Amine-catalyzed, ring-opening polymerization

The overall rate of conversion of e-caprolactam to polymer is higher than the polymerization rate of e-aminocaproic acid by more than an order of magnitude [Hermans et al., 1958, I960]. Step polymerization of e-aminocaproic acid with itself (Eq. 7-57) accounts for only a few percent of the total polymerization of e-caprolactam. Ring-opening polymerization (Eq. 7-58) is the overwhelming route for polymer formation. Polymerization is acid-catalyzed as indicated by the observations that amines and sodium e-aminocaproate are poor initiators in the absence of water and the polymerization rate in the presence of water is first-order in lactam and second-order in COOH end groups [Majury, 1958]. 

Scheme 6.38 Ring-opening polymerization of L-lactide catalyzed by tertiary amine-functionalized thiourea rac-12.

Suzuki et al.[57 explored the Pd-catalyzed ring-opening of the monomer 5,5-dimethyl-6-ethenylperhydro-l,3-oxazin-2-one to give a hyperbranched polyamine. The polymerization was conducted at ambient temperature in THF and catalyzed with Pd2(dba)3-2 dppe, affording after the evolution of carbon dioxide, the desired hyperbranched polyamine. NMR spectroscopy confirmed the high yield conversion of the monomer. The degree of branching was ascertained (from NMR data) as the ratio of tertiary amine units to the total of secondary and tertiary moieties. 

Ring-Opening Polymerization. Polyamides can be formed by ringopening reactions as in the polymerization of caprolactam. The polymerization may be carried out at high temperature with water, amino acid, or amine carboxylate salt as Initiator. The predominate ring-opening mechanism with these initiators is carboxyl-catalyzed aminolysls. In the case of water initiation, hydrolysis occurs (Reaction 10) to form small amounts of the amino acid. The amino acid then reacts with lactam amide groups (Reaction 11). 

The proposed polymerization pathway differs fundamentally from the coordination-insertion mechanism involving metal complexes, see Fig. 3.7 [5, 38]. Indeed, the nucleophilic catalyst only activates the monomer toward ring opening, whereas the metal complex activates the monomer, initiates the polymerization, and remains bound to the growing chain. The polymerization mechanism of a superbase or thiourea-amine catalyzed ROP will be discussed in more detail below. 

Caprolactam melts at about 69°C. It does not polymerize upon heating to elevated temperatures. However, shortly after Carothers developed nylon-6,6, Schlack [51] of I.G. Farben discovered that the ring-opening reaction occurs readily in the presence of amine and carboxyl groups. Thus, -aminocaproic acid, nylon-6,6 salt, or simply water, is employed to hydrolyze lactam to form [COOH] and [NH2] end groups. The [COOH] group catalyzes the addition of [NH2] to the caprolactam ring. This discovery led to the polymerization of caprolactam for nylon-6. 

Polyformaldehyde. Polyformaldehyde or polyacetal is made by two different processes. Delrin is made from formaldehyde by anionic polymerization catalyzed by a tertiary amine. The homopolymer is end-capped with acetic anhydride. Celcon is made from trioxane cationic copolymerization using boron trifluoride catalyst and ethylene oxide (2-3%) as the comonomer. Boron trifluoride is a Lewis acid that associates with trioxane and opens up the six-membered ring. Ethylene oxide provides the end capping. Without an end cap, polyformaldehyde is thermally unstable and loses formaldehyde units. 

Amines. Tertiary amines R3N are catalysts that open the epoxy ring and thus catalyze the polymerization reaction. They may be used with hydroxyl-containing molecules to catalyze homopolymeiization (Fig. 3.26), but more often they are used to catalyze copolymerization of epoxy resins with amine or acid curing agents. Several more specialized amines are also mentioned as catalysts (Fig. 3.27). 

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