ORIGINAL PAPER Theoretical investigation on the mechanism and kinetics of the ring-opening polymerization of ε-caprolactone initiated by tin(II) alkoxides Chanchai Sattayanon & Nawee Kungwan & Winita Punyodom & Puttinan Meepowpan & Siriporn Jungsuttiwong Received: 27 December 2012 /Accepted: 30 September 2013 /Published online: 31 October 2013 # Springer-Verlag Berlin Heidelberg 2013 Abstract A theoretical investigation of the ring-opening polymerization (ROP) mechanism of ε-caprolactone (CL) with tin(II) alkoxide, Sn(OR) 2 initiators (R= n -C 4 H 9 , i -C 4 H 9 , t -C 4 H 9 , n -C 6 H 13 , n -C 8 H 17 ) was studied. The density functional theory at B3LYP level was used to perform the modeled reactions. A coordination-insertion mechanism was found to occur via two transition states. Starting with a coordination of CL onto tin center led to a nucleophilic addition of the carbonyl group of CL, followed by the exchange of alkoxide ligand. The CL ring opening was completed through classical acyl-oxygen bond cleavage. The reaction barrier heights of ε-caprolactone with different initiators were calculated using potential energy profiles. The reaction of ε-caprolactone with Sn(OR) 2 having R= n -C 4 H 9 has the least value of barrier height compared to other reactions. The rate constants for each reaction were calculated using the transition state theory with TheRATE program. The rate constants are in good agreement with available experimental data. Keywords Coordination-insertion mechanism . Density functional theory . ε-caprolactone . Ring-opening polymerization . Tin(II) alkoxides . Transition state theory Introduction Poly(ε -caprolactone) (PCL), synthetic biodegradable and biocompatible polymer, has been extensively studied due to its medical and environmental applications [ 1 ]. The biomedical uses are in areas of controlled release drug delivery systems [2–5] and 3D scaffolds for use in tissue engineering [6]. The environmental friendly uses are in the area of disposable packaging [7]. The most widely used technique for synthesizing this polymer and its related aliphatic polyesters is the ring opening polymerization (ROP) [8]. The ROP of cyclic esters [9] can be achieved by using cationic, anionic, activated monomer, enzymatic, and organocatalytic methods [10]. A large number of experimental studies have been carried out with different catalyst or initiator of metal alkoxides in which metals can be alkali [11, 12], transition [13–16], and lanthanide [17, 18]. Metal alkoxides are the most widely used types of ROP initiator and their ring opening mechanism is coordination-insertion of monomer into the metal-oxygen bond of initiator. To date, the most widely use metal alkoxide both in academia and industry is Sn(Oct) 2 [9, 19–24]. Many research groups have used Sn(Oct) 2 with alcohol to study the ROP of different kinds of monomers. It is widely accepted that the Sn(Oct) 2 initiator and ROH co-initiator react together in situ to form the corresponding tin(II) monoalkoxide, [Sn(Oct)(OR)], and/or dialkoxide, [Sn(OR) 2 ] which are the “true initiator”. However, the true initiator has to be formed prior to ROP initiation and Electronic supplementary material The online version of this article (doi:10.1007/s00894-013-2026-2) contains supplementary material, which is available to authorized users. C. Sattayanon : N. Kungwan : W. Punyodom : P. Meepowpan Center of Excellence for Innovation in Chemistry, Department of Chemistry, Faculty of Science, Chiang Mai University, Chiang Mai, Thailand 50200 N. Kungwan (*) : W. Punyodom : P. Meepowpan Biomedical Polymers Technology Unit, Department of Chemistry, Faculty of Science, Chiang Mai University, Chiang Mai, Thailand 50200 e-mail: naweekung@gmail.com S. Jungsuttiwong Center for Organic Electronic and Alternative Energy, Department of Chemistry and Center of Excellence for Innovation in Chemistry, Faculty of Science, Ubon Ratchathani University, Ubon Ratchathani, Thailand 34190 J Mol Model (2013) 19:5377–5385 DOI 10.1007/s00894-013-2026-2