Cross-linked polymers are characterized by what kind of bonding between chains?

• A. Adjacent chains joined by ionic bonds in a reversible process
• B. Cross-links formed by van der Waals interactions
• C. Adjacent linear chains are joined one to another by covalent bonds
• D. Cross-links formed by physical entanglements that can be undone by solvents

Answer: (C)

Cross-linked polymers form a permanent three-dimensional network because adjacent polymer chains are joined by covalent bonds. Those covalent linkages lock the chains together at many points, greatly restricting their mobility and creating a rigid, thermally stable structure. This is what distinguishes cross-linked networks from simple entanglements or weaker interactions: covalent bonds hold the network in place and resist dissolution or flow.

If the links were just van der Waals interactions, or if the material relied on physical entanglements, the connections would be much weaker and could be undone by heat, solvent, or enough stress. Ionic bonds could also connect chains, but the hallmark, in the usual context of cross-linked polymers, is the formation of covalent bonds that knit the chains into a permanent network.