A homopolymer is derived from a single monomer species. Formally, a homopolymer is obtained by deriving a bivalent group from the monomer, constituting a structural repeat unit (SRU). For example, from the monomer molecule 3-hexylthiophene, the SRU is derived by removing the two H-atoms at position 2 and 5 of the thiophene ring. The following notation encodes the 3-hexylthiophene-derived SRU:


The ring positions 2 and 5 correspond to the node positions 1 and 11 in the string, respectively. The backbone of poly(3-hexylthiophene), PH3T, is formally constructed by connecting the C-atom at node position 1 to the C-atom at node position 11, n times:
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Poly(3-hexylthiophene) backbone
The PH3T polymer with terminating H atoms is encoded as:


The atoms belonging to the SRU are identified by back-scanning from the {+n} annotated node to the first {-} annotated node at the same depth level. If nodes that belong to the SRU are located outside this span— or, if nodes inside this span are not part of the SRU—then they need to be indicated. This is done by using the annotation dictionary keys inc and exc to include and exclude atoms. The following string, an alternative notation to the one given above, illustrates SRU inclusion and exclusion of atoms:


The value of an inc or exc key is a single integer or a comma-separated list of integers specifying atoms by their node position within the notation. In the given example, the S-atom at position 1 is included and the H-atom at position 3 is excluded.
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[1] J. Vohlídal et al.: Glossary of Class Names of Polymers Based on Chemical Structure and Molecular Architecture. Pure Appl. Chem. 2009, 81 (6) 1131-1186 (section 4.11). pdf: www.iupac.org/publications/pac/pdf/2009/pdf/8106x1131.pdf.
[2] A. Drefahl: CurlySMILES: a chemical language to customize and annotate encodings of molecular and nanodevice structures. J. Cheminf. 2011, 3:1; doi: 10.1186/1758-2946-3-1.

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