Цепная Реакция

What is Free Radical Chain Reaction Polymerization?

Free radical chain reaction polymerization is a chain-growth polymerization mechanism where reactive intermediates (free radicals) are generated, which then add monomer units in a chain reaction until termination occurs. The process consists of three main steps: initiation (generation of free radicals from initiator), propagation (sequential addition of monomer to the growing chain), and termination (combination or disproportionation of two radicals). This mechanism is responsible for producing many common polymers including polyethylene, polystyrene, PVC, and PMMA. The kinetic chain length and molecular weight distribution depend on the relative rates of propagation and termination.

Chain Reaction Mechanism

Initiation Step: The initiator I decomposes to form two primary radicals R· with rate constant k_d. The rate of radical generation is R_i = 2f·k_d[I], where f is the initiator efficiency (fraction of radicals that successfully initiate chains).
Propagation Step: A radical adds to a monomer M, forming a new radical at the chain end. This repeats rapidly with rate R_p = k_p[M][R·], where [R·] is the total radical concentration. Each propagation event adds one monomer unit and extends the chain by one repeat unit.
Termination Step: Two radicals combine (combination) or disproportionate, with rate R_t = 2k_t[R·]². Termination is bimolecular and second-order in radical concentration.
Steady-State Approximation: Assuming d[R·]/dt ≈ 0, [R·] = (f·k_d[I]/k_t)^0.5, giving R_p = k_p(f·k_d/k_t)^0.5[M][I]^0.5.

Polymerization Kinetics

Rate Equation: Under steady-state approximation, R_p = k_p(f·k_d/k_t)^0.5[M][I]^0.5. The rate is first-order in monomer and half-order in initiator.
Monomer Conversion: [M] = [M]₀·exp(-k_p[R·]t). Conversion X = 1 - exp(-k_p[R·]t). As initiator depletes, [R·] decreases and the rate slows.
Kinetic Chain Length: ν = R_p/R_i = k_p[M]/(2f·k_d[I])^0.5 under steady-state. This represents the average number of monomer units added per initiating radical.
Degree of Polymerization: For combination termination, X_n = 2ν. For disproportionation, X_n = ν. The number-average molecular weight M_n = X_n × M₀ (monomer molecular weight).

Molecular Weight Distribution

Polydispersity: The most probable distribution for chain-growth polymerization gives polydispersity index PDI = M_w/M_n = 1.5 for disproportionation termination and PDI = 2.0 for combination termination.
Distribution Shape: Low termination rates (long chains) give narrower distributions at high conversion. High termination rates give broader distributions.
Effect of Conversion: As conversion increases, monomer depletion and initiator consumption can broaden the distribution. The Trommsdorff effect (autoacceleration) at high conversion can complicate the kinetics.
Practical Control: Molecular weight is controlled by adjusting initiator concentration, temperature (affects all k values), and using chain transfer agents to limit chain length.

Industrial Applications

Polyethylene (PE): Produced by free radical polymerization of ethylene at high pressure (2000-3000 atm) and temperature (100-300°C). Used in packaging, containers, toys.
Polystyrene (PS): Styrene polymerization initiated by peroxides or AIBN. Used in foam packaging, disposable cups, insulation.
Polyvinyl Chloride (PVC): Suspension or emulsion polymerization of vinyl chloride. Used in pipes, window frames, flooring.
Poly(methyl methacrylate) (PMMA): Known as acrylic glass or Plexiglas. Used in optical lenses, displays, coatings.
Acrylonitrile-Butadiene-Styrene (ABS): Copolymer produced by free radical mechanism. Used in 3D printing filament, automotive parts.

Factors Affecting Polymerization

Initiator Concentration: Higher [I] increases rate but decreases molecular weight (ν ∝ [I]^(-0.5)). Lower initiator gives longer chains.
Monomer Concentration: Higher [M] increases both rate and molecular weight (ν ∝ [M]). Purer monomer gives better results.
Temperature: Increases all rate constants via Arrhenius equation. Higher T increases rate but typically decreases molecular weight (activation energies: E_d > E_p > 0.5E_t).
Chain Transfer: Chain transfer agents (e.g., mercaptans) intentionally terminate growing chains and start new ones, controlling molecular weight and narrowing distribution.
Inhibition: Inhibitors like oxygen or hydroquinone scavenge radicals and must be removed for controlled polymerization.