preferred constitutional repeating unit

initialism: P-CRU
https://doi.org/10.1351/goldbook.09016
Constitutional repeating unit (CRU) written with the subunits in a linear series from the highest (higher) seniority to the lowest (lower) or as close as possible by the shortest path.
Notes:
  1. This definition is based on descriptions of methods to reach the P-CRU.
  2. The P-CRU is chosen with the lowest possible locant(s) for the substituents.
Examples:
  1. \(\ce{-[O-CH2CH2-S-CH2CH2]_{n}\bond{-}}\ \rm{not}\ \ce{-[S-CH2CH2-O-CH2CH2]_{n}\bond{-}}\)
  2. \(\ce{-[O-CH2CH2-O-CH2CH2CH2]_{n}\bond{-}}\ \rm{not}\ \ce{-[O-CH2CH2CH2-O-CH2CH2]_{n}\bond{-}}\)
Source:
PAC, 2021, 93, 963. 'Structure-based nomenclature for irregular linear, star, comb, and brush polymers (IUPAC Recommendations 2020)' on page 966 (https://doi.org/10.1515/pac-2020-0103)