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The general mechanism for anionic ring-opening polymerization. Polarized functional group is represented by X-Y, where the atom X (usually a carbon atom) becomes electron deficient due to the highly electron-withdrawing nature of Y (usually an oxygen, nitrogen, sulfur, etc.). The nucleophile will attack atom X, thus releasing Y-. The newly formed nucleophile will then attack the atom X in another monomer molecule, and the sequence would repeat until the polymer is formed.
Cationic initiators and intermediates characterize cationic ring-opening polymerization (CROP). 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 mechanism is affected by 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 a heteroatom and the chain grows by the addition of cyclic monomers thereby opening the ring system.
CROP can be a living polymerization and can be terminated by nucleophilic 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.
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.
General scheme of the mechanism for ROMP.
Commercially relevant unsaturated polymers synthesized by ROMP include Norsorex (polynorbornene), Vestenamer (polycyclooctene), and Metton (polycyclopentadiene).
The formal thermodynamic criterion of a given monomer polymerizability is related to a sign of the free enthalpy (Gibbs free energy) of polymerization:
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)), ?Hp(xy) and ?Sp(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 (?Gp) may be expressed as a sum of standard enthalpy of polymerization (?Gp°) 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 ?Gp° = ?Hp° - T?Sp° (where ?Hp° and ?Sp° indicate a standard polymerization enthalpy and entropy, respectively), we obtain:
At equilibrium (?Gp = 0), when polymerization is complete the monomer concentration ([M]eq) assumes a value determined by standard polymerization parameters (?Hp° and ?Sp°) 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 ?Gp prevails (thus, when ?Hp° < 0 and ?Sp° < 0, the inequality |?Hp| > -T?Sp is required). Therefore, the higher the ring strain, the lower the resulting monomer concentration at equilibrium.
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