Many polymer molecules are like "chains" where the monomer units are the links. Polymer molecules can be straight-chain, have "branching" from the main chain, or "cross-linking" between chains. As an example of cross-linking, sulfhydryl (-S-H) groups in two cysteine amino acid units in polypeptide chains can bond together to make a disulfide bridge (-S-S-) joining the chains together. 