What is "polyorthoester"

Polyorthoesters are polymers with the general structure –[–R–O–C(R, OR)–O–R–]– whereas the residue R can also be part of a heterocyclic ring with the residue R. Polyorthoesters are formed by transesterification of orthoesters with diols or by polyaddition between a diol and a diketene acetal, such as 3,9-diethylidene-2,4,8,10-tetraoxaspiro[5.5]undecane.