English to Japanese Science Chemistry; Chem Sci/Eng

...substitujted oxetane-2,2-diyl, oxetane-3,3-diyl, oxirane-2,2-diyl, cyclopropane-1,1-diyl, cyclobutane-1,1-diyl, cyclopentane-1,1-diyl, cyclohexane-1,1-diyl, 4,5-
dihydroisoxazole-5,5-diyl, where X-6 contains at most one spiro group, or in each case two adjacent radicals Z16, Z17orZ17, Z18 or Z18, Z19 or Z19, Z20 or Z20, Z21 preferably present -CR13=CR14-, -CR13=N-, -N=CR13-, where X-6 may not contain more than 2 of such groups, which for their part must not be cumulated,....

上記の例文の最後の箇所にあります｢which for their part must not be cumulated｣をどう訳せばいいかわかりません。　よろしくお願いします。