Caprolactam anionic polymerization

Scheme 21 Mechanism of e-caprolactam anionic polymerization initiated by Synhydride.

Nylon-6 can also be produced from molten caprolactam using strong bases as catalysts (anionic polymerization) this is used as the basis of monomer casting and reaction injection mol ding (RIM). Anionic polymerization proceeds much faster than the hydrolytic route but products retain catalysts which may need to be extracted. 

Wichterle,0., Sebenda, J., and Krdlicek,JThe Anionic Polymerization of Caprolactam. Vol. 2, pp. 578-595. 

The anionic polymerization of caprolactam is rapid and can be carried out below die melting temperature of the PA. Care should be taken to work under anhydrous conditions. 

Anionic polymerization of lactams was shown to proceed according to what is called the activated monomer mechanism. With bischloroformates of hydroxy-terminated poly(tetramethyleneglycol) and poly(styrene glycol) as precursors for a polymeric initiator containing N-acyl lactam ends, block copolymers with n-pyrrol-idone and e-caprolactam were obtained by bulk polymerizations in vacuum at 30 and 80 °C, respectively361. ... 

Wichterle, O., Sebenda, and Kralicek, J. The Anionic Polymerization of Caprolactam. 

The details of the anionic polymerization of nylon 6 have been extensively reviewed (1-8) and will only be discussed briefly as they affect the star-polymerization of nylon 6. Nylon 6 is polymerized anionically in a two-step process (Figure 1). The first step, creation of the activated species 3, is the slow step. The e-caprolactam monomer reacts in the presence of a strong base (such as sodium hydride) to form the caprolactam anion 2. This anion reacts with more caprolactam monomer to form 3. The reaction of this activated species with lactam anions occurs rapidly to form the nylon 6 polymer 4. 

We have used an autocatalytic model originally proposed by Malkin et al. [62]. Bolgov et al. [61] found that the originally proposed autocatalytic model [62], which was valid for equal concentration of initiator and catalyst during the anionic polymerization of caprolactam, can be modified for unequal concentration of the initiator and catalyst by an autocatalytic equation of type... 

The terms in Equation 1.3 (Malkin s autocatalytic model) are described in Nomenclature. In Malkin s autocatalytic model, the concentration of the activator, [A], is defined as the concentration of the initiator times the functionality of the initiator. For a difimctional initiator [e.g., isophthaloyl-bis-caprolactam, the concentration of the activator (acyllactam) is twice the concentration of the initiator]. The term [C] is defined as the concentration of the metal ion that catalyzes the anionic polymerization of caprolactam. In a magnesium-bromide catalyzed system, the concentration of the metal ion is the same as the concentration of the caprolactam-magnesium-bromide (catalyst) because the latter is monofunctional. 

Table 1.2 Kinetic Constants for Anionic Polymerization of Caprolactam with Different Catalyst and Initiator Systems...

Viscosity Growth During Anionic Polymerization of Caprolactam... 

Figure 1.13 Isothermal complex viscosity rise during anionic polymerization of caprolactam using caprolactam-magnesium-bromide/isophthaloyl-bis-caprolactam as the catalyst/initiator system. Run numbers and polymerization temperatures are shown in the legend...

For sodium/hexamethylene-l,6-bis-carbamidocaprolactam system, Sibal et al. [64] found the value of the constant k in Equation 1.4 to be 17.5. Note that the values of the constant k in Equation 1.4 that defines the relative complex viscosity rise during anionic ring opening polymerization of caprolactam are comparable for both caprolactam-magnesium-bromide/isophthaloyl-bis-caprolactam and sodium/hexamethylene-l,6-bis-carbamidocapro-lactam as the catalyst/initiator systems even though the kinetic constants for anionic polymerization for these systems are extremely different (see Table 1.2). 

The rheokinetics of polycaprolactam polymerizing in the monomer shows that below 50 percent conversion, the relative complex viscosity versus conversion of the nylon 6 homopolymerization is defined by the phenomenological equation ri / t]Q = exp(19.6 X), where // is the complex viscosity of nylon 6 anionically polymerizing in its monomer, 0 is the viscosity of caprolactam monomer, and X is fractional conversion. 

The main contaminant and its concentration in commercial caprolactam usually is water at <0.1 wt%. Anhydrous caprolactam is produced in small quantity for use in anionic polymerization processes. Commercial product of very high purity is required by the users, ie, the fibers and plastics producers, most of whom utilize technologically advanced processes that are sensitive to monomer quality. 

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