Free radicals are very reactive species which are usually only formed when extra energy, often in the form of ultra-violet light, is applied to the reaction. However, a free radical initiator has a very weak bond which can easily come apart, even using the energy from ordinary temperatures. That is, they fall apart at room temperature or slightly hotter. This produces a pair of free radicals. A free radical will attach itself to one end of the double bond by sharing its unpaired electron with one of the double bond electrons. This creates a free radical at the other end of the double bond. This free radical will seek out another double bond, and consequently extend the chain. This process continues, with the polymer chain getting longer and longer, until the extending end of the chain is capped off by another free radical. Hence, a couple of free radicals from the initiator can cause several thousand monomer molecules to join together. Organic peroxides, with the weak -O-O- linkage, are frequently used as free radical initiators: