Table 2.

Chemical approaches for the preparation of hydrogels with relevant references.

	Type of Reaction	Functional Group or Materials	Refs.
Covalent Chemistry	Radical polymerization	Vinyl monomers	[85,86,87,88,89,90,98,99]
	Radical copolymerization	Vinyl monomers	[91,92,93,94,101]
	Esterification 1	Carboxylic acid–alcohol	[95]
	Amidation 1	Carboxylic acid–amine	[96,97,106,110]
	Imine condensation 1	Imine–aldehydes	[102,106]
	Ketal formation 1	Diol–ketone	[107]
	Boron esterification	Boronic acid–diol	[98]
	Thiol-yne	Thiol–terminal alkyne	[111]
	Diels–Alder	Furan–maleimide	[112]
Non-covalent Chemistry	Metal coordination	Carboxylic acid–metal	[114,133,134,135]
		Sodium borate–diol	[115]
	Self-assembly/crystallization	Small molecule	[116,117,118]
		Polymers	[119]
Miscellaneous	Additive processing	Synthetic dye	[123]
		CaCO3 crystal	[124]
	Molecular imprinting	Acrylamides	[129]
	Multilayered structure	PAA	[130]
	Electrospinning	PAA	[131]
	Hydrothermal method	Graphene	[132,136,137]

¹ Including post-polymerization modification.