Tin octanoate

Much effort has been devoted to the optimization of the polyesterification reaction. For instance, different types of monomeric precursors structurally related to succinic acid (e.g., dimethyl succinate or succinic anhydride) were used. Different kinds of catalysts (e.g., phenolates, titanium alkoxides, tin octanoates) at different concentrations were studied. Different reaction temperatures (130-190 °C) were reached and different procedures for water elimination (vacuum drying under different conditions or toluene distillation) were adopted. Experimental results obtained showed that the use of different catalysts and different monomer precursors (succinic acid derivatives) did not significantly alter the polymerization kinetics or yield, and for this reason, they were abandoned. The procedure finally adopted is summarized below. 

A combination of two different supramolecular forces, namely metal-metal bonds and the ureidopyrimidones were investigated by Schubert et al. [136,137] (Fig. 35). A poly(g-caprolactame) was prepared—with a ter-pyridine moiety at one end and an ureidopyrimidone unit at the other end—via tin-octanoate catalyzed polymerization. Together with iron(ll) ions, double supramolecular polymers formed from chloroform solution. Again, solid-like behavior was observed, similar to results for the purely hydrogen-bonded polymers. 

Reactive sUicone potymers-silanol terminated [Andisil OH 40, OH 75, and OH 2 (Anderson Assoc.)] Epoxy silane cross-linker [KIO (Anderson Assoc.)] Dihutyl tin dilaurate [PC-055 (United Chemical Tech.)], and Andisil TL 10 (Anderson Assoc.) zinc octanoate (PC 040) and tin octanoate [PC 050 (United Chentical Tech.)]... 

Meso-lactide can be obtained from racemic PLA by depolymerization at 180-200°C under reduced pressure and distilling off the meso-lactide (11). Also, lactic acid can be used directly in the presence of catalyst such as tin octanoate, and lithium carbonate (12). 

Recently, Vuorinen et al. have used simple bismuth alkoxides, Bi(OR)j (R Tr, Bu, and CMe Tr), in LA ROP [27]. Their results showed these alkoxides had acceptable activity (Table 7.2, entry 60-62) except Bi(OTr)j which made aggregate and decreased solubility. They also showed that the reactivity of bismuth t-butoxide was higher than 2,3-dimethyl-2-butoxide and both had higher activity than tin octanoate. In addition, their kinetic studies revealed the linear relationship between monomer/catalyst ratio and polymer molecular weight through a classical coordination-insertion mechanism. According to their report, Bi(0 Bu)j also showed good reactivity in CL ROP reaction. 

Poly(e-caprolactone) (3) (23) is synthesized by ring-opening polymerization of e-caprolactone using tin-octanoate as a catalyst (24). It is used, for example, as blown film for compost bags or in materials where it serves as a matrix for the controlled release of pesticides, herbicides, and fertilizers. The low melting point of poly(e-caprolactone) (60°C) (Table 1) limits applications, but it has good mechanical properties and is more hydrophobic than—and compatible with—many biopolymers. It is therefore widely used to modify the properties of other degradable plastics in blends (see below). 

Although strong Lewis acid metal halogenides are known catalysts for cationic ROP, zinc dichloride can initiate ROP of -CL, according to a coordination-insertion mechanism in which the alkyl-oxygen bond of the monomer is cleaved (Fig. 13) (72). The mechanism was assessed by computational calculations, analysis of the two chain-ends by NMR, and agreement between A/ and the monomer/zinc dichloride molar ratio. Zinc carboxylates are ROP promoters in the presence of alcohols (71,73). Similarly to tin octanoate, zinc alkoxide is formed, which is the real initiator (72,74). Side formation of octanoate ester end groups is kinetically less favorable compared with the tin counterpart (73). 

In most cases, through ROP, an OH-bearing specie is used as the ROP cocatalyst, and a cyclic ester (eg, lactide, glycolide, e-caprolactone) is used as a repeating unit for the synthesis of polyesters. The process can be catalyzed by different compounds, among which is tin octanoate, that posses the approval of the Food and Drug Administration (FDA) for biomedical applications. (Fig. 12.2). 

Catalyst Systems A vast number of catalysts have been utilized in the ROP of lactide, of which the most studied are the carboxylates and alkoxides of Sn [111-120] and A1 [121-127]. Of these, stannous 2-ethylhexanoate (tin octanoate) is the most intensively studied. The polymerization mechanism is suggested to involve a preinitiation step, in which stannous 2-ethylhexanoate is converted to a stannous alkoxide by reaction with a hydroxyl-bearing compound. Then, the polymerization proceeds on the tin-oxygen bond of the alkoxide ligand, whereas the carboxylate itself is... 

Catal t to promote reaction of cone with silica (e.g., tin octanoate)... 

As shown in Scheme 8, condensation can take place by the reaction of terminal OH groups of poly(dimethylsiloxane) with tri-21 or tetraalkoxy-22 silane monomers or with polyalkoxysilanes in the presence of a catalyst such as dibutyltin dilaurate or tin octanoate, which are colorless and pourable. The products from these reactions are white or beige, room temperature-vulcanized two-component system (RTV-2) silicone rubbers. Polyalkoxy silane undergoes similar reaction with poly(dimethylsiloxane) to give RTV-2 silicone rubber. The reaction is isothermal and a volatile alcohol forms from the corresponding alkoxy groups. 

Figure 10.1 Structure of tin octanoate, aluminum isopropoxide and zinc lactate.

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