Date: 12 September 2025 | Category: News
Authors: Arjun Raghuraman, Heather A. Spinney, David R. Wilson, Carlos Villa, Sukaran Arora, Ankita Majumder, Julibeth M. Martinez De La Hoz, Manjiri Paradkar, Anthony P. Gies, Masayuki Suzuki, An Keaton, Sukrit Mukhopadhyay, Varinia Bernales, Jean-Paul Masy, and Robert D. Kennedy
Researchers from the Dow Chemical Company, based in Texas, USA, have disclosed an improved ring-opening polymerisation (ROP) of propylene oxide (PO) using the catalyst tris(3,5-bis(trifluoromethyl)phenyl)borane (1), building on previous methods using tris(pentafluorophenyl)borane (BCF) Figure 1.[1] Initial experiments evaluated the performance of different boron-based Lewis acid catalysts with optimisation facilitated using a flow chemistry set-up, which enabled rapid, safe and reproducible reaction conditions. These conditions were then converted to process scale that introduced the use of a semi-batch approach to control the high exotherm of the ROP reaction. Overall, use of borane 1 allowed for lower catalyst loadings and production of polyurethane films with fewer defects and higher tensile strength than those formed using previously disclosed methods.
Figure 1: Synthesis of polyurethanes and polyether polyols. (a) Synthesis of polyurethanes; (b) ROP of alkylene oxides to form polyether polyols; (c) Boron-based catalysts.
Polyurethanes: Extremely versatile and increasingly ubiquitous
Polyurethanes (PUs) are a class of polymers that are prolifically used within foams, elastomers, coatings, adhesives, and fibre composites, as well as appliances, furniture and building materials.[2, 3] Recent restrictions on the use of perfluoroalkyl substances (PFAS) is anticipated to increase the demand for PUs worldwide as a non-halogenated alternative material, for example in the manufacturing of sustainable technologies such as wind turbines and electric vehicles.[4]
PUs are prepared through the combination of polyisocyanates and polyols, Figure 1a. Structural variation in the commercially available polyisocyantes is limited therefore the physical and chemical properties of PU, influenced by molecular weight, functionality, hydrophilicity and stereochemical configuration, are predominantly modulated through variation of the polyol component.[1]
Polyether polyols are commonly used, which are prepared through ROP of alkyne oxides such as ethylene oxide (EO), propylene oxide (PO) or butylene oxide (BO) using a multifunctional initiator in the presence of a catalyst.[5] In industry, potassium hydroxide or double metal cyanides are commonly used as the catalyst, however they often result in sluggish polymerization, high catalyst loadings or a poor primary to secondary alcohol ratio in the product.[1]
Lewis acids in PU preparation
Lewis acid catalysis provides a valuable alternative but has yet to be exploited on a commercial scale, likely due to challenges in control over the ratio of primary to secondary alcohols formed with non-activated epoxide substrates. BCF is a boron-based catalyst that defies this trend, providing good selectivity for primary alcohols during ROP of PO. However, a major drawback of BCF is formation of propionaldehyde (PA) during the reaction, leading to lower yields, byproduct formation and requirement for additional purification steps – factors that limit its industrial applicability.
Vapourtec R-Series allows rapid catalyst screening
The team at Dow Chemical Company used the Vapourtec R-Series system to screen several boron-based catalysts similar to BCF, monitoring the success of the ROP of PO and formation of the undesired PA side product, Figure 2. The use of a flow reactor was particularly advantageous, as it readily addressed safety concerns regarding the exothermic nature of the transformation. In addition, the reactor’s high throughput capabilities and speed of implementation enabled results to be generated quickly and efficiently. Undertaking a kinetic study was simplified through inclusion of an IR module within the flow chemistry set-up, which measured both PO conversion and PA formation.
Figure 2: Simplified set-up of the Vapourtec R-Series system. In the kinetics experiments, an IR module was inserted between the reactor coil and the sampler/scrubber outlet. Proglyde is di(propylene glycol) dimethyl ether.
Overall, it was shown that catalyst 1 (Figure 1) led to less PA formation than BCF, without significantly impacting the kinetics of the polymerization process. While the use of this catalyst resulted in lower regioselectivity for the desired primary alcohol (40 – 50% 1° OH), the formation of primary alcohols in reasonable yield provides a viable alternative to previous methods using potassium hydroxide, which exclusively gives secondary alcohols.
In summary, the use of the Vapourtec R-Series resulted in safe and rapid screening of reaction conditions for the industrially significant ROP of PO. Moreover, the modularity of the Vapourtec set-up enabled inclusion of an IR module, providing insight into the reaction kinetics and potential synthetic pathways.
References:
[1] Triarylborane-Catalyzed Ring-Opening Polymerization of Propylene Oxide: A Pathway to Superior Polyurethanes. (A. Raghuraman, H. A. Spinney, D. R. Wilson, C. Villa, S. Arora, A. Majumder, J. M. Martinez De La Hoz, M. Paradkar, A. P. Gies, M. Suzuki, A. Keaton, S. Mukhopadhyay, V. Bernales, J.-P. Masy, R. D. Kennedy, Ind. Eng. Chem. Res., 2025, 64, 33, 15982–15996). https://doi.org/10.1021/acs.iecr.5c01981
[2] Polyurethanes: Versatile Materials and Sustainable Problem Solvers for Today’s Challenges (H.-W. Engels, H.-G. Pirkl, R. Albers, R. W. Albach, J. Krause, A. Hoffmann, H. Casselmann, J. Dormish, Angew. Chem. Int. Ed., 2013, 52, 9422–9441). https://doi.org/10.1002/anie.201302766
[3] Polyurethane types, synthesis and applications – a review (J. O. Akindoyo, M. D. H. Beg, S. Ghazali, M. R. Islam, N. Jeyaratnam, A. R. Yuvaraj, RSC Adv., 2016, 6, 114453–114482). https://doi.org/10.1039/C6RA14525F
[4] The scale of the problem of replacing ‘forever chemicals’ PFAS. (Chemistry World, 2024, https://www.chemistryworld.com/news/the-scale-of-the-problem-of-replacing-forever-chemicals-pfas/4020175.article (Accessed August 2025).
[5] Polymerization of Ethylene Oxide, Propylene Oxide, and Other Alkylene Oxides: Synthesis, Novel Polymer Architectures, and Bioconjugation (J. Herzberger, K. Niederer, H. Pohlit, J. Seiwert, M. Worm, F. R. Wurm, H. Frey, Chem. Rev., 2016, 116, 2170–2243). https://doi.org/10.1021/acs.chemrev.5b00441