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Polimerización radical en cadena: mecanismo
El mecanismo de polimerización por crecimiento de cadenas radicales consta de tres pasos: inicio, propagación y terminación de la polimerización. La p…
La polimerización por crecimiento de cadena radical consta de tres etapas: iniciación, propagación y terminación.
En la etapa de iniciación, un radical libre generado por la fisión homolítica del iniciador del radical reacciona con el doble enlace del monómero. Genera un enlace sigma entre el iniciador y el monómero. Como resultado, el sitio radical se desplaza al monómero.
La etapa de propagación comienza cuando esta nueva especie radical se suma a otro monómero. Este paso se repite para agregar más monómeros para hacer crecer la cadena polimérica.
Los monómeros se unen preferentemente de tal manera que el sitio radical siempre está presente en el carbono más sustituido.
Finalmente, la terminación de la cadena puede ocurrir por el acoplamiento de radicales en dos cadenas poliméricas en crecimiento.
Alternativamente, el radical de una cadena abstrae hidrógeno del carbono adyacente al sitio radical en la otra cadena de polímeros. Por lo tanto, la polimerización se detiene y produce un par de polímeros terminados en alquilo y terminados en alquenilo.
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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.