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Chain polymerization involving cyclic monomers

In polymer chemistry, ring opening polymerization (ROP) is a form of chain growth polymerization in which the terminus of a polymer chain attacks cyclic monomers to form a longer polymer (see figure). The reactive center can be radical, anionic or cationic. Some cyclic monomers such as norbornene or cyclooctadiene can be polymerized to high molecular weight polymers by using metal catalysts. ROP is a versatile method for the synthesis of biopolymers. Ring opening of cyclic monomers is often driven by the relief of bond angle strain. Thus, as is the case for other types of polymerization, the enthalpy change in ring opening is negative.

Cyclic monomers that are amenable to ROP include epoxides, cyclic trisiloxanes, some lactones and lactides, and amino acid N carboxyanhydrides. Many strained cycloalkenes, e. g norbornene, are suitable monomers via ring opening metathesis polymerization. Even highly strained cycloalkane rings, such as cyclopropane and cyclobutane derivatives, can undergo ROP.

Ring opening polymerization has been used since the beginning of the 1900s to produce polymers. Synthesis of polypeptides which has the oldest history of ROP, dates back to the work in 1906 by Leuchs. Subsequently, the ROP of anhydro sugars provided polysaccharides, including synthetic dextran, xanthan gum, welan gum, gellan gum, diutan gum, and pullulan. Mechanisms and thermodynamics of ring opening polymerization were established in the 1950s. The first high molecular weight polymers (Mn up to 105) with a repeating unit were prepared by ROP as early as in 1976. An industrial application is the production of nylon 6 from caprolactam.

Ring opening polymerization can proceed via radical, anionic, or cationic polymerization as described below. Additionally, radical ROP is useful in producing polymers with functional groups incorporated in the backbone chain that cannot otherwise be synthesized via conventional chain growth polymerization of vinyl monomers. For instance, radical ROP can produce polymers with ethers, esters, amides, and carbonates as functional groups along the main chain.

The monomers can be activated by Bronsted acids, carbenium ions, onium ions, and metal cations. Commercially relevant unsaturated polymers synthesized by ROMP include polynorbornene, polycyclooctene, and polycyclopentadiene.