Ring-opening polymerization

Note:If the monomer is polycyclic, the opening of a single ring is sufficient to classify the reaction as ring-opening polymerization.Penczek S.; Moad, G. Pure Appl. Chem., 2008, 80(10), 2163-2193In polymer chemistry, ring-opening polymerization (ROP) is a form of chain-growth polymerization, in which the terminal end of a polymer chain acts as a reactive center where further cyclic monomers can react by opening its ring system and form a longer polymer chain (see figure). The propagating 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 continues to be the most versatile method of synthesis of major groups of biopolymers, particularly when they are required in quantity. In polymer chemistry, ring-opening polymerization (ROP) is a form of chain-growth polymerization, in which the terminal end of a polymer chain acts as a reactive center where further cyclic monomers can react by opening its ring system and form a longer polymer chain (see figure). The propagating 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 continues to be the most versatile method of synthesis of major groups of biopolymers, particularly when they are required in quantity. The driving force for the ring-opening of cyclic monomers is via the relief of bond-angle strain or steric repulsions between atoms at the center of a ring. Thus, as is the case for other types of polymerization, the enthalpy change in ring-opening is negative. ROP can introduce functional groups such as ether, ester, amide, and carbonate into the polymer main chain, which cannot be achieved by vinyl polymerization affording polymers only with C-C main chain. Polymers obtained by ROP can be also prepared by polycondensation in most cases, but following controlled radical polymerization is possible in ROP, which is difficult in polycondensation. Cyclic monomers that are polymerized using ROP encompass a variety of functional groups, such as: 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. Ring-opening polymerization can proceed via radical, anionic or cationic polymerization as described below. ROP can involve metal catalysts and is best exemplified by the polymerization of olefins while maintaining unsaturation in the resulting polymer. This mechanism is known as ring-opening metathesis polymerization (ROMP). With radical ring-opening polymerization, it is possible to produce polymers of the same or lower density than the monomers. This is important for applications that require constant volume after polymerization, such as tooth fillings, coatings, and the molding of electrical and electronic components. 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. Free radical polymerization allows control radical of molecular weight. Reversible Addition Fragmentation Transfer (RAFT) has been applied to radical ROP of a cyclopropane monomer. For instance, the RAFT polymerization of the cyclic monomer to synthesize polymers with anthracene along the backbone chain has been demonstrated.