Copolymerization of LL-Lactide at Its Living Polymer-Monomer Equilibrium with E-Caprolactone as Comonomer Jaroslav Mosna ´ c ˇ ek, † Andrzej Duda, Jan Libiszowski, and Stanislaw Penczek* Department of Polymer Chemistry, Center of Molecular and Macromolecular Studies, Polish Academy of Sciences, 90-363 Lodz, Sienkiewicza 112, Poland Received September 22, 2004 Revised Manuscript Received December 7, 2004 Introduction. Copolymerization of a given monomer, after it has reached equilibrium with its homopolymer, is a well-known general phenomenon and was studied in the past by several authors. 1-8 The monomer in equilibrium may interact either stronger or weaker with a foreign active center than with its own. In the former instance its equilibrium monomer concentration ([M] eq ) would decrease, whereas in the latter no effect on [M] eq would be observed. The pertinent equations describing the dependence of the copolymer composition on the monomer feed have also been derived in the past for several systems. 9 Thermodynamic treatments of such instances have been summarized in the extensive Sawada’s monograph 1 and the chapter devoted to ther- modynamics of the ring-opening polymerization in Comprehensive Polymer Science. 10 However, we could not find in the available open literature any study of specific copolymerization with the aim of introducing a given monomer into a polymer after living polymer- monomer equilibrium was reached. It is shown in the present paper that LL-lactide (LA) can be completely converted into polymer repeating units with the help of another monomer. This is important because the six-membered LA, as well as some other medium-strained cyclic esters, has relatively high equilibrium monomer concentrations, increasing with the increasing polymerization temperature. 11-13 Since polymerization of LA is conducted often in the monomer/polymer melt, relatively high temperatures are required. In our earlier work, devoted to the thermodynamics of LA polymerization, the correspond- ing dependence of the equilibrium LA concentration ([LA] eq ) on temperature was found, and then the en- thalpy (ΔH lc ) and entropy (ΔS° lc ) of the LA monomer to polymer transformation were determined. 11 For in- stance, at 180 °C, i.e., close to the polymer melting point, [LA] eq is equal to 0.32 mol L -1 . It is therefore important to find conditions to force this monomer, being in a relatively high equilibrium concentration, to enter into the polymer chains. If lowering of the polymerization temperature cannot be applied, copolymerization looks to be the only way to achieve this goal. Indeed, lowering temperature and polymer crystallization would decrease the amount of monomer at equilibrium, but remelting of the system at higher temperature would restate the previous condi- tions. When copolymerization is applied, the repeating units derived from a comonomer will appear at the polymer chain end that would alter in a certain way the final polymer properties. Thus, the comonomer structure has to be chosen in a way preventing deterioration of the properties as little as possible. In another series of papers from this laboratory and related to another system of the ring-opening polymerization, it has been shown that when equilibrium is reached, it is possible by proper comonomer choice to form a periodic or alternating copolymer, depending on the structures of the given comonomers pair. 7,8 Results and Discussion. In this preliminary com- munication we show that LA is able to copolymerize with ǫ-caprolactone (CL) at or below its monomer equilibrium concentration ([LA] eq ) reached in homo- polymerization. We also show that even CL, known to have much lower reactivity than LA in their copoly- merization carried out above equilibrium concentration for both comonomers, 14 enters even faster the chains than LA when [LA] eq is reached. This phenomenon has a thermodynamic origin since LA addition to the active polymer chains bearing a terminal lactide unit is counterbalanced by depropagation (thus, cannot form longer sequences). Therefore, whenever the ...-la* chain end is formed, the probability of formation of the ...-la-cl* unit prevails over that of ...-la-la*. It does not mean that the formation of the ...-la-la* unit is slower than formation of the ...-la-cl* unit (thus, LA is still more reactive monomer), but ...-la-la* depropagates much faster than the ...-la-cl* does. To avoid or at least to minimize transesterification that would not allow achieving our goal because of constant re-formation of LA by its depropagation from the newly created PLA chain ends (for example, by attack of the ...-cl* chain ends on the ...-la-... repeating units), we used as an initiator the reaction product of Al[OCH(CH 3 ) 2 ] 3 trimer (A 3 ) with (S)-(+)-2,2′-[1,1′- binaphthyl-2,2′-diylbis(nitrylomethilidyne)]diphenol [(S)- SB(OH) 2 ]. This choice was based on results of our earlier studies on the relationship of the selectivity of polym- erization on the structure of the growing chain end in the polymerization of CL and LA. 15 It has also been shown in the recently published papers that the A 3 / (S)-SB(OH) 2 complex polymerizes LA to PLA with low polydispersity index (M w /M n ) to high conversion, indi- cating only little transesterification taking place. 16,17 Thus, LA was polymerized first with (S)-SBO 2 Al-O-..., and when the living polymer-monomer equilibrium was reached CL was added, and the dependencies of [LA] and [CL] on the copolymerization time were simul- taneously measured by SEC. Experimental procedures are described in ref 18. Figure 1 shows plots of LA and CL concentration changes during the copolymerization time. Before co- polymerization was started, LA already has reached its equilibrium concentration, i.e., [LA] 0 ) [LA] eq ) 0.055 mol L -1 (at 80 °C, THF solvent). It is clearly seen that at these conditions, in contrast to what is known on the copolymerization of the LA/CL, CL is a “faster mono- mer”, as mentioned already above. This is simply because LA at these conditions is unable to homopoly- merize. Thus, the kinetic equations for such a copolym- erization may be written as in Scheme 1. † On leave from Polymer Institute, Centre of Excellence for Degradation of Biopolymers, Slovak Academy of Sciences, Du- bravska cesta 9, 842 36 Bratislava, Slovak Republic. * Corresponding author. E-mail: spenczek@bilbo.cbmm.lodz.pl. 2027 Macromolecules 2005, 38, 2027-2029 10.1021/ma0480446 CCC: $30.25 © 2005 American Chemical Society Published on Web 02/12/2005