Polymerization without Carboxylic Acid Additives

Polar (dimethylacetamide (DMAC), DMF, NMP, THF) and nonpolar (toluene) solvents are suitable for these reactions and should be selected according to the polymer solubility. DMAc is not a suitable solvent for polymerization and always gives brown soluble material. Addition of carboxylic acid may be beneficial in nonpolar solvents owing to the high polarity of the C-H bond transition states. However, C-H bond activations have been accomplished in toluene and xylenes without carboxylic acids. ... 

The presupposition, that carboxylate anions are the active centers of polymer growth (43, 70, 72) is equally to be rejected the addition of carboxylic acid salts, e. g. sodium acetate is without any effect on the polymerization rate (103). Besides, the concentration of carboxylic groups as found in the resulting polymers used to be very low (28, 29, 60). 

Some lactones can polymerize in the presence of compounds like alcohols, amines, and carboxylic acids without additional catalysts. The reactions, however, are slow and yield only low molecular weight polymers Exceptions are polymerizations of pivalolactone in the presence of cyclic amines that yield high molecular weight polyesters at high conversion. The initiating steps result from formations of adducts, amine-pivalate betaines ... 

Crystallization is able to recover the desired product (in solid or crystal form) without many process control operations. After removal of cell biomass and organic impurities by centrifugation and activated carbon absorption, respectively, a direct vacuum distillation-crystallization is used for succinic acid recovery from broth by Luque et al. (2009). The pH of the aqueous broth is adjusted to 4.2 by addition of hydrochloric acid before vacuum distillation. Some volatile by-product carboxylic acids, such as acetic, formic acids, in broth are removed under vacuum distillation at 60 °C. The followed crystallization of succinic acid is carried out at 4 °C. When this method is used in a simulated broth, the highest succinic acid yield and purity are 75 % and 97 %, respectively. Another direct crystallization method at normal pressiue bases on the principle that carboxylic acids have different distribution with their dissociated and undissociated forms at different pH, and the undissociated carboxylic acid has different solubility (Li et al. 2010b). The solubility of succinic acid is 3 % at 4 °C, pH 2.0, while the other acid by-products, such as lactic acid, acetic acid, and formic acid, are still fully water miscible. Thus, crystallization of succinic acid can be carried out at 4 °C and pH < 2. While acidic by-products remain in the solution, succinic acid could be selectively crystallized. By this one-step recovery technique, succinic acid yield and purity are 70 % and 90 %, respectively. However, much succinate is still residual in the broth, and the product of low piuity cannot be used as a monomer for polymerization. To obtain a higher purity of succinic acid, coupled purification processes such as adsorption or membrane separation are needed for broth to remove residual impurities (Wang et al. 2014). 

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