Design, synthesis, and polymerization of novel heterocyclic monomers as precursors for functional polyesters, poly(ester amide)s and polyamides
The research presented in this dissertation involves the synthesis and polymerization of heterocyclic monomers which pave the way to biodegradable polyester nanocomposites, functional polyesters or poly(ester amide)s, functional polyamides and supramolecular polymers. The key monomers are ε-caprolactone, γ-acetamido-ε-caprolactone, γ-ethylene ketal-ε-caprolactam and α-amino-ε-caprolactam. Poly(ε-caprolactone) organo-modified montmorillonite nanocomposites were prepared by in-situ polymerization using dibutyltin dimethoxide as initiator/catalyst. The montmorillonite was first modified with 1-decyl-2-methyl-3-(11-hydroxyundecyl)-imidazolium cation. The hydroxyl functionality was used for not only initiating polymer chains from the surface of the clay platelets but also for grafting polymer chains to the surface by acting as a reversible chain transfer agent. The molecular weight of polymer chains was controlled by the ratio of monomer to hydroxyl groups. XRD and TEM studies confirmed that highly exfoliated nanostructures were formed. The amount of inorganic component did not affect thermal behavior of the polymer matrix by DSC or TGA. The highly exfoliated clay sheets acted as nucleating agents and increased the degree of crystallinity. DMA revealed an enhancement of storage modulus with increasing clay content above the glass transition temperature. The average mineralization (biodegradation) of the test materials in marine environment was measured by respirometry experiments. The results revealed improved biodegradation rates for the nanocomposites compared to those of the neat polymer. There appears a trend between the addition levels of the organically modified clay and the rate of biodegradation. This can be attributed to highly exfoliated morphology of nanocomposites in which the confinement of polymer chains between the silicate layers is avoided. In addition, the nanocomposites with lower degrees of polymerization are more prone to biodegradation. γ-Acetamido-ε-caprolactone was synthesized in very good yield in five steps starting from 1,4-cyclohexandione monoethylene ketal. The molecular geometry and conformation are discussed based on variable temperature NMR, proton exchange and NOESY experiments. The data suggest that an intramolecular hydrogen-bond exists between the amide proton and the lactone carbonyl. The presence of such a 7-membered intramolecular hydrogen-bond effects molecular geometry and further reactivity of the compound. Despite its lower polymerization reactivity compared to the unsubstituted analog, ε-caprolactone, γ-acetamido-ε-caprolactone has great potential for generating a large library of new materials as indicated by select examples described here. A functional derivative of ε-caprolactam, 5-azepane-2-one-ethylene ketal or γ-ethylene ketal-ε-caprolactam, has been synthesized by a very straightforward and highly efficient Beckmann rearrangement reaction. Homopolymers of this new monomer and its copolymers with ε-caprolactam have been synthesized by anionic ring opening polymerization using N-acetyl-ε-caprolactam and NaH. The ketone groups can be easily released by deacetalyzation, and subsequent reaction leads to complete reduction to iv hydroxyl pendant groups. The ketone containing (co)polymers impart sensitivity to both thermal and photo-crosslinking in this novel class of materials. These new aliphatic polyamides bearing either ketone or hydroxyl pendant groups provide entries into a large number of application areas. Hydrogen-bonded supramolecular polymers were prepared from the derivatives of α-amino-ε-caprolactam, obtained from lysine, a renewable resource. Several self-complementary monomers were synthesized by varying the number of carbons in the spacer between the hydrogen-bonding end groups. Macroscopic properties of these hydrogen-bonded supramolecular polymers were clearly demonstrated by differential scanning colorimetry, solid state NMR and X-ray powder diffraction analyses. The supramolecular behavior was also supported by the fiber formation from the melts of these compounds. Stable glassy copolymers were prepared from the physical mixtures of two different biscaprolactams. The hydrogen-bonding ability of these compounds was also utilized by incorporation of α-amino-ε-caprolactam at the chain ends of low molecular weight Jeffamine ( M n =900 g/mol) through urea or amide linkages. The transformation of a liquid at room temperature into an elastic transparent film clearly showed the improvement in mechanical properties with the introduction of these hydrogen-bonding groups. Last, the use of monomers with a functionality of four gave rise to network formation and materials with enhanced properties.