Yttrium and Rare-earth Initiators

One of the most widely studied initiators is commercially available yttrium iso-propoxide, (291), which exists as an oxo-alkoxide cluster of the formula Yf/is 0)(//2 0 Pr)4(/r3-01Pr)4(01Pr)5, usually abbreviated to Yfm 0)(0 Pr) 3.873 The ROP of rac-LA using several Lndiis 0)(0 Pr)13 complexes (Ln = La, Sm, Y and Yb) has been reported 874 in all cases polymerization occurs at room temperature with rates La Sm Y Yb. The polymerizations 

A similar in situ approach to alkoxide formation employs the readily accessible tris(amide)-Y(NTMS2)3, (293), and PrOI 1.881 In the absence of alcohol the polymerization of CL is fast but not controlled (Mw/Mn 3). However, upon addition of alcohol, a controlled living system with polydispersities 1.1-1.2 results. At least 50 equivalents of PrOH may be added (the excess effects rapid chain transfer) with molecular weights in good agreement with theoretical values. Similar results have been reported using Nd(NTMS2)3, (294), and PrOH.882 The reactions of PrOH with (292) and (293) have both been studied by NMR, and in both cases Y5(/r5-0)(0 Pr)13 is not 

A series of structurally characterized di-yttrium(III) complexes bearing alkoxy-derivatized triazacyclononane ligands have been examined as initiators for lactone ROP.886 Both (296) and (297) are active for the polymerization of rac-LA at RT, but little control is afforded over molecular weights. Chain length distributions are broad, (Mw/Mn= 1.5-2.2) and attempts to identify the initiating group via end-group analysis have not been successful. 

Improved control was observed, however, upon addition of benzyl alcohol to the dinuclear complexes.887 X-ray crystallography revealed that whereas (296) simply binds the alcohol, (297) reacts to form a trinuclear species bearing four terminal alkoxides. The resultant cluster, (298), polymerizes rac-LA in a relatively controlled manner (Mw/Mn=1.15) up to 70% conversion thereafter GPC traces become bimodal as transesterification becomes increasingly prevalent. NMR spectroscopy demonstrates that the PLA bears BnO end-groups and the number of active sites was determined to be 2.5 0.2. When CL is initiated by (298) only 1.5 alkoxides are active and kinetic analysis suggests that the propagation mechanisms for the two monomers are different, the rate law being first order in LA, but zero order in CL. 

As described previously for aluminum, in order to obviate the kinetic complexity arising from aggregation, several groups have examined potentially less complicated single-site monoalkoxides. For example, complexes (299)-(301) are active for the ROP of CL, 6-VL and even /3-PL at 0 °c.888 Polydispersities are low ( 1.10) even up to 90% conversion and Mn increases linearly with conversion for CL, although initiator efficiencies are typically 50-60%. Lanthanocene alkyls, such as (302) and (303), and hydrides, (304), exhibit almost identical reactivity for the polymerization of CL and 5-VL to the alkoxides (299)-(301) (although no activity for /3-PL was observed). 

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