Primary valence bonds connect the atoms in the chain of a macromolecule while secondary valences bonds act between macromolecules and hold them together. Primary valence bonds, also called covalent bonds, are the chemical bonds, while secondary valence bonds are physical in nature. The bond energy is defined as the energy difference between the bonded state of two atoms and that of the same atoms in an unbounded state i.e. When they are infinitely far apart from each other. Primary valence bonds are the chemical bonds that connect the monomer units within a macromolecule or polymer. They can also connect one macromolecule to another through cross-linking. Many monomers, e.g. Styrene, contain double bonds that activate when either energy is supplied, such as heat, or by the split addition of reactive species, such as initiators or catalysts, that can be split and become reactive. The reactive fragments, radical or ions can combine with additional monomers to form macromolecules or polymers. This process is called polymerization. The composition of the macromolecule is identical or very similar to that of the monomer from which it is polymerized. During the reaction of the monomer molecules with each other, low molecular by-products can be split off, as in a poly condensation, or individual atoms can be transferred, as in poly addition resulting in a polymer with a slightly different composition form the monomer.