Marvin JS does not differentiate between SRU, Repeating units, Link nodes, and Copolymers in display or creation.
However, there are restrictions to export a group as a Link node, these are as follows:
There must be one atom in the group.
The repeating pattern must not have been set (or it must be the default value).
The number of repetitions must be a range that starts at 1.
Create a Link Node
With the S-group Tool
Select the S-group tool , then select the atom you would like to change to a link node with it.
Then the S-group Dialog opens. Leave the repetition pattern on the default value and set the repetition range from the value 1 to the number you would like, then click OK.
After the dialog closes, the link node is established.
Select the atom you would like to group with one of the Selection tools then click the S-group tool.
The S-group Dialog opens. Leave the repetition pattern the default value and set the range from the value 1 to the number you would like, then click OK.
After the dialog closes, the repetition unit has been established.
Modifying a Link Node
You can change a Link node to a Repeating unit an SRU by changing the range, or the repeating pattern.
Changing the atom type, in this case, is not possible.
Deleting a Link Node
Right-click on the atom in the brackets, then select Ungroup from the menu.
When you modify the atoms or bonds in the group, it will automatically ungroup.