Anionic Polymerization of Lactams: Some Industrial Applications

Chapter 19

Anionic Polymerization of Lactams: Some Industrial Applications 1

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Downloaded by PENNSYLVANIA STATE UNIV on December 17, 2014 | http://pubs.acs.org Publication Date: June 30, 1998 | doi: 10.1021/bk-1998-0696.ch019

K. Udipi , R. S. Dave , R. L . Kruse , and L. R. Stebbins 1

Monsanto Company, 800 North Lindbergh Boulevard, St. Luois, MO 63167 Morrison and Foerster, 2000 Pennsylvania Avenue, Washington, DC 20006 Bayer Polymers Division, 800 Worcester Street, Springfield, MA 01151

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Anionic ring opening polymerization of lactams to generate polyamides has been studied quite extensively both in academia and in industry. Caprolactam is by far the most studied lactam and the nylon 6 prepared by this route compares favorably in properties with that prepared by conventional hydrolytic polymerization. Fast reaction kinetics, absence of byproducts, and the crystalline nature of the nylon so produced makes anionic polymerization a compelling choice for several industrial applications. This paper will review a few such industrial applications as in reactive extrusion, reactive thermoplastic pultrusion, and reaction injection molding.

Anionic ring opening polymerization of lactams to generate polyamides has been studied quite extensively by Sebenda , Wichterle , and Sekiguchi among others in academia and Gabbert and Hedrick in industry. Caprolactam is by far the most studied lactam and the nylon 6 prepared by this route compares favorably in properties with that prepared by conventional hydrolytic polymerization. Fast reaction kinetics, absence of by-products, and the crystalline nature of the nylon so produced makes anionic ring opening polymerization a compelling choice for several industrial applications such as reactive extrusion, reactive thermoplastic pultrusion, and reaction injection moldings. This paper will review reactive processing of caprolactam in the above three areas. 1

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Anionic ring opening polymerization of lactams follows an activated monomer mechanism as against conventional activated chain end mechanism. That is, the chain growth reaction proceeds by the interaction of an activated monomer (lactam anion) with the growing chain end. A typical reaction path for the polymerization of

©1998 American Chemical Society

In Applications of Anionic Polymerization Research; Quirk, R.; ACS Symposium Series; American Chemical Society: Washington, DC, 1998.

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256 caprolactam initiated by a bisimide (isophthaloyl bis caprolactam) and catalyzed by a Grignard species (caprolactam magnesium bromide) is shown in Figure 1. A large number of initiators and catalysts are mentioned in literature but the most commonly employed initiators are N-acyl lactams while lactamate anions generated by the reaction of a Grignard or sodium with lactams are the preferred catalysts.


REACTIVE EXTRUSION Reactive extrusion as it applies to polymers involves conducting a chemical reaction in an extruder on a preformed polymer to generate functionalized polymers such as compatibilizers for polymer blends or carrying out continuous conversion of low viscosity oligomers or monomers to polymers. Although single screw extruders were employed in the early stages of development, ' in recent years, twinscrew extruders are preferred because of their ability to mix, devolatalize, and pump low viscosity liquids and to add or remove ingredients from the melt at various stages . Several different twinscrew extruders of varying configurations are available, but among those, counter rotating non-intermeshing, counter rotating intermeshing and corotating intermeshing extruders have gained the broadest acceptance. 5 6

Currently almost all of the - 1 . 2 billion lbs of worldwide supply of nylon 6 is manufactured by the hydrolytic, continuous or batchwise polymerization of caprolactam. It takes a few hours at temperatures in the 250-270°C range to produce this polymer. Anionic ring opening polymerization of caprolactam on the other hand is fast, takes only minutes at < 250° C to complete and as suggested earlier with no byproducts formed, it lends itself to a continuous polymerization process in an extruder. It is also less capital intensive. Evidence in literature dates back to 1969 of some early attempts by Illig and others ' to adapt the above chemistry to reactive extrusion. Bartilla " et al explored the sodium salt of caprolactam as a catalyst and acetyl caprolactam as the initiator in their study carried out in a 30mm corotating intermeshing twinscrew extruder. They also fed the ingredients to the extruder as solids at room temperature expecting to get a uniform mixing when melted. Nichols et a l , employed caprolactam magnesium bromide as the catalyst and acetyl caprolactam as the initiator in their study in a 20mm counter rotating non-intermeshing twinscrew extruder. In all the cases cited above, they were able to successfully polymerize caprolactam to nylon 6 comparable in most respects to commercial hydrolytic nylon 6. In our own study (Figure 2) carried out in a counter rotating nonintermeshing twinscrew extruder (L/D=48), we employed caprolactam magnesium bromide as the catalyst and isophthaloyl biscaprolactam (BAIT) as the initiator. It is imperative that the polymerization be carried out under anhydrous conditions and the twinscrew extrusion is quite conducive in this respect. All the materials employed in the polymerization are thoroughly dried and pumped through heat traced transfer lines under moisture-free conditions. The variables examined included initiator concentration (3 mmoles to 5 mmoles/lOOg monomer at a catalyst/initiator ratio of ~ 2.5), feed rates (6 lb/hr to 16 lb/hr), residence time in the extruder( 50 sees to 170 sees), and the extruder temperature 7

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^

J

Ν — C

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+MgBr

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·— Ν

—

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h

ΝH

"Ν—c

//

Figure 1. Bisimide initiated anionic polymerization of caprolactam

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DrMg-f-

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Ο



258 (100 to 230°C). In all cases the rates of monomer conversion were fast and the molecular weights attained were considerably higher than for hydrolytic nylon 6. A major distinguishing factor in favor of nylon 6 produced by reactive extrusion route as against conventional hydrolytic nylon 6 is the ease with which molecular weight is controlled. Thus in hydrolytic nylons, the commonly attained weight average molecular weight M^, is ~30Kg/mole. Although such molecular weights are adequate for most applications, they are not high enough for such applications as blow molding. Conventional nylons with higher molecular weights are obtained by solid state polymerization, wherein nylon pellets are heated below the melting point with either vacuum or a nitrogen sweep. Our studies as well as earlier studies have demonstrated that anionically polymerized nylons can be produced in the molecular weight range of M , , - 80-120 Kg/mole without going through the solid state polymerization step. All the nylon 6 polymers generated by the reactive extrusion route exhibit reasonably high levels of residual caprolactam monomer. This is typical of the ring chain equilibrium associated with ring opening polymerization. Even the nylon 6 produced by the hydrolytic polymerization contains residual monomer which is removed by either washing or application of high vacuum. The level of residual monomer in reactively extruded nylon 6 can be brought down by optimizing the extrusion variables to the levels in commercial hydrolytic nylon 6. Further reduction is feasible by injecting water into the extruder barrel towards the end of polymerization and subsequent devolatalization before exitingfromthe extruder die. In our work we have been able to lower the residual monomer to

Anionic Polymerization of Lactams: Some Industrial Applications

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