Homopolymerization step growth

Random Step Growth Homopolymerization of a Bifunctional Monomer... 

A macromonomer is a macromolecule with a reactive end group that can be homopolymerized or copolymerized with a small monomer by cationic, anionic, free-radical, or coordination polymerization (macromonomers for step-growth polymerization will not be considered here). The resulting species may be a star-like polymer (homopolymerization of the macromonomer), a comblike polymer (copolymerization with the same monomer), or a graft polymer (copolymerization with a different monomer) in which the branches are the macromonomer chains. 

Homopolymerization. In the simplest type of step growth, a bifunctional monomer reacts successively with itself, eventually forming a polymer with a large number of repeating units. The reaction may be an addition, but more commonly is a condensation. Although condensation usually is reversible, its equilibrium is driven toward complete conversion by removal of the small and volatile cast-off molecule ... 

Two major polymerization mechanisms are considered chain growth and step growth. In addition, polymerization can be homopolymerization—a single monomer is used—and copolymerization usually with two monomers with complementary functional groups. 

Step Growth Homopolymerization Mechanism and Kinetics Here any two growing chains can react with each other. The propagation mechanism is an infinite set of reactions ... 

Copolymerization Copolymerization involves more than one monomer, usually two comonomers, as opposed to the single monomer involved in the chain growth and step homopolymerization schemes above. Examples are some nylons, polyesters, and aramids. Here as well there are step growth and chain growth mechanisms, and these are much more complex [see, e.g., Ray in Lapidus and Amundson (eds.), Chemical Reactor Theory—A Review, Prentice-Hall, 1977]. 

Historically, polymerization problems were initially treated using statistical methods. This is because they have the advantage of simplicity when dealing with simple polymerization problems. In using such approaches, a certain amount of intuition is necessary. For example, consider the case of a step-growth homopolymerization of a, co-amino-caproic acid to make nylon 6. This reaction can be schematically represented by... 

Simple AB step-growth homopolymerization AB homopolymerization is shown below ... 

Thiols can be used in two ways with free-radical polymerization. Thiols react with electron-rich enes (allyl ethers) via a step-growth mechanism to create a polymer only if both ene and thiol have functionalities of at least two. The allyl ethers cannot homopolymerize. If thiols are present, the acrylate can homopolymerize and copolymerize with the thiol. Pojman et al studied frontal thiol-ene polymerization using pentaer-ythrytoltriallyl ether (PTE) and trimethylolpropanetris (3-mercaptopropionate) (95%) (TTl). Not surprisingly, the front velocity was a maximum at a 1 1 thiol ene ratio (Figure 35). ... 

In practice, the choice of monomers for copolymerization should be such that aU of them will respond to the catalyst system used for copolymerization. In cases where any of these monomers does not respond to the catalyst, that particular monomer is not incorporated into the copolymer. Thus, a monomer (e.g., styrene) that polymerizes by the radical mechanism does not copolymerize with a monomer sueh as s-caprolactam, which homopolymerizes by the step-growth mechanism. 

The formation of aromatic isocyanate trimers is of economic importance, because rigid insulation foams, having isocyanurate structures built into their network structure, are produced from aromatic diisocyanates. Triphenyl isocyanurates with hydroxyl or carboxyl groups in their p-positions can be obtained on hydrolysis of McsSiO- and McsSiOCO-groups, respectively, with hydrochloric acid °. Such trifunctional compounds are of use in the construction of network polymers. The mechanism of the phenyl isocyanate trimer-ization, using Pd(o) diimide catalysts was elucidated recently. The initial steps of this trimerization reaction involve a chain growth process as encountered in the anionic homopolymerization of isocyanates. 

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