I. Introduction  
In the production of waterborne polyurethane, carboxylic acids used as anionic hydrophilic chain extenders are a class of diol-containing carboxylic acids that have been widely applied due to their unique molecular structure and excellent product performance.  
The main carboxylic chain extenders are 2,2-dimethylolpropionic acid (DMPA) and 2,2-dimethylolbutanoic acid (DMBA). They are unique multifunctional hindered diol molecules with both hydroxyl and carboxyl groups. After neutralization with alkali, the free acid groups effectively improve the water solubility or dispersibility of the resin; introducing polar groups improves coating adhesion and dyeability of synthetic fibers; and increases the alkali solubility of coatings. They can be used in water-soluble polyurethane systems, water-soluble alkyd and polyester resins, epoxy ester coatings, polyurethane elastomers, and powder coatings.  
They can also be used in leather chemicals, liquid crystals, inks, food additives, and adhesives, especially in the manufacture of water-emulsion polyurethane and leather finishes, where they serve both as chain extenders and give polyurethane good self-emulsifying properties, greatly improving the stability of polyurethane aqueous emulsions, and thus are widely used.  
II. Advantages of Using Dimethylol Carboxylic Acids  
Waterborne polyurethane emulsions usually introduce hydrophilic agents into the polyurethane molecular chain, then neutralize with alkali to form salts, and disperse in deionized water by mechanical stirring to form polyurethane aqueous emulsions.  
Hydrophilic agents for waterborne polyurethane mainly include anionic, cationic, and nonionic types. Anionic types mainly include: 2,2-dimethylolpropionic acid, 2,2-dimethylolbutanoic acid, tartaric acid, sulfobutanediol, sodium ethylenediaminoethanesulfonate, and half-esters synthesized from glycerol and maleic anhydride, etc.; cationic types mainly include: methyldiethanolamine, triethanolamine, etc.; nonionic types mainly include: hydroxyl-terminated polyethylene oxide.  
Nonionic hydrophilic agents such as polyethylene oxide require very high content to stabilize the dispersion. Waterborne polyurethane resins made with hydroxyl polyoxyethylene ether as hydrophilic groups have good electrolyte resistance, but poor water resistance of the film, and are therefore not practical;  
Cationic hydrophilic agents such as ethylenediamine-sodium acrylate adducts make the entire reaction system alkaline, involving not only rapid reaction of —NH2 with —NCO but also reaction of —NCO with —NHCOO—, making the reaction hard to control and prone to gelation, with coarse emulsion particles and poor film water resistance, and thus have not been used industrially;  
Among anionic types, dimethylol carboxylic acids themselves contain two hydroxyl groups and also act as chain extenders. This dual role gives them great advantages in preparing self-emulsifying PU emulsions. During urethane synthesis, they make the reaction system acidic; under acidic conditions, the —NCO and —OH reaction is mild, while —NHCOO— does not participate, avoiding gelation. In addition, dimethylol carboxylic acids act as chain extenders so that hydrophilic groups (carboxyl groups) are located in the macromolecular chain segments. Using tertiary amines as neutralizing agents can produce waterborne polyurethane resins with excellent stability and outstanding film water and solvent resistance. Dimethylol carboxylic acids are currently widely used as excellent hydrophilic compounds for preparing waterborne polyurethane resins.  
III. 2,2-Dimethylolpropionic Acid (DMPA) and 2,2-Dimethylolbutanoic Acid (DMBA)  
Among these two dimethylol carboxylic acids, 2,2-dimethylolpropionic acid was used earlier and is currently the more commonly used hydrophilic chain extender. Although it has many advantages, it also has drawbacks, mainly its high melting point (180–185℃), making it difficult to melt by heating, which requires organic solvents such as N-methylpyrrolidone (NMP), N,N-dimethylformamide (DMF), acetone, etc. NMP has a high boiling point and is hard to remove after preparing APU. Moreover, DMPA has low solubility in acetone, requiring large amounts of acetone in synthesis; ketone removal wastes energy and poses safety risks. Therefore, using 2,2-dimethylolpropionic acid not only leads to high production energy consumption but also easily causes organic residues in the product.  
2,2-Dimethylolbutanoic acid is a new hydrophilic chain extender that has emerged in recent years. Compared with 2,2-dimethylolpropionic acid, it has the following main advantages:  
1. Better solubility in organic solvents. The table below shows solubility data of DMBA and DMPA at different temperatures in different solvents;  
Solubility data of DMBA and DMPA at different temperatures in different solvents:

Solubility unit: g/100 g solvent  
Solubility in water: DMBA 48%, DMPA 12%.  
2. High reaction rate, fast reaction speed, and low reaction temperature. For example, synthesis of polyurethane prepolymer takes only 50–60 minutes for DMBA, while DMPA takes 150–180 minutes;  
3. Finer particle size and narrower distribution when used in waterborne polyurethane emulsions;  
4. Low melting point of 108–114℃;  
5. Formulation flexibility, allowing reduced solvent use, thus lowering solvent costs and waste treatment expenses;  
6. Can be used to prepare completely solvent-free polyurethane and polyester systems;  
In actual synthesis, it can be used without any solvent, producing emulsions with good performance and films with excellent mechanical properties, shortening reaction time, reducing energy consumption, and saving energy. Therefore, 2,2-dimethylolbutanoic acid is currently the best-known hydrophilic compound in performance. Industry experts believe that dimethylolbutanoic acid will inevitably replace dimethylolpropionic acid as a green chemical.  
IV. Development of Dimethylol Carboxylic Acids in China  
Research, development, and production of 2,2-dimethylolpropionic acid in China started relatively late, mainly constrained by the progress of resin waterborne conversion. Industrial production of 2,2-dimethylolpropionic acid in China began in 1995. Entering the 21st century, demand for 2,2-dimethylolpropionic acid has increased year by year and formed a certain market scale. Unfortunately, production of 2,2-dimethylolpropionic acid in China has not yet reached scale; only a few manufacturers are still producing it, all at relatively small scale, and current use in China still mainly relies on imports.  
Due to complex production processes, high industrialization difficulty, high production costs, expensive prices, and serious environmental issues, the production development and application promotion of 2,2-dimethylolbutanoic acid have been greatly limited. Production of 2,2-dimethylolbutanoic acid in China began in 2006 with only a few manufacturers, all of which successively shut down for the above reasons, leaving China without available 2,2-dimethylolbutanoic acid for several years. Many waterborne materials manufacturers have been eagerly awaiting a change to this passive situation and hoping to use 2,2-dimethylolbutanoic acid as soon as possible.  
Jiangxi Jiyu New Materials Co., Ltd. was founded against this background. The newly built Jiyu 2,2-dimethylolbutanoic acid production workshop has reached a capacity of 1,200 tons/year, and the 2,2-dimethylolpropionic acid production workshop has reached 3,500 tons/year. Jiyu is determined to revitalize China's 2,2-dimethylolbutanoic acid and 2,2-dimethylolpropionic acid industries and contribute to China's waterborne materials and environmental protection industries.