Effects of Polymerization Catalyst Residues

Many factors, such as molecular weight, moisture, residual and hydrolyzed monomers, oligomers, and residual metals from polymerization catalysts, have been reported to influence PLA thermal decomposition. In particular, the effect of pol mierization catalysts, such as Sn, Zn, and Al compounds is important. The Sn catalyst is of special importance because only Sn 2-ethylhexanoate (Sn(0ct)2) is approved by the FDA as a catalyst for food-contact PLA applications [33]. 

Some reports about the effect of the Sn based catalyst on PLA thermal degradation have been published. Noda et al. [34] evaluated the activity of mono-, di-, tetra-all l and aryl-tin (IV) compounds as intramolecular transesterification catalysts, and found that monobutyltin trichloride was the most active catalyst with some other compounds indicating almost the same depolymerization activity as Sn(0ct)2. Degee et al. [35] also reported that PLA prepared with Sn(0ct)2 was less stable thermally than that prepared with Al alkoxides based on TGA data. However, Cam and Marucci [27] found a contrary effect on thermal decomposition onset temperature in the order of Al Zn Sn. 

Moreover, they demonstrated the random scission of PLA (Sn content 690 ppm) based on a linear relationship between 1/jJeff and time, giving an Eg of 119.4 kj mol [20], 

However, as reported by Wachsen et al. [22] and Babanalbandi et al. [17], sometimes a degradation process with value less than 100 kJ mor is encountered. Kricheldorf et al. [38] reported that the pol3mierization of L,L-lactide most likely proceeds via the relatively reactive Sn-alkoxide group. They further pointed out that these Sn-alkoxide groups will be transformed into -CH-OH end-groups via alcoholysis or hydrolysis. Thus, as-pol5unerized PLA (PLA-ap) and PLA precipitated with methanol (PLA-pr) were prepared from polymerization products leaving an excess amount 

Sodergards et al. [20, 40] reported the stabilization of Sn-rich PLA with various types of peroxides. Since the binding energy for electrons in the Sn 3d level increased for PLA, the stabilization mechanism involves deactivation of the species that catalyze the reversible esterification reaction. 

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