Structural Repeating Unit (SRU) Polymers
It is possible to use the Structural Repeating Group (SRU) type to draw a polymer where the structure-based representation is known.
Each SRU S-group has two or more dedicated bonds, (called crossing bonds) which cross the brackets. The crossing bonds of an SRU show how the repeating units can connect in several ways to each other within the polymer.
Depending on the number of crossing bonds and brackets, the following polymers and connectivities can de differentiated withing the polymer:
- Polymers with two crossing bonds: If a polymer has one crossing bond on each bracket of the SRU there are three possibilities for the repeating pattern:
- Ladder-Type Polymers: Polymers with paired brackets and with two crossing bonds on each bracket are called ladder-type polymers.
It must be specified how the two crossing bonds on each bracket connect to the corresponding bonds of the adjoining repeating units.
Additionally to the head-to-tail, head-to-head connectivity information, there is flip information to specify whether the repeating unit flips around the polymer backbone when it connects to the adjoining SRU.
These types of information are handled only in case of brackets with exactly two crossing bonds on both sides (head and tail side). The following polymer connectivities can be differentiated:
- head-to-tail with no flip
- head-to-tail with flip
- head-to-head with no flip
- head-to-head with flip
- Polymers with three or more brackets: If the polymer has three or more bonds with a separated bracket on each bond, the polymer always has the either/unknown repeating pattern.
The end groups of polymers are often unknown or unspecified which are represented by star atoms (*).
The modified (mod), grafted (grf) and crosslinked (xl) form of a structural repeating unit can be drawn as well.
Draw a Ladder-type Polymer
- Draw the structure that forms the polymer.
- Click the Create Group tool on the toolbar, and select the structure. Atoms to be replaced by star atoms (*) must be left out of selection.
- In the Create Group dialog window choose the SRU polymer type from the Type list.
- Set the polymer repeat pattern if necessary.
- Click OK. The star atoms (*) are added automatically.
- To create a bracket that crosses two bonds select the two brackets each crossing a bond and click Merge Brackets in the contextual menu.