Activated monomer mechanism polymer chain generation

Chain growth polymers, which are often referred to as addition polymers, form via chain addition reactions. Figure 2.2 presents a generic chain addition mechanism. Chain addition occurs when the active site of a monomer or polymer chain reacts with an adjacent monomer molecule, which is added to the end of the chain and generates a new active site. The active site is the reactive end of a monomer or polymer that participates in the polymerization reaction. 

Most emulsion polymerisations are free radical processes (318). There are several steps in the free radical polymerisation mechanism initiation (324), propagation and termination (324, 377, 399). In the first step, an initiator compound generates free radicals by thermal decomposition. The initiator decomposition rate is described by an Arrhenius-type equation containing a decomposition constant ( j) that is the reciprocal of the initiator half-life (Ph). The free radicals initiate polymerisation by reaction with a proximate monomer molecule. This event is the start of a new polymer chain. Because initiator molecules constantly decompose to form radicals, new polymer chains are also constantly formed. The initiated monomeric molecules contain an active free radical end group. 

A prochiral monomer such as propylene offers two faces for coordination to a metal center. The steric environment at the active site, formed by the coordinated ligands and the growing polymer chain after activation with a cocatalyst, determines the orientation of the incoming monomer. In this case, the mechanism of stereoselection is referred to as enantiomorphic site control. The stereochemistry of the polymer is thus determined by the chirality relationship of the two coordination sites of the catalyst. However, every monomer insertion generates a new stereogenic center. As a consequence, chiral induction (enantioface preference) arises from the last-inserted monomer unit in the growing polymer chain. This mechanism is referred to as chain-end control (see Chapter 1 for an introduction to chain-end and enantiomorphic site control mechanisms in iPP synthesis). 

These are the most important. The two double bonds mutually activate each other conjugation is essentially not destroyed by addition to the growing chain end. Therefore the conjugated dienes are difunctional monomers. They are polymerized by a relatively simple mechanism. Of all the polymers generated in living tissues, we have so far been able to imitate most closely natural rubber, poIy-cis-l,4-isoprene. Butadiene, isoprene and chloroprene are the dienes most often employed in macro-molecular chemistry. 

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