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- Tin catalysts (2) (entfernen)
Organisationseinheit der BAM
- 6 Materialchemie (1)
- 6.3 Strukturanalytik (1)
Five new catalysts are prepared from dibutyltin oxide and catechol (HCa),
2,3-dihydroxynaphthalene (NaCa), 4-tert-butyl catechol (BuCa), 4-cyano
catechol (CyCa), and 4-benzoyl catechol (BzCa), but only BuCa gives useful
results. When benzyl alcohol is used as an initiator, linear chains having
benzyl ester end groups are formed in a slow polymerization process. In
contrast to cyclic or noncyclic dibutyltin bisalkoxides, neat BuCa yields
cyclic poly(l-lactide)s via a fast ring-expansion polymerization. Under certain
conditions, a high-melting crystalline phase (Tm = 191 °C) is obtained.
At 160 °C and short reaction times even-numbered cycles are slightly
prevailing, but, surprisingly, at 120 °C, odd-numbered cycles are predominantly
formed. These results definitely prove that a ring-expansion mechanism
is operating.
This article reviews the usefulness of tin(II) and tin(IV) salts and compounds as catalysts for the polymerization of lactides. The text is subdivided into nine parts mainly reflecting different polymerization strategies, such as ring-opening polymerization (ROP), ring-expansion polymerization (REP), ROP combined with simultaneous polycondensation (ROPPOC), various catalysts with unknown polymerization mechanisms, and polycondensation of lactic acid. Since the toxicity of tin salts and compounds is a matter of concern and frequently mentioned in numerous publications, the first section deals with facts instead of myths about the toxicity of tin salts and compounds.