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DOI: 10.1039/D5PY00580A (Paper) Polym. Chem., 2025, Advance Article

A continuous flow based irreversible polycondensation enables synthesis of polycarbonate diols beyond batch limitations†

Jiawen Daia, Shuyuan Luoa, Zhenjiang Lia, Jie Sunb, Haritz Sardonc, Ning Zhu*a, Jin Huang*a and Kai Guoa
aState Key Laboratory of Materials-Oriented Chemical Engineering, College of Biotechnology and Pharmaceutical Engineering, Nanjing Tech University, 30 Puzhu Road South, Nanjing 211816, China. E-mail: ningzhu@njtech.edu.cn; jinhuang@njtech.edu.cn
bCollege of Food Science and Light Industry, Nanjing Tech University, Nanjing 211816, China
cPOLYMAT University of the Basque Country UPV/EHU, Avenida de Tolosa, 72, Donostia-San Sebastián 20018, Spain

Received 10th June 2025 , Accepted 11th July 2025

First published on 14th July 2025

Abstract

Aliphatic polycarbonate polyols have emerged as valued precursors for high-performance polyurethanes due to their superior hydrolytic and thermal stability. These polyols are industrially produced using conventional batch reactors which suffer from limited control over polycondensation and low efficiency of production. In this work we report a continuous-flow platform for the continuous production of polycarbonate diols. This process is based on the irreversible polycondensation of short-chain diols with diphenyl carbonate in the presence of a methanesulfonic acid catalyst. This system enables quasi-first-order kinetics with high yield in short residence time, affording polycarbonate diols with a molecular weight range of Mn = 1200–2600 g mol−1 and a dispersity range of 1.9–2.3. We extend the continuous process for the production of polyurethanes by utilizing the crude polycarbonate diols directly for polyurethane synthesis, enabling an integrated and purification-free process. This work establishes a unified flow platform for the continuous production of step-growth polymers with high precision, providing a promising solution to the longstanding limitations of batch production.

Introduction

Polyols, as key precursors in polyurethane production, are generally classified into four main types: polyether, polyester, polycarbonate, and polyamide polyols. Among them, aliphatic polycarbonate polyols stand out due to the presence of carbonate linkages with delocalized electron clouds formed by three oxygen atoms, which impart excellent hydrolytic stability, thermal resistance, and mechanical strength.1 These properties are markedly superior to those of conventional polyether and polyester polyols, making polycarbonate-based systems highly attractive for high-performance and durable polyurethane applications.2–10

Besides ring-opening polymerization methods11–17 and coupling of CO2 and diols,18–21 step-growth polycondensation of diols with carbonate monomers, including dimethyl carbonate (DMC), diethyl carbonate (DEC), diphenyl carbonate (DPC) and carbonyldiimidazole, is a major approach for the synthesis of polycarbonates. Among them, DMC and DEC are widely employed for the polycondensation with primary diols efficiently affording polycarbonate catalyzed by metal or organo-based catalysts.22–28 In contrast, DPC and carbonyldiimidazole, benefiting from the low nucleophilicity of its by-product, are more suitable for the polycondensation with less reactive secondary diols (e.g., isosorbide),29,30 heterogeneous polycondensation,31,32 and high-molecular-weight and structurally well-defined polycarbonate materials.33–36 However, traditional synthesis of polycarbonate polyols is predominantly carried out via batch polycondensation, which suffers from prolonged reaction times and broad molecular weight distributions. This may negatively impact the mechanical performance, durability, and long-term service stability of the final polyurethane products. Therefore, the development of new synthetic strategies that enable efficient and controllable preparation of polycarbonate polyols is essential for advancing the performance of polyurethane materials.

Microfluidic reactors are powerful platforms for polymer synthesis through microscale effects to enhance mass and heat transfer. They have been widely employed in chain-growth polymerizations, including ring-opening polymerization, free-radical polymerization, and ring-opening metathesis polymerization.37–43 However, step-growth polymerizations remain challenging to implement in microflow systems. This difficulty arises from the in situ generated small-molecule byproducts, which are not readily removed from the confined reaction environment, thereby hindering the forward progression of equilibrium-controlled reactions. Thus, the suppression of the reverse reaction becomes a critical requirement for enabling continuous-flow polycondensation. Achieving this demands a significant kinetic disparity between the forward and reverse reactions.42 Although reversible step-growth polymerizations remain elusive under continuous-flow conditions, a limited number of examples involving irreversible polycondensation reactions have been demonstrated.44–53

Oshimura et al. reported an irreversible polycondensation of diols with DPC catalyzed by dilithium tetra-tert-butylzincate. Production of aliphatic polycarbonates (Mn ≥ 10 kg mol−1) can be achieved under mild conditions (25–70 °C).54 This irreversible polycondensation approach significantly enhanced the structural stability and controllability of the polymers, laying a solid foundation for the continuous industrial production of polycarbonate polyols. We envision that the use of this approach could open new avenues for the production of polycarbonate diols by step-growth polymerization using a continuous microfluidic approach.

In this work, we report for the first time a continuous synthesis method for aliphatic polycarbonate polyols using a series of aliphatic diols and DPC in a microflow reactor system catalyzed by organic acids. Due to the lack of nucleophilicity of the generated phenol, we directly utilize the polycarbonate diols for the production of industrially relevant polyurethanes by reacting them with polyisocyanate precursors. Polyurethanes are ideal synthetic targets as they represent a versatile class of functional polymer materials, extensively applied as elastomers, adhesives, coatings, foams etc. By leveraging the enhanced mass and heat transfer in the microflow system, this approach offers a highly efficient and controllable pathway for the synthesis of polycarbonate polyols, providing new insights into their industrial production and application (Scheme 1).


image file: d5py00580a-s1.tif
Scheme 1 Schematic representation of the synthesis of aliphatic polycarbonate polyol and its corresponding polyurethane using continuous flow chemistry.

Results and discussion

To investigate the catalytic irreversible polycondensation between diols and diphenyl carbonate (DPC), we initially screened several commercially available catalysts, including DBU (1,8-diazabicyclo[5.4.0]undec-7-ene), NaOH, TBD (1,5,7-triazabicyclo[4.4.0]dec-5-ene), sulfamic acid (ASA), and methanesulfonic acid (MSA). The initial attempt was made using 1,6-hexanediol (HDL) and DPC at 140 °C with an equimolar feed ratio of [HDL]0/[DPC]0 = 1[thin space (1/6-em)]:[thin space (1/6-em)]1 and a catalyst loading of 3 mol%. After 4 hours, a portion of the reaction mixture was withdrawn and analyzed by 1H NMR spectroscopy. The results revealed that base-catalyzed systems exhibited relatively low NMR yield (approximately 10–46 mol%, Table 1, entries 1–3) compared to acid catalysts. This is likely attributed to the stronger acidity of the in situ generated phenol relative to HDL, which leads to neutralization of the base catalysts and subsequently suppresses their catalytic activity.1 In contrast, the ASA-catalyzed reaction afforded a yield of 16 mol% (Table 1, entry 4), whereas the MSA-catalyzed system afforded a significantly higher yield of 90 mol%, corresponding to a number-average molecular weight (Mn, NMR) of approximately 1300 g mol−1 (Table 1, entry 5). Upon extending the reaction time to 4–7 hours, the yield further increased to 94–97 mol% (Fig. S1†), with Mn, NMR reaching 1800 g mol−1, confirming the high catalytic efficiency of MSA in promoting the irreversible polycondensation.
Table 1 Polycondensation of 1,6-hexanediol (HDL) and DPC catalyzed by different catalystsa

image file: d5py00580a-u1.tif
Entry Catalyst NMR yieldb/mol% DPc Mn, NMRd/g mol−1
a Unless otherwise stated, the polycondensation was carried out at 140 °C with an equimolar feed ratio of [HDL]0/[DPC]0 = 1[thin space (1/6-em)]:[thin space (1/6-em)]1 and a catalyst loading of 3 mol%.b Polycarbonate NMR yield was determined by 1H NMR spectroscopy in CDCl3.c Determined from 1H NMR, DP = I4.12/I3.63.d Determined from 1H NMR, Mn, NMR = M(144 g mol−1) × DP.
1 DBU 14 <1 —
2 NaOH 20 <1 —
3 TBD 46 <1 —
4 ASA 16 <1 —
5 MSA 90 9 1300

Having confirmed the catalytic activity of MSA, we next investigated the polymerization kinetics of the reaction in a batch reactor. The monomer conversion increased progressively with time evolution, under the defined conditions (140 °C, [HDL]0/[DPC]0 = 1[thin space (1/6-em)]:[thin space (1/6-em)]1, catalyst loading = 3 mol%). The kinetic plot exhibited a linear correlation, indicating that the MSA catalyzed reaction followed apparent first-order kinetics (Fig. 1, kobs = 0.025 min−1). As the temperature increased, kobs rose accordingly. At 160 °C, a maximum kobs of 0.050 min−1 was observed, suggesting that elevated temperatures significantly promote the polycondensation process (Fig. 1a). The kobs values obtained at different reaction temperatures were further used to construct an Arrhenius plot, from which the apparent activation energy (ΔEa) for the MSA-catalyzed reaction was calculated. The resulting value of ΔEa = 18.9 kcal mol−1 indicates a relatively moderate activation barrier that is compatible with the experimental conditions used (120–160 °C) for the MSA-catalyzed polycondensation process (Fig. 1b).


image file: d5py00580a-f1.tif
Fig. 1 (a) Catalytic kinetic processes of methanesulfonic acid catalysis at different temperatures (120–160 °C); (b) Arrhenius plot of MSA catalyzed polycondensation reaction. Experimental conditions: [HDL]0/[DPC]0 = 1[thin space (1/6-em)]:[thin space (1/6-em)]1, catalyst loading = 3 mol%.

To gain mechanistic insight into the MSA-catalyzed coupling reaction between aliphatic diols and diphenyl carbonate (DPC), density functional theory (DFT) simulation was performed. Coulembier and Taton revealed a dual activation mechanism of the ROP of cyclic ester using benzonic acid as a catalyst.55 Therefore, we postulate that MSA serves as a bifunctional catalyst to facilitate the alcoholysis of DPC through both the hydrogen bond donor and acceptor (Fig. S2†). Overall, the reaction is thermodynamically favorable (ΔG ≈ −18.01 kcal mol−1), where the rate-determining step is a nucleophilic attack through TS1 (ΔG‡ = 22.18 kcal·mol−1), which is in good agreement with the experimentally determined apparent activation energy ΔEa (18.9 kcal mol−1) (Fig. 2a).


image file: d5py00580a-f2.tif
Fig. 2 (a) The DFT simulation of initial coupling reaction between propanol and DPC catalyzed by MSA, leading to the generation of a tetrahedral intermediate (Int2). (b) The transition state for phenol generation with a high energy barrier for the reverse reaction. To reduce computational cost, n-propanol was employed as a model alcohol. Geometry optimizations were calculated at the ωB97XD/6-31+G(d) level, and single-point energies were refined at the ωB97XD/6-31+G(d′,p′) level.

DPC is activated by protonated MSA via hydrogen bonding, while the alcohol is simultaneously activated through interaction with MSA's sp2 oxygen, establishing a dual activation mechanism. Phenol elimination to form Int4 is both kinetically and thermodynamically favorable, with a low forward barrier (9.75 kcal mol−1) and a high reverse barrier (29.2 kcal mol−1), due to phenol's low nucleophilicity (Fig. 2b). This irreversibility supports the potential for non-equilibrium polycondensation in continuous-flow irreversible carbonate synthesis.

Based on the results of the batch polycondensation process, we next studied the polycondensation using HDL and DPC in a continuous-flow reactor. According to the Carothers equation,56 to obtain a polycarbonate polyol with an Mn of approximately 2000 g mol−1, the molar ratio of [HDL]0/[DPC]0 was set at 1[thin space (1/6-em)]:[thin space (1/6-em)]0.9, introducing a slight stoichiometric imbalance to limit the degree of polymerization. The initial investigation of the effect of the flow rate on polymerization performance was performed with a catalyst loading = 3 mol%, T = 160 °C, using a reactor with an inner diameter of 1 mm (d = 1 mm) and a fixed residence time of 4 h. The influence of the flow rate on continuous-flow irreversible polycondensation was systematically evaluated. As the flow rate increased from 0.004 to 0.033 mL min−1, the monomer conversion gradually increases from 61 to 96 mol%, accompanied by a notable growth in Mn, NMR from 200 to 2000 g mol−1. The increase in molecular weight with higher flow rates may be attributed to reduced polymer chain entanglement under rapid flow conditions (Fig. 3a). This enhancement is more accurately attributed not to polymer chain disentanglement, which is negligible at such low flow velocities, but rather to the hydrodynamic behavior characteristic of laminar flow regimes. Under these conditions, residence time distribution (RTD), axial dispersion, and parabolic flow profiles can significantly affect the spatial homogeneity of reaction progress along the channel. Higher flow rates can mitigate these effects by reducing axial dispersion and suppressing RTD broadening, leading to more uniform reaction conditions and thus higher conversions and narrower molecular weight distributions.57,58


image file: d5py00580a-f3.tif
Fig. 3 (a) NMR yield and Mn, NMR for the polycondensation of HDL and DPC using MSA at 160 °C utilizing continuous-flow reactors with different flow rates for a 1 mm tube diameter (d = 1 mm); (b) the influence on polymerization performance using continuous-flow reactors with different tube diameters; (c) kinetic plots for the polycondensation of HDL and DPC catalyzed by MSA at 160 °C using continuous-flow reactors (d = 1 mm).

The inner diameter of the tubular reactor is a critical design parameter in continuous-flow systems, as it directly influences fluid dynamics, mass and heat transfer efficiency, reaction rate, and system pressure drop. To evaluate the effect of tube diameter on the polymerization performance, experiments were conducted at a fixed flow rate of 0.026 mL min−1. When the reactor diameter was set to 0.5 mm, the NMR yield reached 82 mol%, with a number-average molecular weight (Mn, NMR) of approximately 400 g mol−1. Increasing the diameter to 0.8 mm resulted in a slight drop in yield to 79 mol%, with Mn, NMR around 500 g mol−1. Interestingly, further expansion of the tube diameter to 2.0 mm led to a marked increase in yield (94 mol%) and molecular weight (Mn, NMR ≈ 1400 g mol−1). Notably, the highest yield (97 mol%) and molecular weight (Mn, NMR ≈ 2600 g mol−1) were achieved at a tube diameter of 1.0 mm (Fig. 3b). This suggests that a 1.0 mm diameter provides an optimal balance between mass transfer and intrinsic reaction kinetics. Under these conditions, the polymerization is likely dominated by the inherent catalytic kinetics rather than mass transfer limitations, resulting in maximum yield and chain growth. The polymerization followed pseudo-first-order kinetics with respect to monomer consumption, as evidenced by the linear evolution of ln[1/(1 − p)] versus time (Fig. 3c).

To investigate the feasibility of synthesizing polycarbonate polyols in a continuous-flow reactor, we evaluated a series of diols with varying chain lengths. Diols including 1,7-heptanediol (1,7-HD), 1,8-octanediol (1,8-OD), 1,9-nonanediol (1,9-ND), 1,10-decanediol (1,10-DD), and 1,4-cyclohexanedimethanol (CHDM) were selected for continuous flow polycondensation under the given conditions. The corresponding polycarbonate polyols were obtained, with number-average molar masses ranging from 1400 to 2600 g mol−1 (ĐM = 1.9–2.1), showing unimodal distributions (Fig. 4a and b). Thermal analysis revealed distinct melting temperatures (Tm) and crystallization behaviors depending on the diol structure. For linear aliphatic diols (from 1,6-hexanediol to 1,9-nonanediol), a clear odd–even effect on melting temperature was observed (23.8–42.4 °C)59–61 (Fig. 4c). The melting points were generally lower than those of higher-molecular-weight analogues, which can be attributed to chain-end disruption and the presence of smaller crystalline domains in low-molecular-weight polyols.62 All long-chain polycarbonate polyols exhibited crystallization temperatures (Tc), which increased with the length of the aliphatic spacer (−12.3–33 °C) (Fig. 4d). In contrast, the polycarbonate polyol derived from 1,4-cyclohexanedimethanol showed no detectable melting transition, likely due to the inability of the rigid cycloaliphatic moieties to pack into an ordered crystalline lattice. Notably, the glass transition temperature (Tg) of the obtained polycarbonate polyol was not observed. This could have resulted from the low molecular weight of the polyols and their high chain flexibility, thereby leading to a broad and weak thermal transition.63,64


image file: d5py00580a-f4.tif
Fig. 4 (a) and (b) The SEC curves of various polycarbonate polyols using a continuous flow reactor. (c) The 2nd heating DSC curves of obtained polycarbonates. (d) The cooling DSC curves of the obtained polycarbonates.

Based on the results of the synthesis of polycarbonate diols in micro-flow reactors and concerning the much lower reactivity of phenol with isocyanates compared to aliphatic alcohols, we directly employed the crude mixture of polyols and the by-product phenol for polyurethane synthesis under micro-flow conditions. We selected the polyaddition between poly(hexamethylene carbonate) diol and 4,4′-methylene-bis(cyclohexyl isocyanate) (HMDI) as a model reaction. Given that 3 mol% MSA enables nearly complete conversion in under 1 second, we reduced the catalyst loading to 0.1 mol% to investigate the polymerization kinetics. The reaction was monitored in situ using FTIR-ATR with a flow-through cell by following the band associated with carbamate group formation at ∼1600 cm−1. The results show that the inner diameter of the tubular reactor strongly influences the reaction rate. A diameter of 0.8 mm yielded an observed rate constant of kobs = 0.14 min−1, significantly higher than those of 1 mm (kobs = 0.09 min−1) and 2 mm (kobs = 0.07 min−1). This enhancement is likely due to increased shear and improved diffusion in narrower channels, which facilitate more efficient mixing between the polycarbonate diol and HMDI (Fig. 5a–d).


image file: d5py00580a-f5.tif
Fig. 5 (a) FTIR spectroscopy characterized polycondensation between polycarbonate and HMDI with time evolution using a continues-flow reactor (d = 0.8 mm); (b) the diameter of the continuous-flow reactor is 1 mm; (c) d = 2 mm; experimental conditions for all kinetic studies: [poly(hexamethylene carbonate) diol]0/[HMDI]0/[MSA]0 = 1[thin space (1/6-em)]:[thin space (1/6-em)]1.1[thin space (1/6-em)]:[thin space (1/6-em)]0.001 at 25 °C; (d) semilogarithmic plot of polycondensation; (e) the 2nd heating (10 °C min−1 under Ar) DSC curve of the obtained polycarbonate-based polyurethane; and (f) the SEC curve of polycarbonate-based polyurethane using a continuous flow reactor.

Polyurethane pressure-sensitive adhesives (PU-PSAs) offer tunable adhesion properties by tailoring the match between the polyol and diisocyanate components, enabling their application across diverse fields including electronic encapsulation, medical dressings, high-end labeling, and removable tapes.65–68 Taking 1,6-hexadiol based polycarbonate as an example due to its commercial and industrial availability, we synthesized a polyurethane prepolymer using a micro-flow reactor and subsequently crosslinked them with trimethylolpropane (TMP) to obtain PU-PSA networks. The polyurethane adhesives were coated using a precision coating machine to form thin films with low viscosity, and then cured at 80 °C in a convection oven for 24 hours. The resulting PU-PSA samples were characterized for their adhesive performance using 180° peel strength testing on stainless steel in accordance with ISO 29862:2024 (Scheme 2). The results revealed a non-linear correlation between the peel strength and degrees of crosslinking (Fig. 6a). At a low molar ratio of 0.11, the adhesive showed limited cohesion strength, yielding a modest peel force. As the crosslinker ratio increased to 0.22, the peel strength reached a maximum (∼0.054 N cm−1), suggesting the formation of an optimal crosslinked network that balanced interfacial adhesion and cohesive integrity. However, further increasing the crosslinker content led to a noticeable decline in peel strength. These results indicate that a moderate crosslinking level is critical for achieving high peel resistance in polyurethane adhesives based on polycarbonate polyols (Fig. 6b–f).


image file: d5py00580a-s2.tif
Scheme 2 Schematic illustration of the preparation of polyurethane adhesive samples and the 180° tape peel test.

image file: d5py00580a-f6.tif
Fig. 6 (a) The comparison of the adhesive strengths of polyurethane with different ratios of TMP; (b) [polycarbonate diol]0/[HMDI]0/[TMP]0 = 1[thin space (1/6-em)]:[thin space (1/6-em)]1.5[thin space (1/6-em)]:[thin space (1/6-em)]0.11; (c) [polycarbonate diol]0/[HMDI]0/[TMP]0 = 1[thin space (1/6-em)]:[thin space (1/6-em)]1.5[thin space (1/6-em)]:[thin space (1/6-em)]0.16; (d) [polycarbonate diol]0/[HMDI]0/[TMP]0 = 1[thin space (1/6-em)]:[thin space (1/6-em)]1.5[thin space (1/6-em)]:[thin space (1/6-em)]0.22; (e) [polycarbonate diol]0/[HMDI]0/[TMP]0 = 1[thin space (1/6-em)]:[thin space (1/6-em)]1.5[thin space (1/6-em)]:[thin space (1/6-em)]0.27; and (f) [polycarbonate diol]0/[HMDI]0/[TMP]0 = 1[thin space (1/6-em)]:[thin space (1/6-em)]1.5[thin space (1/6-em)]:[thin space (1/6-em)]0.33.

Conclusion

Aliphatic polycarbonate polyols are promising and valuable building blocks for high-performance polyurethanes due to their superior hydrolytic and thermal stability. However, conventional batch step-growth polymerizations suffer from limited control over the polymer structure and low efficiency of production. We developed a continuous-flow strategy for the synthesis of polycarbonate diols via the MSA-catalyzed irreversible polycondensation of diols and diphenyl carbonate. Mechanistic and kinetic results revealed that MSA operates through dual hydrogen-bond activation, enabling a quasi-first-order reaction with high NMR yield in short residence time. The resulting polycarbonate diols exhibited controlled molecular weights and could be directly applied to polyurethane synthesis in flow. This study establishes a practical, scalable, and continuous platform for the efficient production of functional polyurethanes.

Conflicts of interest

There are no conflicts to declare.

Data availability

The data supporting this article have been included as part of the ESI.†

Acknowledgements

The authors are thankful for the financial support from the National Key Research & Development Program of China (2021YFC2101900), the National Natural Science Foundation of China (No. U24A20492) and Nanjing Municipal Science and Technology Bureau, and also appreciate the high performance center of Nanjing Tech University for supporting the computational resources.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d5py00580a
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