5.1 Free radical polymerization: mechanism and kinetics

Free radical polymerization is a crucial process in polymer synthesis. It involves three main steps: initiation, propagation, and termination. These steps work together to create long polymer chains from individual monomer units.

The kinetics of free radical polymerization are influenced by various factors. Temperature, initiator concentration, and monomer structure all play roles in determining the rate and outcome of the polymerization process. Chain transfer can also impact the final polymer properties.

Free Radical Polymerization Mechanism

Steps of free radical polymerization

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• Initiation
  • Decomposition of initiator (I) into free radicals (R•) occurs through homolytic cleavage ($I \rightarrow 2R \bullet$)
  • Addition of free radical to monomer (M) forms an active center ($R \bullet + M \rightarrow RM \bullet$)
• Propagation
  • Rapid sequential addition of monomer units to the active center leads to polymer chain growth
    • $RM \bullet + M \rightarrow RMM \bullet$
    • $RMM \bullet + M \rightarrow RMMM \bullet$
  • Polymer chain continues to grow until termination occurs
• Termination
  • Combination involves two active centers reacting to form a single polymer chain ($RM_n \bullet + \bullet M_mR \rightarrow RM_{n+m}R$)
  • Disproportionation involves transfer of a hydrogen atom from one active center to another ($RM_n \bullet + \bullet M_mR \rightarrow RM_n + RM_m$)

Kinetics and Factors Affecting Free Radical Polymerization

Kinetics and rate factors

• Rate of polymerization ($R_p$) depends on the concentration of monomer [M] and active centers [M•] according to the equation $R_p = k_p[M][M \bullet]$, where $k_p$ is the propagation rate constant
• Steady-state assumption states that the rate of initiation equals the rate of termination ($R_i = R_t$)

Temperature and concentration effects

• Increasing temperature increases the rate of initiator decomposition and propagation, leading to faster polymerization
• Higher initiator concentration leads to more active centers and faster polymerization
• Monomers with electron-donating substituents (vinyl acetate) increase the rate of polymerization compared to those with electron-withdrawing groups (methyl methacrylate)

Chain transfer in polymers

• Chain transfer occurs when an active center is transferred from a growing polymer chain to another molecule, such as a solvent (toluene), monomer, or a deliberately added compound (mercaptans)
• Chain transfer reduces the molecular weight of the polymer by causing shorter polymer chains due to premature termination
• Chain transfer increases the polydispersity index (PDI), resulting in a broader molecular weight distribution
• Chain transfer can introduce functional end groups on the polymer chains, depending on the nature of the chain transfer agent (CTA)
• The effectiveness of a CTA is measured by the chain transfer constant ($C_s$), which is the ratio of the rate constant for chain transfer ($k_{tr}$) to the propagation rate constant ($k_p$). Higher $C_s$ values indicate a more effective CTA.