I am finding conflicting information regarding this cross-link. Some mechanisms I find online show it to be hydrogen bonding, while others show a covalent bond as a dehydration reaction.
The journal Chemical and Engineering News (C&EN) has written several articles on the popular slime experiment. See the following link for one of these articles: https://cen.acs.org/physical-chemistry/chemical-bonding/Errors-CEN-graphic-revea l-widespread/96/i28. You can search the online journal archives for additional articles. I'm not an expert in this field, but there appears to be good evidence for covalent bonding.
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