Glossary

5.1 Free radical polymerization: mechanism and kinetics

2 min read•july 23, 2024

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

The kinetics of free radical polymerization are influenced by various factors. , 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→2R∙I \rightarrow 2R \bullet)
    • Addition of free radical to monomer (M) forms an active center (R∙+M→RM∙R \bullet + M \rightarrow RM \bullet)
  • Propagation
    • Rapid sequential addition of monomer units to the active center leads to polymer chain growth
      • RM∙+M→RMM∙RM \bullet + M \rightarrow RMM \bullet
      • RMM∙+M→RMMM∙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 (RMn∙+∙MmR→RMn+mRRM_n \bullet + \bullet M_mR \rightarrow RM_{n+m}R)
    • Disproportionation involves transfer of a hydrogen atom from one active center to another (RMn∙+∙MmR→RMn+RMmRM_n \bullet + \bullet M_mR \rightarrow RM_n + RM_m)

Kinetics and Factors Affecting Free Radical Polymerization

Kinetics and rate factors

  • (RpR_p) depends on the [M] and active centers [M•] according to the equation Rp=kp[M][M∙]R_p = k_p[M][M \bullet], where kpk_p is the propagation
  • Steady-state assumption states that the rate of initiation equals the rate of termination (Ri=RtR_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 (CsC_s), which is the ratio of the rate constant for chain transfer (ktrk_{tr}) to the propagation rate constant (kpk_p). Higher CsC_s values indicate a more effective CTA.

Key Terms to Review (19)

Acrylic acid: Acrylic acid is a colorless, liquid organic compound with the chemical formula C3H4O2, which is primarily used as a key building block in the production of superabsorbent polymers and coatings. It plays a critical role in free radical polymerization, where it undergoes polymerization to form polyacrylic acid, a widely used material known for its absorbent properties. The unique reactivity of acrylic acid facilitates the creation of various polymers that have applications in numerous industries, including adhesives, paints, and textiles.
Arrhenius Equation: The Arrhenius equation is a formula that expresses the relationship between the rate of a chemical reaction and temperature, highlighting how reaction rates increase with temperature due to higher energy collisions among molecules. It shows how temperature affects polymerization processes, with the activation energy influencing the kinetics of both free radical and step-growth polymerizations. Understanding this equation helps in analyzing viscoelastic properties and the time-temperature superposition principle in polymers.
Azo compounds: Azo compounds are organic compounds characterized by the presence of a functional group that contains a nitrogen-nitrogen double bond (–N=N–), known as the azo group. These compounds are important in the field of polymer science as they can act as colorants or dyes, and can also be involved in free radical polymerization reactions where they serve as initiators or intermediates to generate radicals.
Chain reaction: A chain reaction refers to a process in which the products of a reaction initiate further reactions, leading to an exponential increase in the number of products formed. In the context of free radical polymerization, this mechanism is crucial as it allows for the rapid growth of polymer chains through a series of sequential reactions driven by free radicals.
Coatings: Coatings are protective or decorative layers applied to surfaces to enhance durability, appearance, and resistance to environmental factors. These materials can be made from various polymers and are often utilized in industrial and consumer products to provide additional functionality, such as corrosion resistance, moisture barrier properties, and aesthetic appeal. The application of coatings can significantly impact the properties of the underlying material and is closely related to both polymerization techniques and the structure of the final polymer product.
Concentration of monomer: The concentration of monomer refers to the amount of monomer present in a solution, usually measured in moles per liter (M). It plays a crucial role in the kinetics of free radical polymerization, as it influences the rate of polymerization and the properties of the resulting polymer. Higher concentrations can lead to increased reaction rates and the formation of longer polymer chains, while lower concentrations can result in shorter chains and different material characteristics.
Degree of Polymerization: Degree of polymerization (DP) refers to the number of repeating units in a polymer chain, which directly influences the molecular weight and properties of the polymer. A higher DP typically indicates longer polymer chains, which can lead to enhanced mechanical strength, viscosity, and thermal stability. Understanding DP is crucial for predicting the behavior and performance of polymers in various applications.
Free Radical Polymerization: Free radical polymerization is a widely used method for producing polymers through the reaction of monomers with free radicals, which are highly reactive species with unpaired electrons. This process involves three main steps: initiation, propagation, and termination. The versatility and efficiency of free radical polymerization make it an essential mechanism in polymer science, especially for synthesizing a variety of polymeric materials used in everyday applications.
Initiation: Initiation is the first step in the polymerization process, where reactive species are generated to start the chain reaction that leads to polymer formation. This step is crucial as it sets the stage for the subsequent propagation and termination steps, influencing the overall kinetics and characteristics of the resulting polymer. Understanding initiation helps in grasping the different mechanisms of polymerization, particularly how different initiators can affect molecular weight and distribution.
Ionic polymerization: Ionic polymerization is a type of chain-growth polymerization that involves the reaction of monomers with an ionic initiator, resulting in the formation of a polymer through the sequential addition of monomer units. This method can be classified into cationic and anionic polymerization, each driven by positively or negatively charged species, respectively. The unique feature of ionic polymerization is its ability to control molecular weight and polymer structure more effectively than some other methods, like free radical polymerization.
Peroxides: Peroxides are chemical compounds that contain an oxygen-oxygen single bond (–O–O–), and are often used as initiators in free radical polymerization processes. These compounds decompose to produce free radicals, which are crucial for initiating the polymerization of various monomers. Their ability to generate free radicals makes peroxides important for controlling the rate and efficiency of the polymerization reaction.
Plastics: Plastics are synthetic materials made from polymers, which are long chains of repeating molecular units. They are versatile and can be molded into various shapes, making them useful in countless applications, from packaging to construction. Understanding plastics involves exploring their formation through processes like free radical polymerization, which is crucial for the synthesis of many common plastic materials.
Propagation: Propagation refers to the step in the polymerization process where monomer units are added to a growing polymer chain. This process is crucial in determining the final molecular weight and structure of the polymer, as it involves the reaction of active species with monomers, leading to an increase in the length of the polymer chain. Understanding propagation is key to grasping how different polymerization mechanisms function and influence the characteristics of the resulting materials.
Rate constant: The rate constant is a proportionality factor in the rate law of a chemical reaction that relates the reaction rate to the concentrations of the reactants. It is crucial in determining how fast a reaction occurs and is influenced by factors like temperature and the nature of the reactants, making it central to understanding reaction kinetics in processes like free radical polymerization.
Rate of polymerization: The rate of polymerization refers to the speed at which monomers are converted into polymer chains during a polymerization reaction. This rate is influenced by various factors such as the concentration of reactants, temperature, and the presence of catalysts or initiators. Understanding the rate of polymerization is crucial in controlling the properties and characteristics of the resulting polymer material.
Styrene: Styrene is a colorless, volatile liquid hydrocarbon that is a key building block in the production of various polymers and copolymers. It is particularly important in the context of free radical polymerization, where it serves as a monomer that can be polymerized to create materials like polystyrene, which are widely used in packaging, insulation, and consumer goods.
Temperature: Temperature is a measure of the average kinetic energy of the particles in a substance, influencing its physical properties and behavior. In polymers, temperature plays a crucial role in determining how these materials respond to stress, their solubility, their processing conditions, and their stability over time.
Termination: Termination refers to the process by which a polymer chain ceases to grow, leading to the end of polymerization. This process can occur through various mechanisms, including combination and disproportionation, and plays a crucial role in determining the final molecular weight and properties of the polymer. Understanding termination is essential because it affects not only the efficiency of the polymerization process but also the characteristics of the resulting materials.
Termination mechanism: The termination mechanism refers to the processes by which a growing polymer chain ends its growth during free radical polymerization. This is a critical step in determining the molecular weight and structure of the resulting polymer, as it effectively stops the chain from elongating further. Understanding how these mechanisms operate helps in controlling the properties of the final polymer product, influencing aspects like stability and reactivity.
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