That the optionally substituted arylene group is linked by each of its two ends to two other optionally substituted arylene groups via a direct C-C linkage, is an essential feature of the recurring units (R) ; thus, an arylene recurring unit which is linked by at least one of its two ends to a group other than an arylene group such as phenylene recurring units ?? 1; ?? 2 and ?? 2 > below : are not