Ep9 Cationic and Anionic Polymerization - UC San Diego - NANO 134 Darren Lipomi

The instructor begins by welcoming students to the fourth week of the Nanoscience Chemistry 134 course, expressing hope that they are still engaged after the exam. The focus for the week is on chemical structures, with an emphasis on ionic polymerization, a key method of chain growth polymerization. The lecture will cover the differences between cationic and anionic polymerization, which are mediated by pairs of electrons instead of single electrons like in radical polymerization. The instructor promises a transition from organic chemistry to thermodynamics, discussing how the theory of two solvents translates to polymers and their molecular structure, ultimately affecting their thermo mechanical behavior.

The summary delves into the specifics of ionic polymerization, contrasting it with radical polymerization. The instructor explains that anionic polymerization is considered 'living' because it lacks termination steps like combination or disproportionation, which are common in radical polymerization. The concept of formal charge is reintroduced, and the stability of carbocations is discussed, with secondary and tertiary cations being more stable than primary ones. The role of the conjugate base in cationic polymerization is highlighted, emphasizing the need for a non-reactive, delocalized charge. The summary also touches on the importance of the solvent and the dielectric constant in influencing the propagation rate of the polymerization reaction.

This paragraph explores the impact of the solvent's dielectric constant on the polymerization process. A high dielectric constant allows for better charge screening, reducing the electrostatic attraction between the growing polymer chain and the counter ion. This results in a faster reaction rate as monomers can more easily add to the chain. The instructor provides a detailed example of how varying the dielectric constant by adjusting the solvent composition significantly increases the reaction rate constant. The summary also explains the kinetic chain length and its relation to the degree of polymerization in the context of ionic polymerization.

The instructor introduces ring-opening polymerization (ROP), a process that allows for the formation of polymers with heteroatoms along the chain. The mechanism involves the use of an initiator, which can be a proton or the growing chain end, and the attack of the oxygen's lone pair on this positive charge. The summary explains how the positive charge becomes delocalized, leading to the opening of the ring and the formation of polyethylene oxide. The paragraph also mentions the potential for resonance structures and the importance of understanding the reactivity of the carbon atom in the polymerization process.

Anionic polymerization is highlighted as a highly useful method for creating a variety of polymeric structures, including block copolymers. However, it is extremely sensitive to water, which can terminate the reaction. The process typically begins with an alkyl lithium reagent, such as butyl lithium, which is noted for its potential dangers in the lab. The instructor emphasizes the need for a dry environment and the use of less reactive lithium reagents for safety. The summary also discusses the propagation step in anionic polymerization and the importance of quenching the reaction with water to terminate the chains.

The instructor contrasts the termination steps of cationic and anionic polymerization. Cationic polymerization can terminate through unimolecular or bimolecular pathways, involving the loss of a hydrogen atom or the transfer of a proton to another monomer. In contrast, anionic polymerization is described as 'living' and can only be terminated by the addition of water. The summary explains that anionic polymerization maintains reactive chain ends, allowing for the potential creation of block copolymers and other complex structures once more monomers are added.

The role of solvent polarity in anionic polymerization is emphasized, with the rate of polymerization increasing with the polarity of the solvent. This is attributed to the ability of polar solvents to allow the counter ion to dissociate farther from the growing chain end, facilitating the addition of more monomers. The instructor also clarifies the difference between electron-donating and electron-withdrawing groups, noting their impact on the reactivity of the carbon atom in the polymerization process.

In the final paragraph, the instructor prepares to conclude the chemistry topics for the course, with plans to address any remaining questions about electron donating and electron withdrawing on Wednesday. The summary also mentions the instructor's intention to provide a supplemental lecture on resonance structures to clarify why positive charges end up in certain positions. The course will then move beyond chemistry, presumably to other areas of study.