Several significant addition polymers are manufactured through ionic mechanisms. While the procedure encompasses initiation, propagation, and termination stages. The developing unit is an ion rather than a radical.
The electron distribution around the carbon atom (marked with an asterisk in Figure 1) in a growing chain can exhibit various forms. In Figure 1(a), there’s an unshared electron, functioning as a free radical. Figure 1(b) depicts a positively charged carbonium ion, which is unstable due to the absence of a shared electron pair. In Figure 1(c), there’s a negatively charged carbanion, also unstable because of the presence of an unshared electron pair.
Carbonium ions and carbanions can serve as active centers for chain growth in polymerization reactions, specifically in cationic and anionic polymerization, respectively. The mechanisms of these reactions are less understood than free radical polymerizations due to the high polymerization rates, making kinetic studies challenging. Traces of certain ingredients, termed cocatalysts, can significantly influence the reaction. Monomers with electron-donating groups on one of the double-bond carbon atoms tend to form carbonium ions in the presence of proton donors and can be polymerized using cationic methods. Conversely, those with electron-attracting substituents can undergo anionic polymerization. Free radical polymerization is intermediate and possible with substituents having moderately electron-withdrawing characteristics. Many monomers can undergo polymerization through more than one mechanism.
Cationic polymerization, employed industrially for polyformaldehyde, polyisobutylene, and butyl rubber, is catalyzed by Friedel-Crafts agents like aluminum chloride (AlCl₃), titanium tetrachloride (TiCl₄), and boron trifluoride (BF₃), which are strong electron acceptors. These catalysts operate in the presence of a cocatalyst. High molecular weight products can be rapidly obtained at -100°C. While the reactions are not fully understood, it is believed that the initial stage involves the interaction of the catalyst with a cocatalyst (such as water) to generate a complex acid.
This involves proton donation to the monomer, resulting in the formation of a carbonium ion (see Figure 2).
Subsequently, this ion engages with another monomer molecule, leading to the formation of another reactive carbonium ion (refer to Figure 3).
The process is iteratively repeated, resulting in the rapid expansion of a long chain ion. Termination can occur through the rearrangement of the ion pair (as depicted in Figure 4) or via monomer transfer.
The anionic polymerization process was initially employed around fifty or more years ago in the sodium-catalyzed synthesis of polybutadiene (Buna Rubbers). Common catalysts include alkali metals, alkali metal alkyls, and sodium naphthalene, which can be utilized for initiating polymerization by opening either a double bond or a ring structure. While the process is not extensively utilized in plastic materials production, it holds significant importance in the manufacturing of synthetic rubbers. Moreover, this method exhibits specific features that make it particularly noteworthy.
Currently, the term “anionic polymerization” encompasses a range of mechanisms initiated by anionic catalysts. It is widely applied to all polymerizations initiated by organometallic compounds, excluding those involving transition metal compounds. Importantly, anionic polymerization doesn’t necessarily indicate the existence of a free anion along the developing polymer chain.
Anionic polymerization is more prone to occur when the monomer contains electron-withdrawing substituents, such as -CN, -NO, and phenyl. In principle, initiation can occur either by the addition of an anion to the monomer, as follows:
or by the addition of an electron to generate an anion radical.
The alkyl and aryl derivatives of alkali metals, particularly alkyl lithium catalysts, commonly initiate reactions. The bond between the metal and the hydrocarbon part may vary in covalency, affecting the attachment of the counterion to the anion. Strong attachment can restrict monomer addition to the growing chain, resulting in more regular structures in polymerizations compared to free radical processes. Factors like steric effects and the choice of solvent also impact the metal-hydrocarbon bond and influence polymer structure in anionic polymerizations, often of the solution type. The significance of alkyl lithium catalysts underscores the directing influence of the metal-hydrocarbon bond.
In anionic polymerizations without impurities, there is often no termination step, allowing the monomer to continuously grow until fully consumed. In specific conditions, adding more monomer, even after weeks, can reactivate the dormant polymerization, known as living polymerization, resulting in living polymers. Notably, the additional monomer can be a different species, facilitating the production of block copolymers. This technique is valuable for certain thermoplastic elastomers and specialized styrene-based plastics.
Tobe continued….
–> Part 2
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