Sustainable and Scalable Synthesis of Acetal‐Containing Polyols as a Platform for Circular Polyurethanes

Abstract

Polyurethanes (PUs) are highly versatile polymers widely utilized across industries. However, chemical recycling of PU poses significant challenges due to the harsh conditions required and the formation of complex mixtures of oligomers upon depolymerization. Addressing this inherent lack of recyclability, we developed closed‐loop recyclable PU materials by integrating cleavable acetal groups. We present a sustainable and scalable synthesis method for acetal‐containing polyols (APs) through aldehyde‐diol polycondensation, utilizing reusable heterogeneous catalysts. Three APs with different hydrolytic stabilities depending on the structure of acetal groups were synthesized from formaldehyde, acetaldehyde, and propionaldehyde with 1,6‐hexanediol (H16). These APs were employed alongside 4,4′‐methylene diisocyanate (MDI) for preparation of PU materials. The resulting PUs exhibited mechanical properties comparable to or surpassing those of conventional PUs, while demonstrating excellent recyclability under acidic conditions. Notably, hydrolysis of PU materials based on acetaldehyde‐derived APs yielded remarkable monomer recovery rates, with 89 % for H16 and 84 % for 4,4′‐methylenedianiline, a precursor to MDI. Furthermore, we successfully demonstrated closed‐loop recycling by synthesizing APs from recovered H16, resulting in PU materials with identical properties to the original PU. This achievement highlights the potential for establishing a closed‐loop recycling system for acetal‐containing PUs, contributing to the advancement of a sustainable and circular economy.

Polyacetal polyols were synthesized via the polycondensation of aldehydes and diols, and subsequently used for the preparation of closed‐loop recyclable polyurethanes. The synthesis of acetal‐containing polyols via the polycondensation of aldehydes and diols as a platform for closed‐loop recyclable polyurethanes is presented. These polyurethane materials show outstanding mechanical properties and can be recycled back into original monomers in excellent yield. The recovered monomers can be used for the preparation of new polyols and polyurethanes with identical properties to the original material.

Introduction

Polyurethanes (PUs) are among the most widely produced polymers globally, finding application in a diverse range of materials such as rigid or flexible foams, elastomers, adhesives and thermoplastics, which all play important roles in various industries.[ ¹ , ² , ³ ] However, despite the widespread use of PU materials in our daily lives, the recycling of PU waste remains an issue of great societal concern.

The most common recycling method for PU waste involves reprocessing by mechanical recycling, which causes deterioration of the mechanical properties as compared with pristine materials. The obtained downcycled plastics lose a significant amount of their value and can only be used in some less demanding applications (i. e., as fillers in carpet materials). ^[4] To preserve the value of the PU materials, chemical recycling such as glycolysis,[ ⁵ , ⁶ , ⁷ , ⁸ , ⁹ , ¹⁰ , ¹¹ ] hydrolysis,[ ¹² , ¹³ , ¹⁴ ] acidolysis[ ¹⁵ , ¹⁶ ] or aminolysis[ ¹⁷ , ¹⁸ , ¹⁹ , ²⁰ ] offer alternatives to mechanical recycling. However, the current chemical recycling of conventional PU materials has two major shortcomings: First of all, recycled products contain mixtures of high molecular weight oligomers, which cannot be separated. Furthermore, such processes require harsh conditions (i. e., combination of high temperature and pressure or a large excess of reactants). Consequently, the implementation of chemical recycling of PU waste remains limited, with the majority of the PU waste ending up in landfills or being incinerated.[ ²¹ , ²² ]

In addressing the challenges posed by the chemical recycling of the current PU materials, the development of new PU materials featuring cleavable chemical bonds emerges as a potential solution. Such cleavable bonds could permit the recycling of the material under specific stimuli into well‐defined molecules, with the acetal bond standing out as an illustrative example.[ ²³ , ²⁴ , ²⁵ , ²⁶ , ²⁷ ] The acetal bond exhibits excellent stability under neutral or basic conditions and can be hydrolyzed under acidic conditions. Thus, recyclable polyurethanes can be designed and developed by incorporating acetal bonds in the material in the form of acetal‐containing polyols (APs).[ ²⁸ , ²⁹ ]

We have recently reported a solvent‐free synthetic pathway for the preparation of APs via the polyaddition of divinyl ethers and diols (Figure 1A). ^[30] During the synthesis, heterogeneous catalyst could be employed instead of the homogeneous catalysts (i. e., p‐toluenesulfonic acid) in order to make the synthesis of APs more amenable to industrial processes and more in line with the principles of the green chemistry, such as bulk conditions and the use of reusable and safe catalysts. However, the utilization of divinyl ethers makes this synthetic pathway disadvantageous. The industrial production of divinyl ethers uses high pressure acetylene gas, thus having a relatively high environmental and economic cost.[ ³¹ , ³² ]

Figure 1.

Strategies for AP synthesis and utilization of APs in closed‐loop recyclable polyurethane. (A) Previous reported strategies for the synthesis of APs via the addition reaction of alcohol and vinyl ether groups.[ ²⁹ , ³⁰ ] (B) The novel synthesis method of APs via the condensation of 1,6‐hexanediol with various aldehydes using a heterogeneous acid catalyst. (C) Closed‐loop recycling of acetal‐containing PU materials consisting of three steps: 1 Synthesis of acetal‐containing polyols via polycondensation of 1,6‐hexanediol with an aldehyde (acetaldehyde), 2 PU synthesis utilizing the acetal‐containing polyol and MDI, 3 hydrolysis of PU material into original monomers, thereby enabling closed loop recycling.

In this work we describe an alternative, sustainable and scalable synthesis pathway for APs which involves the polycondensation of aldehydes and diols (Figure 1B). One notable advantage of synthesizing APs via polycondensation is the ability to customize the chemical structure of the APs over a wide range by simply varying the types of aldehyde and diol used. Moreover, the low cost and high availability of aldehydes and diols add further to the benefits of this synthetic approach. This work was partially inspired by the synthesis of various acetal structures as reported in prior work. For example, small cyclic acetals have been reported for use as fuel additives synthesized via the condensation of tri‐functional alcohols with aldehydes. ^[33] Amphiphilic polyacetal polymers were prepared by Endo et al. by reacting polyethylene glycol (PEG) with lilial using a homogeneous catalyst. ^[34] Zhu et al. prepared a dual dynamic PU network containing both acetal and Diels‐Alder moieties. ^[35] These degradable networks were prepared using acetal‐containing diols synthesized from diols and aldehydes utilizing homogeneous catalysts in solution. The synthesis process involved various organic solvents and extensive workup steps, including neutralization and extractions. These factors make their method less suitable for industrial application. Additionally, polymers containing spiroacetals have been prepared by reacting a dialdehyde with a four‐functional alcohol, yielding a rigid polymer structure with both a high glass transition and melting temperature.[ ³⁶ , ³⁷ , ³⁸ , ³⁹ ] However, to the best of our knowledge, the synthesis of APs using polycondensation of aldehydes and diols has not been previously reported.

Herein, we present a synthetic procedure of APs via polycondensation of various aldehydes and 1,6‐hexandiol (H16) using acidic heterogeneous catalyst (Siral 70, a silica‐alumina hydrate, or MOF C‐300, a metal organic framework). The utilization of heterogeneous catalyst allows for the easy filtration of the reaction mixture, enabling the recovery and reuse of the used catalyst and yielding pure polyols. The resulting APs, featuring various acetal groups, were used in the preparation of PUs based on 4,4′‐diphenylmethane diisocyanate (MDI). The resulting acetal‐containing PUs showed significant differences in mechanical properties and hydrolytic stability. Furthermore, the PUs could be hydrolyzed under acidic conditions into H16 and primary diamine building blocks with high yield and purity. The main monomer (H16) was reused to produce new APs and PU material with properties comparable to the original AP‐containing PU. Thus, this work provides an efficient, inexpensive, and scalable route to closed‐loop recyclable polyurethanes (Figure 1C).

Results and Discussion

Synthesis and Characterization of Acetal‐Containing Polyols

Acetal‐containing polyols (APs) with various acetal groups and an average molecular weight of 1000 g/mol were synthesized via the polycondensation of 1,6‐hexanediol (H16) with different aldehydes in the presence of an acidic heterogeneous catalyst, which was removed by filtration after the reaction (Figure 1B). In our previous work, two catalysts (K10 and Siral 70) were highlighted as suitable catalysts for the synthesis of polyacetals via the polyaddition of diols with divinyl ethers. ^[30] These catalysts were also found to be suitable for the polycondensation reactions. The polycondensation of H16 and various aldehydes was studied under bulk conditions in order to optimize some key parameters including selectivity, reaction speed, the molecular weight, acid value and color of the polyol product after filtration. The optimized catalyst, reaction time, temperature, and average molecular weight of the APs synthesized from the reaction between H16 and aldehyde (polymers, trimers, or monomer) are listed in Table 1. The molecular weight of the APs was determined using proton nuclear magnetic resonance (¹H‐NMR) spectroscopy (Figure S1 2 and 3) and OH value titration. The APs obtained after filtration were transparent and colorless with an acid value below the detection limit of the titration method of 0.08 mg KOH/g. When paraformaldehyde or paraldehyde are used (AP1 and AP2, respectively), prior to the condensation reaction with the H16, the aldehyde needs to be liberated in‐situ from the aldehyde precursor. A good catalyst is therefore required to provide a balance between the aldehyde releasing and aldehyde consuming reaction (condensation). Too fast in‐situ production of aldehyde will lead to huge losses in volatile aldehyde monomers during the reaction. On the other hand, too slow liberation will limit the reaction speed of the polycondensation reaction due to a lack of free aldehyde in the reaction mixture. Both Siral 70 and K10 were active as catalysts. However, the use of K10 catalyst resulted in decomposition of the precursors that was much faster compared to the polycondensation reaction, thus creating a large amount of free aldehyde in the reaction mixture, and subsequently leading to side reactions and excessive evaporation of the aldehyde (Table S1). In comparison, Siral 70 has a better balance between the liberation and condensation reactions and was therefore chosen as the catalyst for AP1 and AP2 (Table 1). In the case of AP1 paraformaldehyde was employed which is nonvolatile and consequently the reaction temperature could be increased to 110 °C under vacuum without evaporation of the monomers. When the temperature was increased to 120 °C yellowing of the product occurred. In the case of AP2, the volatility of paraldehyde prevents the use of high temperature or vacuum. The reaction temperature was limited to 74 °C as yellowing of the reaction mixture was observed around 80 °C. AP3 is directly formed from the reaction between H16 and the aldehyde monomer, propionaldehyde. In the synthesis of AP3, a metal organic framework (MOF) C‐300 was found to be a better catalyst than K10 and Siral 70 in terms of improved reactivity and higher selectivity. Using C‐300 as the catalyst, aldol condensation products started to be detected at temperatures above 85 °C, so the temperature should not exceed 85 °C (Figure S4). C‐300 was also investigated as the catalyst for the synthesis of AP1 and AP2. However, it was found that C‐300 did not effectively catalyze the decomposition of the precursors into free aldehydes, thus conversion was very low when using C‐300 (Table S1). The selection of H16 as the diol in this reaction stems from the fact that it is the smallest diol that completely avoids the formation of cyclic acetals during polycondensation with aldehydes. Notably, when utilizing shorter diols such as 1,5‐pentanediol or shorter, small cyclic acetal structures were formed as side products (Table S1 and S2). The reusability of the recovered catalyst after filtration was verified by repeating the synthesis of AP1 four times using the recovered catalyst. The filtered catalyst could be reused after a simple washing and drying step. The molecular weight of obtained polyols and reaction time of the reactions are shown in Figure S5. The catalyst Siral 70 showed excellent performance during reuse and only a minor increase in reaction time (from 2–3 hours) was observed for full conversion after each reuse. In addition, the molecular weight remained relatively constant after each reuse. The high purity of the polyol product after four cycles was confirmed by H‐NMR spectroscopy (see Figure S6–S9).

Table 1.

APs obtained from the polycondensation of different monomers.

Polyol	Monomers	Catalyst	Temperature (°C)	Reaction time (h)	Mn from 1H‐NMR (g/mol)	Mn from OH value (g/mol)
AP1	H16+paraformaldehyde	5 wt % Siral 70	110	3	1100	1020
AP2	H16+paraldehyde	5 wt % Siral 70	74	30	1110	1090
AP3	H16+propionaldehyde	3 % C‐300 MOF	70–85	20	1000	1050

In order to further assess the structure of the polyols produced, ¹H‐NMR spectroscopy and matrix‐assisted laser desorption and ionization‐time of flight‐mass spectroscopy (MALDI‐TOF‐MS) were performed on AP1–AP3 (Figure 2 and Figure S1–S3). From the ¹H‐NMR spectra (Figure 2) characteristic acetal peaks can be observed, such as the acetal bridging protons at 4.66 (s), 4.66 (q) and 4.38 ppm (t) for AP1, AP2 and AP3 respectively. Furthermore, the peak of the CH₂ adjacent to the OH end group at 3.64 ppm (m) and the absence of unidentified signals is further evidence of the high purity of the produced APs. The MALDI‐TOF‐MS spectra show peaks with differences of 130.09, 144.12 and 158.13 between major peaks (m/z) coinciding exactly with the molar mass of the H16‐aldehyde repeating units in AP1, AP2 and AP3, respectively. Additionally, the absolute value of the peaks matches the theoretical molecular weight of the polymer chains with different number of repeating units plus an additional sodium cation (e. g., [M+Na]⁺ for AP1, x=6 corresponds to the peak at m/z=921.74).

Figure 2.

¹H‐NMR spectra (400 MHz, CDCl₃) and MALDI‐TOF mass spectra of AP1 (A, B), AP2 (C, D) and AP3 (E, F).

Gel permeation chromatography (GPC) analysis was performed to verify the molecular weight distribution of the polyols (Figure S10). The GPC traces of APs did not show the presence of significant amounts of monomers and the molar mass of APs all follow a distribution curve characteristic for a polyol synthesized via polycondensation. Differential scanning calorimetry (DSC) analysis was performed to determine the thermal properties of AP1–AP3 (Figure S11). AP2 and 3 showed no crystallization or glass transition (T _g) between −80 and 100 °C, while AP1 showed a crystallization peak at −10 °C.

The hydrolytic stability of AP1–AP3 under acidic conditions was investigated by in‐situ ¹H‐NMR spectroscopy at 40 °C (Figure S12–14). In 0.6 g acetone‐d ₆, 0.1 g of AP was added, and the hydrolysis was started by dosing 0.1 ml of 5 mM p‐toluenesulfonic acid solution into the NMR tube. Based on the resulting ¹H‐NMR spectra the percentage of remaining acetal was calculated and plotted over time (Figure 3). AP1 remained stable under these conditions, showing no significant hydrolysis even after 14 h, while the hydrolysis of AP2 and AP3 started instantly. The amount of acetal bonds in AP2 and AP3 decreased to below 10 %, at which point an equilibrium was reached. This observed difference in the hydrolytic stability of AP1–3 follows the trend found in literature for the stability of the different acetal groups under acidic conditions. ^[40]

Figure 3.

Stability of AP1–AP3 under acidic condition measured by ¹H NMR over a period of 14 h (0.1 g polyol in 0.6 g acetone‐d ₆ with 0.1 g 5 mM p‐toluenesulfonic acid solution).

Synthesis and Characterization of Acetal‐Containing PUs

Three acetal‐containing PUs, namely PU AP1, PU AP2 and PU AP3, were prepared by reacting AP1–AP3 with 4,4’‐methylene diphenyl diisocyanate (4,4’‐MDI) in bulk,  using H16 as a chain extender (the molar ratio of NCO:OH was 1.01 : 1) to produce 30 % hard phase contents (hard phase content was calculated according to Equation 2 shown in supporting information). The GPC traces of the acetal‐containing PUs show that all samples have a molecular weight between 50000 and 90000 g/mol (Figure S15). The Fourier‐transform infrared spectroscopy (FT‐IR) measurements of the three PUs (Figure S16) confirm the completion of the isocyanate reaction evidenced by the total disappearance of the N=C=O stretching vibration band at 2260 cm⁻¹. Additionally, a distinct carbonyl bond (C=O) signal of the urethane group is observed. The carbonyl band exhibits two distinct stretching vibration peaks: one at 1730 cm⁻¹ attributed to a non‐hydrogen bonded carbonyl, and another at 1700 cm⁻¹ attributed to a hydrogen bonded carbonyl. This observation allows for a qualitative comparison of the degree of phase separation within different PU materials, as the majority of hydrogen bonding of the carbonyl occurs in the hard phase, while the non‐hydrogen bonded urethanes reside in the soft phase.[ ⁴¹ , ⁴² ] Analysis of the FT‐IR spectra indicate that PU AP3 has the highest amount of hydrogen bonding. We propose that this difference in the phase separation is due to lower polarity of AP3 which promotes phase separation from the polar hard phase. The more polar AP1 and AP2 have more compatibility with the hard phase and therefore show less pronounced hydrogen bonding.

The thermal and mechanical properties of acetal‐containing PUs were analyzed by thermogravimetric analysis (TGA), DSC, tensile test, and dynamic mechanical analysis (DMA) (Figure 4).

Figure 4.

(A) TGA (B) DSC, (C) stress–strain and (D) DMA curves of the acetal‐containing PUs.

All acetal‐containing PUs showed good thermal stability with an initial decomposition temperature (T_d5%) ranging from 310–320 °C, which is close to the typical value of 295 °C for polyether based aromatic PUs (Figure 4A). ^[41] PU AP1 has a much broader degradation range than PU AP2 and PU AP3, which is likely due to the greater thermal stability of the methylene bridge compared to the acetal bridge with methyl or ethyl side groups. The DSC measurements reported a T _g temperature of around −50 °C for the AP‐containing PUs (Figure 4B).

The mechanical properties shown in Figure 4C are also summarized in Table 2. Among the PU samples, PU AP1 exhibits the highest elongation (1075 %) and stress at break (25 MPa). The elongation at break of PU AP2 and PU AP3 is slightly lower and comparable to each other, with PU AP3 displaying the lowest stress at break at 16.8 MPa. Notably, all three PU samples reach or even surpass conventional PU materials based on polyether polyols, such as PTHF in terms of elongation and stress at break. ^[42] The Young′s modulus of the AP‐containing PUs is approximately 19 MPa, significantly higher than that of typical polyether‐based PU materials. ^[42] Furthermore, the AP‐containing PUs demonstrate high toughness, with PU AP1 exhibiting the highest toughness due to its greater stress and elongation at break combined with the high Young′s modulus. Thermomechanical characterization of the acetal‐containing PUs was performed by DMA (Figure 4D). The T _g values for the acetal‐containing PUs obtained from the maxima of the tan(δ) curves are similar at −30 °C for AP1, −24 °C for AP2, and −25 °C for AP3. The storage moduli of all PUs went through a smooth transition from the glassy plateau to the rubbery plateau. Melting of the acetal‐containing PU materials was indicated by the sudden decrease in storage modulus, which starts around 120 °C.

Table 2.

Mechanical properties of the acetal‐containing PU materials.

Sample	Elongation (%)	Stress (MPa)	Young′s modulus (MPa)	Toughness (MJ/m3)
PU AP1	1075±95	25.0±1.7	19.9±0.5	134±19
PU AP2	876±55	20.1±1.2	18.5±0.2	100±10
PU AP3	885±57	16.8±0.7	19.6±1.3	90±9

To evaluate the hydrolytic stability of PU AP1, tensile samples were subjected to prolonged water exposure at 80 °C for 24 h. After drying the water exposed specimens (PU AP1 W) at 80 °C for 16 h tensile testing was performed. PU AP1 W exhibited a Young′s modulus and tensile strength nearly identical to the original PU AP1 (Figure S17 and Table S3).

Closed‐Loop Recycling of Acetal‐Containing PUs

As AP1–AP3 are hydrolyzable under acidic conditions, it is possible to chemically depolymerize the acetal‐containing PUs back into H16, aldehyde, and a urethane‐rich hard phase. These three components were separated in a straightforward manner in high purity. Subsequently, the isolated hard phase was depolymerized into H16 and MDA (the precursor of 4,4’‐MDI) (Figure 5A).

Figure 5.

(A) Synthesis and chemical recycling of PU AP2. (B) Photographs of PU AP2 at various stages of recycling using 0.3 M HCl. (C) ¹H‐NMR spectra (400 MHz, DMSO‐d ₆) of the original (bottom) and recovered (top) H16. (D) ¹H‐NMR spectra (400 MHz, DMSO‐d ₆) of commercial (bottom) and recovered (top) MDA.

Depolymerization was achieved under various conditions depending on the type of acetal group present in the material. The hydrolytic stability of the PUs under acidic conditions follows the same trend as the APs stability in acidic solution (Figure 3), PU AP2 and PU AP3 having similar hydrolytic stability and PU AP1 being much more stable towards hydrolysis. To achieve full hydrolysis within 24 h, either 1.2 M H₃PO₄ or 0.3 M HCl could be used for PU AP2 and PU AP3 while 2 M HCl was required for PU AP1. During this process, the produced aldehyde was released as a gas, which can be easily recovered by distillation when the reaction is performed on industrial scale. ^[43] For example, during the hydrolysis of PU AP3 using 0.3 M HCl, propionaldehyde was isolated from the gas stream in a non‐quantitative manner (¹H‐NMR spectra and photographs of the collected products are shown in Figure S18–31). The high purity of the recovered aldehyde based on ¹H‐NMR (Figure S30) shows the potential for the industrial separation of the aldehyde from the hydrolysis mixture. In addition, separation of the H16 was possible as H16 is dissolved in the aqueous phase, and the urethane‐rich hard phase remains as an insoluble fine powder. A large laboratory sample of PU AP2 (66 g) was hydrolyzed using 0.3 M HCl yielding 27.6 g H16 and 30.1 g of hard phase. Further hydrolysis of the urethane‐rich hard phase into MDA and H16 was also performed. Previous work showed that TBD could be used to hydrolyze the urethane into free amines and diols with minimal side reaction. ^[30] Using a 74 : 15 : 11 ratio of hard phase to water to TBD, the reaction mixture was refluxed at a temperature of 140 °C for 24 h. The addition of water to the cooled reaction mixture resulted in an aqueous phase containing mostly TBD and H16, and an organic layer containing MDA and a mixture of ureas and urethanes. The aqueous phase was passed through an ion exchange resin (Amberlyst 36) to remove the TBD, followed by vacuum distillation to isolate 8.3 g H16. The organic fraction was directly vacuum distilled to isolate MDA. The final purity of the MDA was further improved by recrystallization from hot water, yielding 12.4 g of MDA. Photographs of PU AP2 at the various stages of the closed‐loop recycling are shown in Figure 5B. The combined amount of H16 was 35.9 g representing an 89 % yield, the 12.4 g MDA corresponds to a yield of 84 % with both an excellent purity based on ¹H‐NMR (Figure 5C and D and S32–34).

The successful recycling and recovery of monomers from a realistic waste stream are crucial for the practical implementation of acetal‐containing PU recycling. To assess this, a selective recycling experiment was conducted by mixing PU AP2 with two types of conventional PU materials, one based on polyether polyols and the other on polyester polyols (Figure 6). Under applied depolymerization conditions (0.3 M HCl heated to 80 °C for 5 h), full hydrolysis of the acetal bonds within the PU AP2 sample was achieved. Subsequently, the mixture underwent filtration using a porous funnel, effectively separating the conventional PUs from the fine particles of the hard phase and the H16 solution. Both conventional PUs samples remained fully intact with less than 1 wt % mass lost. Subsequently, the straightforward separation of H16 and the hard phase was achieved through filtration, yielding high purity monomers based on ¹H‐NMR (see Figure 6 G and Figure S35 and S36).

Figure 6.

Photographs illustrating the selective recycling of PU AP2 under acidic conditions alongside two conventional PU materials (one polyether based and the other polyester based) at different stages of the recycling process. (A) The simulated waste mixture, (B) the reaction mixture, (C) the reaction mixture after 5 hours, (D) the recovered conventional PU material, (E) the conventional PU material after drying, unaffected by the recycling process, (F) the filtered reaction mixture containing H16 and hard phase in the form of a fine precipitate, (G) the separated H16 and hard phase.

The recovered H16 was subsequently used to synthesize a new batch of polyol (Figure S37). This AP recycled (OH value=106.3 mg KOH/g) was used in the preparation of new PU material. The properties of this new PU were compared to the original PU AP2 (Figure 7), demonstrating nearly identical properties to the original material based on GPC, tensile testing, and DMA results. The elongation and stress at break of the recycled material were 950 %±117 and 20 MPa±3.0, respectively while T _g obtained from DMA was −23 °C for both the virgin and recycled material. The high yield and purity of recycled H16 and MDA, in combination with the identical properties of the recycled material demonstrate that the closed‐loop recycling of the AP‐containing PUs was successful.

Figure 7.

Comparison between the properties of the original and recycled PU material. (A) GPC curves (B) tensile curves, (C) DMA curves.

Conclusions

Acetal‐containing polyols with different acetal groups with a molecular weight of around 1000 g/mol have been prepared via the polycondensation of various aldehydes with 1,6‐hexanediol using heterogeneous catalysts. The application of heterogeneous catalysts paved the way for a straightforward synthetic pathway, leading to polyols characterized by both high synthesis efficiency and exceptional purity. Furthermore, the catalyst was easily recovered and reused for subsequent reactions. Therefore, this method is applicable to the large‐scale production of acetal containing polyols. The acetal‐containing polyols showed different hydrolytic stabilities depending on the structure of acetal groups, whereby the formaldehyde‐based polyol (AP1) was much more resistant to hydrolysis than the polyols based on acetaldehyde and propionaldehyde (AP2 and AP3). The acetal‐containing polyols were further used for the preparation of acetal‐containing PUs. These PU materials exhibited excellent mechanical and thermal properties similar to classical polyether (PTHF) based PUs. Moreover, it was shown that the acetal containing‐PUs could be efficiently depolymerized into the original monomers in a two‐step hydrolysis process, in the case of PU AP2 yielding MDA and H16 with yields of 84 % and 89 %, respectively. The recovered H16 was then used for the synthesis of new polyols, and PU based on this recycled polyol showed nearly identical properties to the original PU. This shows that closed‐loop recycling of the PU systems can be achieved using acetal‐containing polyols.

The polycondensation of aldehydes and diols described in this work provides a way to build up a library of acetal‐containing polyols with different structures resulting in PUs with various properties. In addition to variation in the aldehyde, many different diols, including those derived from bio‐based sources, can be employed to formulate acetal‐containing polyols. This approach holds the potential to integrate a substantial amount of bio‐based content into recyclable polyurethanes.

Conflict of Interests

The authors declare no conflict of interest.

Supporting information

As a service to our authors and readers, this journal provides supporting information supplied by the authors. Such materials are peer reviewed and may be re‐organized for online delivery, but are not copy‐edited or typeset. Technical support issues arising from supporting information (other than missing files) should be addressed to the authors.

Supporting Information

Schara P., Cristadoro A., Sijbesma R. P., Tomović Ž., ChemSusChem 2025, 18, e202401595. 10.1002/cssc.202401595

Data Availability Statement

The data that support the findings of this study are available from the corresponding author upon reasonable request.