2-Methyl-1,3-propanediol, abbreviated as MPD, has the molecular formula C?H??O?. It exhibits good hygroscopicity and excellent thermal stability. The molecule contains two terminal hydroxyl groups, enabling it to undergo condensation reactions with dicarboxylic acids and isocyanates. It is primarily used in the synthesis of unsaturated polyester resins, water-based polyurethane resins, and powder coating resins, and also serves as a moisturizing agent in cosmetics and a high-boiling-point solvent in inks.

Environmental fate, ecotoxicology and toxicology of 2-methyl 1,3-propanediol

2-Methyl-1,3-propanediol (MPD, CAS RN: 2163-42-0; EC 412-350-5) is a colorless low viscosity liquid with a unique molecular structure. It is a low molecular weight branched aliphatic diol with two primary hydroxyls. MPD is water soluble at room temperature and it also has a low volatility and high flashpoint. As an isomer of 1,3-butyleneglycol, MPD offers similar performance characteristics. MPD is produced by Lyondell Chemical Company in a proprietary, multi-step reaction from propylene oxide. 2-Methyl-1,3-propanediol can be employed in a wide variety of applications. MPD has undergone extensive evaluation and been determined of low hazard, therefore it has been approved in Europe and the U.S. for use in personal care products. It can be used in a variety of products such as antiperspirants, nail polish, shaving creams and sunscreens. MPD can be used as a neutralizer, emollient, emulsifier and humectant, as well as a fragrance enhancer and carrier solvent. 2-Methyl-1,3-propanediol can also be used in the synthesis of ortho-, iso-, and terephthalate-based unsaturated polyester resins with increased production rates, improved styrene solubility, improved corrosion performance and improved mechanical performance. In addition, MPD can be used in the production of polyester polyols for OEM, refinish, and coil coatings. An acute inhalation study in Wistar rats, conducted according to the OECD403 Guideline, exposed rats, nose-only, to MPD aerosols, resulting in a calculated LC50 of >5.1 mg/l (Muijser, 1998). A second inhalation study, conducted according to OECD403 Guideline exposed Wistar rats (5/sex), nose only, to 2-methyl-1,3-propanediol aerosols for 4 h, resulting in a calculated LC50 of >5.4 mg/L.[1]

Removal of MPD from surface waters by abiotic degradation is not expected to be a significant process, since the absence of hydrolysable moieties in its molecular structure prevents hydrolysis from occurring. Significant microbial degradation has been shown in several studies, confirming biodegradability as a major mechanism for removal of 2-methyl-1,3-propanediol from the environment. Three separate “ready biodegradability” studies have been conducted, two according to a carbon dioxide evolution, modified Sturm test method and one according to a closed bottle method. Due to the unexplained differences between the high and low concentration results, it was felt that these tests were not fully reliable and a further study was conducted to clarify biodegradation potential of MPD. 2-Methyl-1,3-propanediol was found to present a low toxicity hazard to aquatic species, with low potential for bioaccumulation or persistence. While some findings are present in human skin sensitization and developmental toxicity studies, when considering the lack of reproducibility and low prevalence rate of the human skin reactions, these findings are considered to represent a low risk to human health. Similarly, the historical control incidence of rat and rabbit developmental findings, leads to the conclusion that there were no findings of toxicological hazard significance for 2-methyl-1,3-propanediol. The overall hazard assessment across all media and test systems indicate a lack of toxicity or environmental hazard that would warrant classification by the WHO GHS, EU, or US chemical classification schemes.

2-Methyl-1,3-propanediol on bio-based poly(propylene furandicarboxylate) copolyesters

Recently, numerous advanced works of furan-based polyesters have been published with a view of processing them to have suitable properties and improving their performance over that of fossil-based resources. PPF has poorer elongation properties than PET, restricting its range of applications. Hongzhon Xie et al. reported that its toughness could be increased by incorporating 1,5-pentanediol. A chemical with an asymmetric pendant group, 2-methyl-1,3-propanediol (MPO), is an intermediate in the organic synthesis of byproducts by the isomerization of propylene oxide to alkyl alcohol, followed by hydrogenation. The structures of 2-methyl-1,3-propanediol and 1,3-propanediol (PPO) are similar, but the lack of a methyl group in PPO caused them to have different mechanical properties, crystalline structures, and thermal behaviors. PPF, PMePF, and a series of PPMF copolyesters with different compositions were synthesized by conventional melt polymerization in a steel autoclave reactor. The structures and compositions were identified using 1H and 13C NMR and FT-IR spectroscopy, and the thermal properties, mechanical properties, crystallinity, and segmental motions were investigated using DSC, TGA, tensile testing, gas permeability analysis, XRD, DMA, and rheological analysis. The influence of an asymmetric substituent group in PPF (or the influence of the molar ratio of PPO to MPO in PPF) was investigated, and the mechanical and gas barrier properties of PPF copolyesters that had been modified by 2-methyl-1,3-propanediol molecules still supported the great potential for their application in next-generation packing materials in the near future.[2]

A series of bio-based PPF, PMePF, and PPMF copolyesters were successfully synthesized by conventional melt polymerization. The structure of each was identified using the FT-IR, 1H NMR, and 13C NMR analysis. From PPF to PPMFs and then to PMePF, the Tm showed a decreasing tendency because the asymmetric 2′-methyl group of 2-methyl-1,3-propanediol disrupted the crystallinity. Tg could be adjusted in the range of 73.3–83.0 °C by controlling the comonomer ratio. Additionally, the magnitude of the β peak and fractional free volume were increased when the lateral methyl group was copolymerized, lowering the gas barrier in the order PPF > PMePF > PPMF30. That is to say, the asymmetric lateral group influenced the main-chain arrangement and increased the motion of the backbone. From micro perspectives, the small-scale molecular motion also increased, making the oxygen permeate the film more easily. In conclusion, the 2-methyl-1,3-propanediol molecule copolymerized into PPF copolyesters not only modified the thermal properties but also increased the range of motion of the molecular chain, changing the gas permeability. Even though the gas permeability tended to decrease from PPF to PMePF to PPMF30, these can all be regarded as eco-friendly copolyesters for use in food packaging, bottling, and low-melting fiber applications.

References

[1]Fowles, J., Lewis, C., & Rushton, E. (2017). Studies on the environmental fate, ecotoxicology and toxicology of 2-methyl 1,3-propanediol. Regulatory Toxicology and Pharmacology, 91, 240–248. https://doi.org/10.1016/j.yrtph.2017.10.031

[2]Yang, Z.-Y., Chen, C.-W., & Rwei, S.-P. (2020). Influence of asymmetric substituent group 2-methyl-1,3-propanediol on bio-based poly(propylene furandicarboxylate) copolyesters. Soft Matter, 16(2), 366–374. https://doi.org/10.1039/C9SM02081K