15.3 Hz), 5.84 (d, 2H, -S-CH=CH-(cis), 10.1 Hz), 7.19 (d, 2H, -S-CH=CH- (cis), 10.1 Hz), 7.70 (d, 2H, -S-CH=CH- (trans), 15.2 Hz) (Fig. S11).

poly(mETP-alt-BDT) [E/Z = 71/29]

1H NMR (400 MHz, DMSO-d6, δ, ppm): 1.73 (brs, 4H, -S-CH2-CH2-), 2.81–3.01 (m, 4H, -S-CH2-CH2-), 3.63 (brs, 4H, -O-CH2-CH-), 3.97–4.07 (m, 2H, -O-CH2-CH(-OH)-CH(-OH)-CH2-O-), 4.20–4.29 (m, 2H, -O-CH2-CH(-OH)-CH(-OH)-CH2-O-), 5.11 (brs, 2H, -OH), 5.84 (d, 2H, -S-CH=CH-(trans), 15.2 Hz), 5.89 (d, 2H, -S-CH=CH-(cis), 10.1 Hz), 7.45 (d, 2H, -S-CH=CH- (cis), 10.2 Hz), 7.80 (d, 2H, -S-CH=CH- (trans), 15.1 Hz) (Fig. S12).

poly(mETP-alt-EBTG) [E/Z = 53/47]

1H NMR (400 MHz, DMSO-d6, δ, ppm): 3.62 (brs, 4H, -O-CH2-CH-), 3.77 (d, 4H, -S-CH2- (trans), 2.6 Hz), 3.94 (d, 4H, -S-CH2- (cis), 2.6 Hz), 3.99–4.07 (m, 2H, -O-CH2-CH(-OH)-), 5.13 (brs, 2H, -O-CH2-CH(-OH)-), 4.31 and 4.33 (2s, 4H, -COO-CH2-CH2-COO-), 5.14 (brs, 2H, -OH), 5.87 (d, 2H, -S-CH=CH-(trans), 15.1 Hz), 5.96 (d, 2H, -S-CH=CH-(cis), 10.1 Hz), 7.42 (d, 2H, -S-CH=CH- (cis), 10.0 Hz), 7.80 (d, 2H, -S-CH=CH- (trans), 15.0 Hz) (Fig. S13).

poly(IMDP-alt-EBTG)[E/Z = 47/53]

1H NMR (400 MHz, DMSO-d6, δ, ppm): 3.62–3.72 (m, 4H, -S-CH2-), 3.79 (s, 2H, sugar H-1a and H-6a), 3.96 (s, 2H, sugar H-1b and H-6b), 4.31 (brs, 4H, -COO-CH2-), 4.65 (m, 2H, sugar H-3 and H-4), 5.05 (brs, 2H, sugar H-2 and H-5), 5.90 (d, 2H, -S-CH=CH-(trans), 15.3 Hz), 5.70–5.99 (d, 2H, -S-CH=CH-(cis), 10.2 Hz), 7.48 (d, 2H, -S-CH=CH- (cis), 9.9 Hz), 7.81 (d, 2H, -S-CH=CH- (trans), 15.3 Hz) (Fig. S14).

poly(IMDP-alt-MAIM) [E/Z = 48/52]

1H NMR (400 MHz, DMSO-d6, δ, ppm): 3.62–3.77 (m, 4H, -S-CH2-), 3.80–4.00 (m, 8H, sugar H-1 and H-6), 4.64 (m, 4H, sugar H-3 and H-4), 4.98–5.12 (brm, 4H, sugar H-2 and H-5), 5.91 (d, 2H, -S-CH=CH-(trans), 15.2 Hz), 5.99 (d, 2H, -S-CH=CH-(cis), 10.2 Hz), 7.47 (d, 2H, -S-CH=CH- (cis), 10.2 Hz), 7.79 (d, 2H, -S-CH=CH- (trans), 15.2 Hz) (Fig. S15).

Measurement of enzymatic degradation by total organic carbon (TOC) testing

Each sample (5.0 mg) was dissolved in its respective optimal solvent, added to a test tube, and allowed to dry for several days. Two aliquots were prepared for each sample: 2.0 ml of standard buffer (pH 6.86 ± 0.02) was added to both, and 50 units of immobilized lipase were added to one of the aliquots. In the other tube, the hydrolysis test was performed in the absence of the lipase as a negative control. The TOC values were measured three times for each solution, and the average value was recorded. All test tubes were shaken in an incubator at 37 °C for 24 h. After the solution was filtered, the total organic carbon (TOC) content was measured using a total organic carbon analyzer (TOC-2300, HIRANUMA Corporation, Mito, Japan).

Biodegradation of poly(ester-thioether)s by biochemical oxygen demand (BOD) testing

Approximately 10 mg of each poly(ester-thioether) sample dissolved in 10 ml of THF or DMF was placed in two test tubes and dried at room temperature for several days. We checked the BOD values every day. Activated sludge (30 mg l−1) obtained from the Nagoya City Waterworks and Sewerage Bureau was added to basal culture medium comprising the following (mg l−1): K2HPO4, 218; KH2PO4, 85; Na2HPO4, 334; NH4Cl, ; CaCl2, 28; MgSO4•7H2O, 23; and FeCl3•6H2O, 0.25 (pH = 7.4). The solution was injected into each test tube to give a poly(ester-thioether) concentration of 100 mg l−1. The test bottles were stirred in a water bath at 25 °C for 30 days, and the oxygen consumption of each test bottle was recorded. Biochemical oxygen demand (BOD) was measured according to ISO guidelines (ISO 14851) by oxygen consumption using a BOD device (Model 200F, TAITEC Co., Koshigaya-shi, Japan). The biodegradability (%) of the sample was calculated as [(BODsample − BODblank)/TOD] × 100. Here, BODsample and BODblank are the actual observed values of the sample and blank tube, respectively, and TOD is the theoretical value of the oxygen demand of the sample calculated assuming that the sample was completely oxidized. For the control sample, the BOD of aniline was also measured. The oxygen consumption for each sample was recorded as the average of the values from the two test bottles.

Cross-linking of poly(ester-thioether) by thiol-ene addition

In a Teflon mold, poly(BDP-alt-EBTG) [synthesized under the conditions of Run 5] (1.0 mmol, 202 mg), EBTG (0.5 × 10−1 mmol), TEA (20 mol% to EBTG), and 1 ml of THF were added and dissolved thoroughly. The mold was set on a hot plate at 50 °C and reacted for 5 h. The gel was removed from the mold, soaked in 30 ml of THF for 1 day, removed from the THF and soaked in fresh THF. After this washing process was performed three times, the THF was removed in a vacuum dryer at 50 °C (size after drying: 27 × 11 × 2 mm). Then, tensile testing was performed to determine the modulus of elasticity, which was 4.5 (N/mm2). The measurements were performed using AGS-X (SHIMADZU Co., Kyoto, Japan) at a rate of 10 mm/min.

Results and discussion

First, the esterification of BD in 4 equiv. of PA (Scheme 1) was carried out at 60 °C for 21 h with stirring to obtain the expected 1:2 adduct (BDP) of BD and 2 equiv. of PA as the dialkynyl monomer in 69% yield. The use of the Sc(OTf)3 catalyst enabled the synthesis of dialkynyl monomers by direct esterification without any other condensation reagents. Monomers based on mET and IM were also synthesized according to a similar procedure, and the expected 1:2 adducts mETP and IMDP were obtained in 41% and 38% yields, respectively. The structures were confirmed by 1H and 13C NMR and ESI-MS measurements (Figs. S1–S8).

Subsequently, as shown in Table 1 and Scheme 2, we carried out the polyaddition of BDP with BDT (Runs 1–3), EBTG (Runs 4–6), DTT (Run 7), or DODT (Run 8) in CHCl3 in the presence of Et3N at room temperature for 72 h. After the indicated time, the reaction mixture was precipitated in hexane to yield the polymeric material. The 1H NMR spectra indicated that the expected poly(ester-thioether)s were prepared (Figs. 1 and S9–S15), and we also confirmed that the polyaddition proceeded via a thiol-Michael addition from the SEC analyses. In theory, the Mw/Mn reached 2.0 when the reactivity was 100%. From the SEC analyses, the Mw/Mn indices are 1.75–2.00, which are consistent with the theoretical values. The Tgs range from −27 to 27 °C. We concluded that Runs 1–3 and 4–6 indeed show large molecular weight differences due to the imbalance of the stoichiometry using small volumes of the ingredients, in which it seems that polymerization in CHCl3 resulted in a higher Mn under these experimental conditions.

Table 1 Thiol-Michael polyaddition of BDP, mETP or IMDP with dithiolsa
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Scheme 2
scheme 2

Schematic of the synthesis of unsaturated poly(ester-thioether)s via Michael-type thiol-yne polyaddition

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By this method, we successfully produced poly(BDP-alt-BDT), poly(BDP-alt-EBTG), poly(BDP-alt-DTT), and poly(BDP-alt-DODT) (Runs 1–8 in Table 1). According to the geometry of the internal carbon‒carbon double bonds, in Runs 2 and 5 and in Runs 3 and 6, the polymerization solvent was changed to THF and DMF, respectively, under the same polymerization conditions, and poly(BDP-alt-BDT)s with different geometric structures were synthesized. The higher the polarity of the reaction solvent was, the greater the percentage of the Z configuration obtained (Fig. 2), which is consistent with previous reported work dealing with thiol-yne click polymerization (see also Fig. S16) [24]. In this study, the geometric structure of the internal double bonds was calculated from the 1H NMR peak intensity ratio (Figs. 1 and 2 and S9–16). To synthesize the saturated poly(ester-thioether)s as the control samples, the dialkenyl monomer, 1,4-butanediol diacrylate (BDA) was also used for thiol-ene click polymerization to give saturated poly(ester-thioether)s, poly(BDA-alt-BDT) (Mn = 3.4 × 103, Mw/Mn = 1.25) and poly(BDA-alt-EBTG) (Mn = 5.8  × 103, Mw/Mn = 1.42) (see also Table S1), which were used for subsequent biodegradation tests.

Fig. 2
figure 2

Comparison of 1H NMR spectra of poly(BDP-alt-BDT) obtained using different reaction solvents

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Similar thiol-yne click polymerizations were also carried out using mETP and IMDP as dialkynyl monomers to improve the biobasedness and biodegradability of the samples. Here, mETP was combined with BDT, and EBTG and IMDP were coupled with EBTG and MAIM, respectively, to give the corresponding unsaturated poly(ester-thioether)s. Using THF and DMF as reaction solvents, unsaturated poly(ester-thioether)s with different geometric structures were synthesized (Runs 9–16). Chloroform was not used because of the insolubility of the monomers. We believe that poly(ester-thioether)s consisting of mETP and IMDP have different solubilities in good solvents during purification using reprecipitation solvent compared to BDP as the monomer. In this study, we could confirm no byproduct except for the corresponding oligo(ester-thioether)s. Unexpectedly, in Runs 15 and 16, the percentage of the Z configuration was smaller in the samples with DMF(E/Z = 67/33) than in the samples with other solvents (E/Z = 48/52), which is different from the previous tendency of a low Z ratio in polar solvents [24,25,26,27]. It seems that the steric hindrance of the monomers as well as the polar conditions influenced the E/Z ratio during click polymerization. Comparing Runs 9–14, a higher Tg was observed for higher molecular weights, and for Runs 15 and 16, where the molecular weights were almost the same, it seems that a higher Tg was observed for higher E configuration values. It seems that the introduction of a dianhydrosugar moiety into the polymer backbone strongly influenced the Tgs compared with the E/Z configuration. Using IMDP as the dialkyne, the highest Tg (49 °C) was confirmed, and it seems that the relatively restricted molecular motion of the cyclic structure of IMDP increased the Tg.

Before conducting the BOD measurements, we performed an enzymatic degradation test and TOC measurements (Fig. 3) [28, 29]. All the samples exhibited higher TOC values when lipase was added than when in the absences of lipase, confirming that enzymatic degradation proceeded. Comparing the results of the addition of lipase among the samples using BDP as the dialkynyl monomer (Runs 1–8), Runs 4–6 showed the highest TOC values, which is considered to be due to the high number of ester groups per repeating unit. On the other hand, the difference between the TOC value and that in the absence of lipase was small, indicating that hydrolysis as well as enzymatic degradation occurred. Compared with 4–6, which have the same structure, it seems that the lower molecular weight unsaturated poly(BDP-alt-EBTG)s have higher values for both hydrolysis and enzymatic degradation. In Runs 9–16, where biobased mETP and MAIM were introduced, the TOC values of all the samples increased remarkably with the addition of lipase, indicating that they were degradable by the lipase. The differences in the observed TOC values for the same poly(ester-thioether) appear to be ascribed to differences in Mn, Tg temperature, and geometric structure, but a remarkable tendency has not been identified. However, from the results of Runs 13–16, it was revealed that poly(IDMP-alt-EBTG) and poly(IDMP-alt-MAIM) showed greater enzymatic degradation in buffer (pH 6.8).

Fig. 3
figure 3

Results of TOC measurements of poly(ester-thioether)s (Runs 1–16) after incubation at 37 °C for 24 h in buffer (pH 6.8) in the absence (black bar) and presence (red bar) of lipase

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Next, actual biodegradation tests of the synthesized unsaturated poly(ester-thioether)s were conducted using activated sludge. First, the results for the polymers obtained from Runs 1, 4, 7, and 8 are shown (Fig. 4). The biodegradabilities of Runs 1, 4, 7, and 8 were dependent on the type of dithiol used as the monomer, although all the samples showed low biodegradabilities of 5–13%. This may be due to the high rigidity of the unsaturated poly(ester-thioether)s due to the internal double bonds in the main chain. The biodegradation curves (BOD/TOD curves) of poly(BDP-alt-BDT) and poly(BDP-alt-EBTG) were compared with those of the corresponding saturated poly(ester-thioether)s, poly(BDA-alt-BDT) (Fig. S17) and poly(BDA-alt-EBTG) (Fig. S18), accompanied by aniline (Fig. S19) as a positive control in the biodegradation test. The results indicated that unsaturated poly(ester-thioether)s show lower biodegradabilities than the corresponding saturated poly(ester-thioether)s. Unfortunately, satisfactory biodegradabilities and dependence on the geometry were not obtained either, but unsaturated bonds can play an important role in postpolymerization chemical modification. The orthogonal reaction enabled us to introduce the expected functional groups into the polymer backbone. As a preliminary study, we succeeded in fabricating cross-linked gels by adding dithiols (Figs. S20 and 21).

Fig. 4
figure 4

BOD/TOD profiles of the biodegradation of Runs 1, 4, 7, and 8 in activated sludge (25 °C)

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The results of the unsaturated poly(ester-thioether)s synthesized in Runs 1–6 were used to compare the biodegradability of different geometric structures (Fig. 5). However, as shown in Fig. 5, the biodegradability of all the samples was low, and therefore, a marked difference in biodegradability could not be confirmed, as shown in Fig. 5. In particular, mETP-based poly(ester-thioether)s (Runs 9–12) showed little biodegradability (data not shown). These results indicate that mET is a sugar-derived biomass that contributes to improved thermal properties via internal hydrogen bonding between pendent hydroxyl groups but is not expected to be biodegradable due to its increased rigidity, reflecting higher Tgs (see Table 1, Runs 11 and 12).

Fig. 5
figure 5

BOD/TOD profiles of the biodegradation of Runs 1–6 in activated sludge (25 °C)

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Finally, the results of the biodegradation test of the unsaturated poly(ester-thioether)s (Runs 13–16) synthesized using IMDP as the dialkyne monomer are shown (Fig. 6). No biodegradability was observed in Runs 13 and 14, even though EBTG was used for dithiols. On the other hand, Runs 15 and 16, in which MAIM was used as the dithiol, showed 5–10% biodegradability after 28 days. A difference in biodegradability between Run 15 and Run 16 was also observed. It seems that the difference in biodegradability can be ascribed to the geometric structure of the internal carbon‒carbon double bonds. The results also agreed well with our previous report dealing with the biodegradability of unsaturated poly(ester)s, in which unsaturated polyester containing 30% of the Z-configuration showed the highest biodegradability [16,17,18]. The results revealed that isomannide-derived MAIM is a promising biomass resource that contributes to improving the thermal properties of poly(ester-thioether) and, depending on the type of dithiols in the polymer, provides a certain degree of biodegradability.

Fig. 6
figure 6

BOD/TOD profiles of the biodegradation of Runs 13–16 in activated sludge (25 °C)

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Conclusions

In this study, the Tg was improved by introducing carbon‒carbon double bonds into the poly(ester-thioether) main chain. In addition, this study has provided new insights into the synthesis of unsaturated poly(ester-thioether)s via thiol-yne Michael addition reactions, including a simple and facile method for accessing dialkynyl monomers. Furthermore, the synthesized unsaturated poly(ester-thioether)s showed some biodegradability in activated sludge. Geometry-dependent biodegradability was also expected to occur among the unsaturated polyesters. Unfortunately, satisfactory biodegradabilities and dependence on the geometry were not obtained either, but unsaturated bonds can play an important role in postpolymerization chemical modification. The orthogonal reaction enabled us to introduce the expected functional groups into the polymer backbone. In our next paper, we will discuss the physical properties of these materials. This basic research should provide further insight for the synthesis of a new type of biodegradable polymer.