Modified from the earlier definition.
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.
- 1 Monomers
- 2 History
- 3 Mechanisms
- 4 Copolymerization
- 5 Thermodynamics
- 6 See also
- 7 Additional reading
- 8 References
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:
- compounds containing heteroatoms in the ring:
- oxygen: ethers, acetals, esters (lactones, lactides, and carbonates), and anhydrides,
- sulfur: polysulfur, sulfides and polysulfides,
- nitrogen: amines, amides (lactames), imides, N-carboxyanhydrides and 1,3-oxaza derivatives,
- phosphorus: phosphates, phosphonates, phosphites, phosphines and phosphazenes,
- silicon: siloxanes, silathers, carbosilanes and silanes.
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 catalystsand 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).
Radical ring-opening polymerization
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.
Examples of monomers that undergo radical ROP include vinyl substituted cyclic monomers, methylene substituted cyclic monomers, bicyclobutanes, spiro monomers (which undergo double ring-opening). Degradable polyester can be synthesized via radical ring-opening homo- and copolymerization.
In free radical ROP, the cyclic structure will undergo homolytic dissociation rather than undergoing heterolytic dissociation (as is the case for any ionic ROP). There are two typical mechanistic schemes in radical ROP.
Scheme 1: The terminal vinyl group accepts a radical. The radical will be transformed into a carbon radical stabilized by functional groups (i.e. halogen, aromatic, or ester groups). This will lead to the generation of an internal olefin.
Anionic ring-opening polymerization
Anionic ring-opening polymerizations (AROP) are involve nucleophilic reagents as initiators. Monomers with a three-member ring structure - such as epoxide, aziridine, and episulfide - undergo anionic ROP.
Common nucleophilic reagents used for the initiation of AROP usually will include organometals (e.g. alkyl lithium, alkyl magnesium bromide, alkyl aluminum, etc.), metal amides, alkoxides, phosphines, amines, alcohols and water. The monomers that undergo AROP will contain polarized bonds (ester carbonate, amide, urethane, and phosphate), which respectively leads to the production of the corresponding polyester, polycarbonate, polyamide, polyurethane and polyphosphate.
The general mechanism of propagation for anionic ROP relies on the nucleophilic attack of a propagating chain end to a monomer.
Transfer and termination
Termination in AROP can be described as chain transfer reactions to monomer that is available. The active centers of AROP monomers are nucleophilic and also act as bases to abstract protons from the monomer, initiating new chains. Thus, AROP often results in low molecular-weight polymers. A possible method to increase the molecular mass of the polymer products is by adding crown ethers as complexing agents for counter-ions in the polymerization system. This causes the free-ions to preferentially add to monomer rather than abstract protons.
Cationic ring-opening polymerization
Cationic ring-opening polymerization (CROP) is characterized by having a cationic initiator and intermediate. Examples of cyclic monomers that polymerize through this mechanism include lactones, lactams, amines, and ethers. CROP proceeds through an SN1 or SN2 propagation, chain-growth process. The predominance of one mechanism over the other depends on the stability of the resulting cationic species. For example, if the atom bearing the positive charge is stabilized by electron-donating groups, polymerization will proceed by the SN1 mechanism. The cationic species is an heteroatom and the chain grows by the addition of cyclic monomers thereby opening the ring system.
Not all cyclic monomers containing an heteroatom undergo CROP. Ring size influences whether the cyclic monomer polymerize through this mechanism. For example, 4, 6 and 7-membered rings of cyclic esters polymerize through CROP. When considering the ring size of the monomer, the reactivity toward polymerization is dictated by the ability to release the ring strain. Therefore, cyclic monomers with small or lacking ring strain will not polymerize.
In CROP, three mechanisms are distinguished by the propagating species.
- When the cationic species is a secondary ion, polymerization proceeds by ring expansion. This mechanism is observed when the monomer is in low concentration.
- When it is a tertiary ion, polymerization proceeds by linear growth.
- The monomer can likewise be activated (i.e. cationic) and the propagation step will proceed via electrophilic addition of the activated monomer to the growing chain.
CROP can be considered as a living polymerization and can be terminated by intentionally adding termination reagents such as phenoxy anions, phosphines or polyanions. When the amount of monomers becomes depleted, termination can occur intra or intermolecularly. The active end can "backbite" the chain, forming a macrocycle. Alkyl chain transfer is also possible, where the active end is quenched by transferring an alkyl chain to another polymer.
Ring-opening metathesis polymerization
The mechanism for ROMP follows similar pathways as olefin metathesis. The initiation process involves the coordination of the cycloalkene monomer to the metal alkylidene complex, followed by a [2+2] type cycloaddition to form the metallacyclobutane intermediate that cycloreverts to form a new alkylidene species.
Copolymerization is the process of combining two polymers that are different. This is an industrial process that creates a substance that has long chains of molecules. In terms of ROP, the stoichiometric equation for copolymerization includes two or more of comonomers.
The following figure shows an example of such a copolymerization. By varying the ratio of monomers and the mode of initiation, many and varied polymers can be obtained, optimized for their use in agricultural, medicinal or pharmaceutical fields.
As an example of copolymerization of non-homopolymerizable monomers, γ-butyrolactone (BL) and ε-caprolactone (CL) show that the copolymerization provides high molar mass polymers: The BL/CL copolymer synthesis is viable despite the fact that BL monomer addition to its own –bl* active chain ends was highly reversible, as the –bl* unit could be blocked via a practically irreversible CL addition.
Similarly, the earlier studies of S8 copolymerization with thiiranes (propylene sulfide; PS), at temperatures below Tf for S8 homopolymerization, revealed that the average sulfur rank in the copolymer increased from 1 to 7 when 8[S8]0/[PS]0 ratio was increasing from 0 to 10.
where x and y indicate monomer and polymer states, respectively (x and/or y = l (liquid), g (gaseous), c (amorphous solid), c’ (crystalline solid), s (solution)), ΔHp(xy) and ΔSp(xy) are the corresponding enthalpy (SI unit: joule per kelvin) and entropy (SI unit: joule) of polymerization, and T is the absolute temperature (SI unit: kelvin). The free enthalpy of polymerization (ΔGp) may be expressed as a sum of standard enthalpy of polymerization (ΔGp°) and a term related to instantaneous monomer molecules and growing macromolecules concentrations:
where R is the gas constant, M is the monomer, (m)i is the monomer in an initial state, and m* is the active monomer. Following Flory–Huggins solution theory that the reactivity of an active center, located at a macromolecule of a sufficiently long macromolecular chain, does not depend on its degree of polymerization (DPi), and taking in to account that ΔGp° = ΔHp° - TΔSp° (where ΔHp° and ΔSp° indicate a standard polymerization enthalpy and entropy, respectively), we obtain:
At equilibrium (ΔGp = 0), when polymerization is complete the monomer concentration ([M]eq) assumes a value determined by standard polymerization parameters (ΔHp° and ΔSp°) and polymerization temperature:
Polymerzation is possible only when [M]0 > [M]eq. Eventually, at or above the so-called ceiling temperature (Tc), at which [M]eq = [M]0, formation of the high polymer does not occur.
For example, tetrahydrofuran (THF) cannot be polymerized above Tc = 84 °C, nor cyclo-octasulfur (S8) below Tf = 159 °C. However, for many monomers, Tc and Tf, for polymerization in the bulk, are well above or below the operable polymerization temperatures, respectively. The polymerization of a majority of monomers is accompanied by an entropy decrease, due mostly to the loss in the translational degrees of freedom. In this situation, polymerization is thermodynamically allowed only when the enthalpic contribution into ΔGp prevails (thus, when ΔHp° < 0 and ΔSp° < 0, the inequality |ΔHp| > -TΔSp is required). Therefore, the higher the ring strain, the lower the resulting monomer concentration at equilibrium.
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