Ring Repeating Unit: An Upgraded Structure Representation of Linear Condensation Polymers for Property Prediction

Unique structure representation of polymers plays a crucial
role
in developing models for polymer property prediction and polymer design
by data-centric approaches. Currently, monomer and repeating unit
(RU) approximations are widely used to represent polymer structures
for generating feature descriptors in the modeling of quantitative
structure–property relationships (QSPR). However, such conventional
structure representations may not uniquely approximate heterochain
polymers due to the diversity of monomer combinations and the potential
multi-RUs. In this study, the so-called ring repeating unit (RRU)
method that can uniquely represent polymers with a broad range of
structure diversity is proposed for the first time. As a proof of
concept, an RRU-based QSPR model was developed to predict the associated
glass transition temperature (Tg) of polyimides
(PIs) with deterministic values. Comprehensive model validations including
external, internal, and Y-random validations were
performed. Also, an RU-based QSPR model developed based on the same
large database of 1321 PIs provides nonunique prediction results,
which further prove the necessity
of RRU-based structure representation. Promising results obtained
by the application of the RRU-based model confirm that the as-developed
RRU method provides an effective representation that accurately captures
the sequence of repeat units and thus realizes reliable polymer property
prediction by data-driven approaches.