A Dilatometric Study of the Kinetics of the Ring-Opening Polymerisation of %-Caprolactone

Dilatometry is a well-established technique for studying the kinetics of polymerisation by following the volume change which accompanies polymer formation. In this work the kinetics of the ring-opening Polymerisation of ε-caprolactone at 90 °C using 0.02 mole % stannous octoate as initiator were studied. After an initial induction period, the results showed a close adherence to first-order kinetics with a first-order rate constant, k1, of 2.42x10 -4 min. Methodology: An appropriate amount of purified ε-caprolactone monomer together with 0.02 mole % of stannous octoate were accurately weighed into a 10 ml ‘Quickfit’ conical flask. A small magnetic bar was added and a capillary inserted into the neck of the flask so that the monomer meniscus level rose to a suitable height in the capillary. The complete dilatometer assembly was then clamped accurately vertical in a water bath at 90°C (zero time, t=0) and a cathetometer focussed on the monomer meniscus level. From the cathetometer scale readings of meniscus height (h) obtained as function of time (t), the % conversion and rate of polymerisation (rp ) were calculated from the equations[1]: ∆V ∆h(t) % conversion = x 100 = x 100 V 0 -V ∞ ∆h(t = ∞) ∆h(t) [M] 0 rp = ∆h(t = ∞) t where [M] 0 = initial monomer concentration (mol l ) Results, Discussion and Conclusions: The zero-order and first-order rate plots for ε-caprolactone polymerisation with 0.02 mole % stannous octoate as initiator at 90°C are compared in Figs. 1 and 2. The resultant plots indicate that after an initial inductor period, the polymerisation reaction adheres more closely to first-order kinetics. From the linear portion of the first-order rate plot, a value for the first-order rate constant, k1, of 2.42 x 10 -4 min is obtained. The mechanism of the polymerisation is generally accepted being a coordination-insertion type mechanism in which successive ε-caprolactone monomer units are first coordinated with and then inserted into the Sn-O bond of the stannous octoate initiator [2]. References: [1] H.R. Allcock and F.W. Lampe, Contemporary Polymer Chemistry, Prentice-Hall, Inc., New Jersey, 1981. [2] A.Duda, A. penczek, A. Kowalski and J. Libiszowski. Macromol. Symp., 153, 41-53 (2000).