More specifically, each R2 is independently a C1-6 monovalent organic group, in particular methyl, each Ar is independently a C6-12 arylene group wherein the bonds are directly connected to an aromatic moiety, in particular phenylene, each R3 is independently a divalent C6-12 organic group, in particular an alkylenephenylene such as propylenephenylen and E is an integer having an average value of