Polymerization of cyclic esters has attracted a lot of interest for the synthesis of biodegradabale/ biocompatible polymeric materials, such as polylactide, polyglycolide and also other polymer compound such as linear polycarbonate. Ring opening polymerization of cyclic ester compounds is better than step polymerization for obtaining high Mw polymers in a controlled “living” fashion, while avoiding the formation of by-products.
Tin octoate (tin(II) bis(2-ethyl hexanoate)), Sn(Oct)2) is one of the most used initiator in the polymerization of cyclic esters. There have been various mechanisms proposed for the initiation and propagation mechanism of cylic ester polymerization initiated by Sn(Oct)2 as these mechanisms are the least being understood. The mechanisms were summarized below (see Scheme 1 - 4):
1. Cationic polymerization
Proton and cationic species were formed during the propagation process and Sn is not bound to the propagating chain (Scheme 1 ).
2. Activated monomer mechanism.
Activated monomer was added to –OH group of macromolecules and ROH is strongly complexed to Sn(Oct)2, involving only the free orbitals of the catalyst
3. Formation of anhydride end group from direct reaction of Sn(Oct)2 with monomer
The initiation and propagation steps proceed without co-initiator.
4. Metal alkoxide initiated-like polymerization from conversion of Sn(Oct)2 into alkoxide compound
Sn(Oct)2 reacts with hydroxide groups of another compounds added or present in the reaction and formed the actual initiator, tin(II) alkoxide or hydroxide.
Scheme 1. Mechanism based on cationic species formation
Scheme 2. Mechanism based on activated monomer
Scheme 3. Mechanism based on direct reaction of Sn(Oct)2 a...
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...n initiated with Sn(Oct)2 were studied by conducting various reactions incorporating various ratio and amount of reagents, co-initiator and trapping agents. Furthermore, reaction kinetics were monitored and intermediate /trapped compounds were isolated and identified. The results indicated that propagation did not proceed as cationic polymerization. Reaction was co-initiated by alcohol or alkoxide compound and Sn was attached to the propagating group.
These findings lead to the scheme shown in Scheme 4 as the adopted mechanism of the cyclic ester polymerization by Sn(Oct)2. The polymerization proceeds on the metal (Sn(II)) alkoxide bond by insertion, like in other metal alkoxide initiators, where at least one carboxylate group in Sn(Oct)2 is replaced by alkoxide group and then polymerization proceeds by monomer insertion into the resulting tin(II)-alkoxide bond.
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