Multichain Polymerization

Aramid polymers are much more expensive than the aliphatic polyamides. The use of aramid polymers is limited to those applications that justify the high cost. The present 

market is about 20 million pounds per year. The applications are those where one needs very high flame resistance (clothing for firefighters and welders, welder s protective shield, upholstery and drapes), heat resistance (ironing board covers, insulation film for electrical motors and transformers, aerospace and military), dimensional stability (fire hose, V- and conveyor belts), or strength and modulus (circuit boards, bulletproof vests, fiber optic and power lines, ship mooring ropes, automobile tire cord, puncture-resistant bicycle tires). 

Wallace Carothers and coworkers at DuPont synthesized aliphatic polyesters in the 1930s [Furukawa, 1998 Hounshell and Smith, 1988]. These had melting points below 100°C, which made them unsuitable for firber use. Carothers then turned successfully to polyamides, based on the theoretical consideration that amides melt higher than esters. Polyamides were the first synthetic fibers to be produced commercially. The polyester and polyamide research at DuPont had a major impact on all of polymer science. Carothers laid the foundation for much of our understanding of how to synthesize polymeric materials. Out of that work came other discoveries in the late 1930s, including neoprene, an elastomer produced from chloro-prene, and Teflon, produced from tetrafluoroethylene. The initial commercial application for nylon 6/6 was women s hosiery, but this was short-lived with the intrusion of World War II. The entire nylon 6/6 production was allocated to the war effort in applications for parachutes, tire cord, sewing thread, and rope. The civilian applications for nylon products burst forth and expanded rapidly after the war. 

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Fig. 2-14 Weight fraction distribution plot for multichain polymerization. Plot I. / 1 plot 2, / = 2 ...

Whelan and Bailey were also able to clarify the polymerization mechanism of the enzymatic polymerization with phosphorylase [124], Their results showed that the polymerization follows a multichain scheme in contrast to a single-chain scheme that was also proposed by some authors. In the multichain polymerization scheme, the enzyme-substrate complex dissociates after every addition step, whereas in the single-chain scheme each enzyme continuously increases the length of a single primer chain without dissociation. 

The molar-mass distributions for such multichain polymerizations have been calculated and shown to be narrower than for a single-chain polymerization since the probability of formation for very long chains is reduced. The polydispersity for chain growth in the presence of a multifunctional reagent Ay with functionality / then becomes, as the conversion, p, approaches unity. 

A polymerization that provides a transition into a discussion of gelation is the condensation of an excess of A-B with a small amount of an /-functional monomer, R-A/, that contains / equivalent functional groups of Type A, but no functional groups of Type B.[3 Linear chains are obtained when / is 1 or 2, but multichain condensation polymers are produced when/>2. At high conversion the polydispersity index depends only on/. 

The formation of multichain condensation polymers using an excess of A-B with a small amount of the multifunctional monomer R-A/, under conditions where no A can react with another A was described above in Sect. 1.2.1. Polyfunctional condensation polymerization using multifunctional monomers in other contexts can produce infinite... 

Coombes and Katchalski [29] have considered a slightly more complex version of this mechanism in which a second propagation coefficient operates above a critical degree of polymerization. Katchalski et al. [30] calculated the molecular weight distribution obtained in a system following scheme (12) but also including a bimolecular termination step. Various authors have analysed more complex systems in which the initiator is a polymeric species. Thus Gold [31] has shown that initiation by a poly a-amino acid with a Poisson distribution leads to a polymeric product with an over-all Poisson distribution, and Katchalski et al. [32] demonstrated that in multichain polymers synthesized from polyfunctional initiators Poisson distributions also arise. 

Molecular weight distribution in the above type of multichain step polymerization would be expected to be much narrower than for a linear polymerization. Consider, for example, polymerization of bifunctional monomer A—B with a small proportion of an /-functional substance Ay leading to... 

Synthesis of the skeletal chains must proceed, as do all enzyme-controlled polymerization reactions, either by a single or a multichain reaction mechanism, through a series of "growing cycles in which the... 

Most early reports on brush and branched polypeptides utilized conventional NCA polymerization techniques. The first graft polypeptides were published in 1956 by Sela etal Using poly(L-lysine) or poly(D/L-omithine) as a multifunctional initiator for the ROP of a number of different NCAs, these authors prepared a variety of graft polypeptides, which they termed multichain poly(amino acids) (eqn [18]). Since the NCA polymerizations were carried out in aqueous dioxane. 

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