Density Functional Theory Study of Mechanism of Epoxy-Carboxylic Acid Curing Reaction Uyen Q. Ly, [a] My-Phuong Pham, [a,b] Maurice J. Marks, [c] and Thanh N. Truong * [b] A comprehensive picture on the mechanism of the epoxy- carboxylic acid curing reactions is presented using the density functional theory B3LYP/6-31G(d,p) and simplified physical molecular models to examine all possible reaction pathways. Carboxylic acid can act as its own promoter by using the OH group of an additional acid molecule to stabilize the transition states, and thus lower the rate-limiting barriers by 45 kJ/mol. For comparison, in the uncatalyzed reaction, an epoxy ring is opened by a phenol with an apparent barrier of about 107 kJ/ mol. In catalyzed reaction, catalysts facilitate the epoxy ring opening prior to curing that lowers the apparent barriers by 35 kJ/mol. However, this can be competed in highly basic cat- alysts such as amine-based catalysts, where catalysts can enhance the nucleophilicity of the acid by forming hydrogen- bonded complex with it. Our theoretical results predict the activation energy in the range of 71 to 94 kJ/mol, which agrees well with the reported experimental range for catalyzed reactions. V C 2017 Wiley Periodicals, Inc. DOI: 10.1002/jcc.24779 Introduction Carboxylic acids are an important class of hardenings for cur- ing epoxy resins. They have been increasingly used in powder coatings, consuming amongst the highest tonnage of epoxy curing agents in this application. This development is due to their relatively low price, widespread availability as raw materi- als, and excellent physical and chemical properties of the cured coatings such as flexibility and heat resistance. [1] The main process of this crosslinking reaction is to yield a b-hydroxypropyl ester. Under extreme conditions that product may react with a second carboxylic acid to yield a diester. The hydroxyl ester can also undergo polymerization by reaction of its secondary hydroxyl group with an epoxy group. [1,2] This curing occurs at a reasonably high temperature of around 2008C or at that and lower temperatures with the use of cata- lysts such as tertiary amines. [2] In practice tertiary amine cata- lysts are usually employed in these formulations. The curing reaction can be represented by the main reaction steps below: There have been a number of speculations on the mechanism of this reaction at the molecular level details based on experi- mental observations. However, to the best of our knowledge, there has not been any theoretical study on the mechanism of the epoxy-carboxylic acid curing system done to date to confirm any of such speculations. The main objective of this paper is to perform a systematic theoretical study to provide a comprehen- sive molecular-level picture on the mechanism of these epoxy- carboxylic acid curing reactions using density functional theory (DFT). In particular, all possible reaction pathways and the roles of catalysts in catalyzing this curing reactions were investigated. Furthermore, by probing the potential energy surface (PES) of possible reaction pathways at the same theoretical level enables the development of a more accurate kinetic model for epoxy- carboxylic acid curing system. Epoxy (denoted as E), carboxylic acid (RCOOH), tertiary amine catalyst (R3N) can form three hydrogen-bonded precur- sors namely acid-acid, acid-epoxy, and amine-acid and some are found to be involved in a number of epoxy-acid curing reaction pathways. Similar to our previous study of the epoxy- phenol curing reaction, [3] the uncatalyzed epoxy-acid reaction can also follow two possible pathways as illustrated in Scheme 1. First is the isolated pathway (designated as C-I), wherein epoxy reacts with an acid curing agent alone. This pathway was suggested in several previous studies. [2,4–6] Second is the self-promoted reaction pathway (C-P), in which an additional [a] U. Q. Ly, M.-P. Pham Institute for Computational Science and Technology, Ho-Chi-Minh City, Vietnam [b] M.-P. Pham, T. N. Truong Department of Chemistry, University of Utah, 315 South 1400 East, Rm 2020, Salt Lake City, Utah 84112 E-mail: thanh.truong@utah.edu [c] M. J. Marks Consultant, Lake Jackson, Texas 77566 Contract grant sponsors: Dow Chemical Company and the Institute for Computational Science and Technology via Ho-Chi-Minh City Office of Science and Technology, Vietnam; Contract grant sponsor: Vietnam’s Ministry of Education (to M.P.P.) V C 2017 Wiley Periodicals, Inc. Journal of Computational Chemistry 2017, 38, 1093–1102 1093 FULL PAPER WWW.C-CHEM.ORG