A polymer as claimed in claim 1, wherein: R1 is identical or different on each occurrence and is in each case a linear or branched alkyl or alkoxy chain having from 1 to 8 carbon atoms, or is an aryl group having from 6 to 10 carbon atoms, which are also substituted by one or more nonaromatic radicals R1; n are identical or different and are each 1 or 2. 15.