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Biopolymers from Renewable Resources (Macromolecular Systems - Materials Approach)
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Initiators for Polymerization of Lactide
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Ring opening polymerization of lactide with various different initiators are
being studied extensively in order to get polylactides with various useful
properties, including high rate of stereocontrol, higher molecular weight,
controlled polydispersity index, high rate of polymerization, and to
develop initiators with low toxicity and biocompatibility.
Main factors that influence the rate of ring-opening polymerization and
the molecular weight of polylactide polymers include:

- purity of monomers,
- type of catalyst,
- amount of water and alcohol in the system,
- polymerization temperature,
- polymerization time, and the
- monomer:catalyst ratio.
Polymerization of Lactides and Lactones by Tin(II)
2-ethylhexanoate catalyst

Ring opening polymerization of lactones are usually done with Tin(II)
2-ethylhexanoate (stannous octoate, stannous 2-ethyl hexanoate,
Sn(Oct)2, tin(II) bis(2-ethylhexanoate)) catalyst due to its solubility in
molten lactide, high reactivity, low rate of racemization of the polymer
and low toxicity.
Structure of tin(II) octoate


In general, the polymerization mechanism of lactide with tin octoate is
reported to be through the coordination-insertion mechanism (1).

Polymerization of Lactides and Lactones by Metal-Free
Initiators

Polymerization of lactones with metal free initiators were studied by Li
et al.,(2) and found that the bromide derivative of hexaalkylguanidinium
salt (HAGs) showed a comparable activity with tin(II)octanoate
catalyst.

Further, mixing of the bromide derivative with sodium carboxylate salts
resulted a higher catalytic activity in (3S)-L-Lactide polymerization with
65% enantioselectivity.
References:
1. Kricheldorfet al., Chemosphere 43,(1), 2001, pp 49-54
2. Li. et al. Ind. Eng. Chem. Res., 2005, 44 (23), pp 8641-8643
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