21.9
Polimerização por Radicais de Crescimento em Cadeia: Mecanismo
O mecanismo de polimerização por radicais de crescimento em cadeia consiste em três etapas: iniciação, propagação e terminação da polimerização. A pol…
A polimerização do crescimento da cadeia radical consiste em três estágios: iniciação, propagação e término.
Na etapa de iniciação, um radical livre gerado pela fissão homolítica do iniciador de radicais reage com a ligação dupla do monômero. Ele gera uma ligação sigma entre o iniciador e o monômero. Como resultado, o local radical muda para o monômero.
A etapa de propagação começa quando essa nova espécie radical se soma a outro monômero. Esta etapa se repete para adicionar mais monômeros para aumentar a cadeia polimérica.
Os monômeros se ligam preferencialmente de modo que o local do radical esteja sempre presente no carbono mais substituído.
Finalmente, a terminação da cadeia pode ocorrer pelo acoplamento de radicais em duas cadeias poliméricas em crescimento.
Alternativamente, o radical de uma cadeia abstrai hidrogênio do carbono adjacente ao local do radical na outra cadeia polimérica. Assim, a polimerização pára e produz um par de polímeros terminados em alquil e terminados em alquenil.
View the full transcript and gain access to JoVE Core videos
Q1: What happens during the initiation step of radical chain-growth polymerization?
During initiation, a free radical generated from a radical initiator undergoes homolytic fission and reacts with the monomer's double bond. This creates a sigma bond between the initiator and monomer, shifting the radical site to the newly bonded monomer. This activated monomer is now ready to begin the propagation phase of polymerization.
Q2: How does the propagation step build the polymer chain?
During propagation, the radical species adds to another monomer's unsaturated bond, creating a new radical site on the freshly added monomer. This step repeats cyclically, with each addition extending the polymer chain. The number of propagation cycles directly determines the final molecular weight of the polymer chain produced.
Q3: Why do monomers add in a head-to-tail manner during radical polymerization?
Monomers preferentially bond such that the radical site remains on the more substituted carbon. In monosubstituted monomers like vinyl chloride and styrene, this positioning places the unpaired electron on the substituted carbon (tail), stabilizing the growing chain and favoring head-to-tail addition throughout polymerization.
Q4: What are the two main pathways for terminating radical polymerization?
Termination occurs through radical coupling, where two growing chains' radical sites combine directly, or through radical disproportionation, where one chain's radical abstracts hydrogen from the other chain's alpha carbon. Coupling produces a single chain with a saturated bond, while disproportionation yields one alkyl-terminated and one alkenyl-terminated polymer.
Q5: How does radical disproportionation differ from radical coupling in chain termination?
In radical coupling, two radical sites bond directly to form a single polymer with a saturated linkage. In radical disproportionation, one radical abstracts hydrogen from the alpha carbon of another chain, producing two distinct polymers: one ending in an alkyl group and one ending in an alkenyl group.
Q6: What role do chain transfer reagents play in radical polymerization termination?
Chain transfer reagents terminate polymer chain growth by removing the radical site from the growing chain or reducing its reactivity. Unlike coupling and disproportionation, chain transfer does not eliminate the radical entirely but instead transfers it elsewhere, allowing polymerization to continue on a new chain or with reduced efficiency.
Q7: How does radical chain-growth polymerization compare to other polymerization mechanisms?
Radical chain-growth polymerization uses free radicals to initiate and propagate chain growth through three distinct stages. Other mechanisms like anionic chain-growth polymerization and cationic chain-growth polymerization employ different reactive intermediates and initiation strategies, offering alternative pathways for synthesizing polymers with distinct properties.