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25.19: Polymerization - Addition Polymers

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  • We enjoy the benefits of Styrofoam containers, but don't often think about where they end up. Styrofoam materials do not break down quickly under exposure to the elements. When buried in a landfill, styrofoam will remain intact for a long time. The good news is that there is not a lot of this pollutant found in landfills (maybe about \(0.5\%\) by weight of the total mass of garbage). There is no good way to recycle Styrofoam at present, but someday a creative scientist may come up with one.

    Polymerization - Addition Polymers

    Polymers are very different from the other kinds of organic molecules that you have seen so far. Whereas other compounds are of relatively low molar mass, polymers are giant molecules of very high molar mass. Polymers are the primary components of all sorts of plastics and related compounds. A polymer is a large molecule formed of many smaller molecules covalently bonded in a repeating pattern. The small molecules which make up the polymer are called monomers. Polymers generally form either from an addition reaction or a condensation reaction.

    Addition Polymers

    An addition polymer is a polymer formed by chain addition reactions between monomers that contain a double bond. Molecules of ethene can polymerize with each other under the right conditions to form the polymer called polyethylene.

    \[n \ce{CH_2=CH_2} \rightarrow \ce{-(CH_2CH_2)}_n-\]

    The letter \(n\) stands for the number of monomers that are joined in repeated fashion to make the polymer and can have a value in the hundreds or even thousands.

    Mechanism of the synthesis of polyethylene
    Figure \(\PageIndex{1}\): Polyethylene synthesis.

    The reactions above show the basic steps to form an addition polymer.

    1. Initiation - a free radical initiator \(\left( \ce{X}^* \right)\) attacks the carbon-carbon double bond (first step above). The initiator can be something like hydrogen peroxide. This material can easily split to form two species with a free electron attached to each: \(\ce{H-O-O-H} \rightarrow 2 \ce{H-O} \cdot\). This free radical attacks a carbon-carbon double bond. One of the pi electrons forms a single bond with the initiator while the other pi electron forms a new free radical on the carbon atom.
    2. Propagation - the new free radical compound interacts with another alkane, continuing the process of chain growth (second step above).
    3. Termination occurs whenever two free radicals come in contact with one another (not shown). The two free electrons form a covalent bond and the free radical on each molecule no longer exists.

    Polyethylene can have different properties depending on the length of the polymer chains and on how efficiently they pack together. Some common products made from different forms of polyethylene include plastic bottles, plastic bags, and harder plastic objects such as milk crates.

    Several other kinds of unsaturated monomers can be polymerized and are components in common household products. Polypropylene is stiffer than polyethylene and is in plastic utensils and some other types of containers.

    Structure of polypropylene
    Figure \(\PageIndex{2}\): Polypropylene structure.

    Polystyrene is used in insulation and in molded items such as coffee cups.

    Net reaction for the synthesis of polystyrene
    Figure \(\PageIndex{3}\): Polystyrene synthesis and structure.

    Polyvinyl chloride (PVC) is extensively used for plumbing pipes.

    Net reaction for the synthesis of polyvinyl chloride(PVC)
    Figure \(\PageIndex{4}\): Polyvinyl chloride.

    Polyisoprene is a polymer of isoprene and is better known as rubber. It is produced naturally by rubber trees, but several variants have been developed which demonstrate improvements on the properties of natural rubber.

    Polyisoprene is better known as rubber
    Figure \(\PageIndex{5}\): Polyisoprene.

    Contributors and Attributions

    • CK-12 Foundation by Sharon Bewick, Richard Parsons, Therese Forsythe, Shonna Robinson, and Jean Dupon.