Scandium and Yttrium Phosphasalen Complexes as Initiators for Ring-Opening Polymerization of Cyclic Esters

The synthesis and
characterization of novel scandium and yttrium
phosphasalen complexes is reported, where phosphasalen refers to two
different bis­(iminophosphorane) derivatives of the more ubiquitous
salen ligands. The activity of the complexes as initiators for the
ring-opening polymerization of cyclic esters is presented. The scandium
complexes are inactive for lactide polymerization but slow and controlled
initiators for ε-caprolactone polymerization. The lack of activity
toward lactide exhibited by these compounds is probed, and a rare
example of single-monomer insertion product, unable to undergo further
reactions with lactide, is identified. In contrast, the analogous
yttrium phosphasalen complex is a very active initiator for the ring-opening
polymerization of rac-lactide (kobs = 1.5 × 10–3 s–1 at 1:500 [yttrium initiator]:[rac-lactide], 1 M
overall concentration of lactide in THF at 298 K). In addition to
being a very fast initiator, the yttrium complex also maintains excellent
levels of polymerization control and a high degree of isoselectivity,
with the probability of isotactic enchainment being Pi = 0.78 at 298 K.