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January 09, 2025 | UR Gate

Addition polymerization of Styrene by Chain Reaction with Experimental Procedure


Dive into addition polymerization of styrene, its mechanisms, and polystyrene applications in this comprehensive scientific overview.

Introduction:

Chain degradation: It is the reaction or linkage of multiple monomer molecules without the formation of any secondary by-products.

This process is specific to compounds containing double or triple bonds between their atoms, such as ethylene (CH₂=CH₂) and acetylene (ethyne) (CH≡CH). It is also relevant to compounds containing unstable cyclic structures with oxidizing elements.


The most distinctive feature of this type of polymerization is that the initiating bond drives the growth of the polymerized molecular chain. The stages involved in the chain polymerization process are:



The initiation process is very important. Chain polymerization that occurs through free radicals (Free Radical) is called radical polymerization, while polymerization that occurs through positive and negative ions is called anionic polymerization (Anionic) and cationic polymerization (Cationic), respectively.

To activate the monomer, a certain amount of energy must be provided. The initiation process can occur either through physical methods, such as using heat, light, or ionizing radiation like alpha, beta, or gamma rays, or through chemical methods by adding substances that help generate free radicals upon decomposition. These substances are called initiators.

One of the most common types of initiators is thermal initiators, with benzoyl peroxide being the most widely used thermal initiator. It decomposes at a temperature of 80°C to produce free radicals. Other types of initiators also exist, such as AIBN (α,α′-Azoisobutyronitrile).


The initiator creates active centers necessary for the initiation process.

Note:
Benzoyl Peroxide is considered one of the most important initiators.

Note:
The monomer must be purified before starting polymer preparation to remove additives added to the monomer that prevent polymerization, known as inhibitors. These substances are added to the monomer when it needs to be stored for an extended period. Examples of polymerization inhibitors include hydroquinone and trinitrobenzene.



Procedure:

  1. Place 4 g of styrene monomer in a test tube. Similarly, place another 4 g of styrene monomer in a separate test tube.
  2. Heat both test tubes in a water bath at 80°C, then add 0.1 g of benzoyl peroxide to each test tube. Continue heating them in the water bath. After 30 minutes, you will notice that the solution becomes more viscous due to the polymerization process.
  3. Continue heating the second test tube for an additional 30 minutes.
  4. Precipitate the contents of both test tubes gradually using ethanol, then dry them.
  5. Determine the percentage yield of the prepared polymer samples compared to the monomer weight and record all observations regarding the heating duration and its effect on the polymerization process.

Discussion

Q1: Write the equation for styrene polymerization, illustrating the initiation, propagation, and termination steps.

  • Already mentioned above.

Q2: What is the effect of temperature on initiator decomposition and reaction rate? Discuss this.


  • Effect of temperature on initiator decomposition: Increasing the temperature enhances the decomposition of the initiator (e.g., benzoyl peroxide) into free radicals because thermal energy overcomes the activation energy required for the molecule's decomposition. This leads to a higher number of free radicals, which in turn accelerates the polymerization process.
  • Effect of temperature on reaction rate: As the temperature increases, the chemical reaction speeds up because molecules gain more kinetic energy, leading to a higher frequency of effective collisions between molecules.

However, if the temperature becomes excessively high, it may cause chain degradation or the formation of undesirable by-products.

Q3: In which sample is the polymer yield higher, and why? What is its relationship to the molecular weight of the polymer?

  • The sample heated for a longer duration (60 minutes) will have a higher polymer yield.
  • Reason: Longer heating duration allows the polymerization process to continue for a longer time, forming more polymer chains and thus increasing the overall polymer mass.
  • Relationship to molecular weight: In radical polymerization, if the chains are allowed to grow for a longer period without termination, the molecular weight of the polymer increases. If the reaction terminates early, the molecular weight will be lower due to shorter polymer chains.

Q4: What are the main differences between the prepared samples?

  1. Polymer viscosity: The sample heated for 60 minutes will have higher viscosity due to the formation of longer polymer chains.
  2. Molecular weight: The polymer produced after 60 minutes of heating will have a higher molecular weight because the chains have had more time to grow.
  3. Percentage conversion: The sample with a longer heating duration will show a higher monomer-to-polymer conversion rate.
  4. Visual and physical characteristics: The longer-heated sample may appear denser and have a higher observed viscosity, indicating the formation of a more extensive polymer network.

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