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| The functionality of a Mer refers to the number of covalent bonds that may bond with other mers. In order to form a long chain molecule a functionality of two is necessary, an example being the ethylene mer of polyethylene. Chains of this type can be terminated by monofunctional groups such as -OH. To form networks a functionality of at least three is required. Phenol-formaldehyde is an example of a tri-functional mer and forms the network polymer Bakelite. Carbon in the (sp)3 electronic configuration is an example of a tetra-functional 'mer' that forms the crystalline material, diamond. | ||||||