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In support of programs to identify polyhydroxyalkanoates with improved materials properties, we report on our efforts to characterize the mechanical and thermal properties of copolyesters of 3-hydroxybutyrate (3HB) and 3-hydroxyhexanoate (3HHx). The copolyesters, having molar fraction of 3HHx ranging from 2.5 to 35 mol % and average molecular weights ranging from 1.15 × 10⁵ to 6.65 × 10⁵, were produced by fermentation using Aeromonas hydrophila and a recombinant strain of Pseudomonas putida GPp104. The polymers were chloroform extracted and characterized by solution-state and solid-state nuclear magnetic resonance (NMR) spectroscopy and a variety of mechanical and thermal tests. Solution-state ¹H NMR data were used to determine polymer composition-of-matter, while solution-state ¹³C NMR data provided polymer-sequence information. Solvent fractionation and NMR spectroscopic characterization of these polymers showed that polymers containing up to 9.5 mol % 3HHx had a Bernoullian compositional distribution. By contrast, polymers containing more than 9.5 mol % 3HHx had a bimodal polymer composition. Solvent fractionation of these 3HHx-rich polyesters produced two polymer fractions, each of which was again consistent with Bernoullian polymerization statistics. Solid-state NMR relaxation experiments provided insight into aging in poly(3HB-co-3HHx) copolymers, demonstrating increased polymer-chain motion with increasing 3HHx content. The elongation-to-break ratio in the polyesters increased with increasing molar fraction of 3HHx monomers. Aging properties of the poly(3HB-co-3HHx) copolymers were very similar to copolymers of 3HB and 3-hydroxyvalerate (3HV). However, poly(3HB-co-3HHx) exhibited increased activation energy to thermal degradation with increasing 3HHx content.

This paper describes the biosynthesis and properties of bacterial poly(hydroxyalkanoate)s (PHA) with predominantly hydroxyl end groups. Hydroxy termination is achieved by the addition of low molecular weight diols to the culture. It is found that low molecular weight diols of various structures can be easily incorporated as chain ends, when used during fermentation, by a variety of microorganisms. Incorporation of a chiral diol does not appear to be stereospecific; both (R)- and (S)-1,2-propanediols were incorporated into the polymer. Moreover, both primary and secondary hydroxyl groups of 1,2-propanediol were found to have reacted. It was found that an increase in the hydroxy termination in P(3HB) leads to an increase in the thermal stability, most likely by prolonging the condensation reaction and delaying the degradation reaction.

Burkholderia sp. was grown on sucrose-containing mineral salts medium with phosphate limitation to induce poly(hydroxyalkanoate) (PHA) accumulation. Under these conditions the cultures accumulated 3-hydroxybutyric acid (3HB) and 3-hydroxy-4-pentenoic acid (3HPE) containing polyesters. Solvent fractionation of the purified polyester indicated the presence of two homopolymers, poly(3HB) and poly(3HPE), rather than a co-polyester with random monomer distribution as has been reported previously [Rodrigues, M. F. A.; da Silva, L. F.; Gomez, G. C.; Valentin, H. E.; Steinbüchel, A. Appl. Microbiol. Biotechnol. 1995, 43, 880]. The simultaneous accumulation of two homopolyesters by Burkholderia sp. was confirmed by NMR spectroscopic analysis. Therefore, this is the first report on accumulation of a poly(3HPE) homopolyester and its accumulation from structurally unrelated carbon sources. Purified poly(3HPE) was cross-linked by UV radiation and subjected to epoxidation using 3-chloroperoxybenzoic acid. Introduction of epoxides into the 3HPE homopolyester was found to increase the glass transition temperature.

The preparation, rheological behavior, and solution properties of branched poly(3-hydroxy butyrate)-co-(3-hydroxy valerate), PHBV, are described. Branching is produced by the use of dicumyl peroxide (DCP) during the reactive extrusion of PHBV. The shear viscosities of reactively extruded PHBV and the straight-chain polymer of similar molecular weight are nearly identical, but the reactively extruded PHBV's elongational viscosity is considerably higher. High elongational viscosity of these polymers results from a branched structure produced when extrusion is carried out in the presence of small amounts of DCP (0.025−0.3 wt %). Strain hardening is observed in the elongational response of the branched PHBV at all deformation rates and increases with increasing concentrations of DCP. Study of the solution properties of the linear and branched PHBV indicates larger hydrodynamic volume compared to that of polystyrene of comparable molecular weight. Mark−Houwink plots, observed for PHBV, exhibited increasing nonlinearity with increasing DCP content. Trifunctional branching frequencies computed on the basis of intrinsic viscosity ratios indicated explosive growth in branching, possibly due to the generation of microgel, when branching reactions are initiated with a DCP level exceeding 0.2 wt %.

Mesomorphic polyesters were synthesized from 4, 4'-dihydroxy-α-methylstilbene and adipic acid (P-6) or (+)-3-methyl adipic acid (P-6M). P-6 forms a thermotropic nematic phase and P6-M a thermotropic cholesteric phase. The nematic phase of P-6 could easily be identified by optical microscopy. For both polymers we observed a biphasic region in which the isotropic and liquid crystalline phases coexist. Bright colors were obtained by increasing the pitch of P6-M by admixture with either a low molecular weight nematogen or with polymer P-6, and also by synthesizing a copolymer containing the two dibasic acids. The copolymeric cholesteric phase, which is stable between 199 and 282d`C, had a predominately planar texture, and these features could be retained in the solid state by quenching to produce a film having a deep blue color at room temperature. The role of the degree of polymerization upon the development of organization, and parameters, of the mesophase is discussed.

Alternate feeding of glucose and propionic acid to phosphate-depleted batch cultures of Ralstonia eutropha (reclassified from Alcaligenes eutrophus) produced poly(3-hydroxybutryate-co-3-hydroxyvalerate) [P(3HB-co-3HV)] with thermal properties markedly different from random copolymers of similar monomer content. These polymers exhibited a single glass transition and a single melting peak that was significantly higher than expected for a random copolymer. The polymers were found, by solvent fractionation and NMR, to be a mixture of poly(3-hydroxybutyrate) [P(3HB)] and a random copolymer, P(3HB-co-3HV). When compared with random copolymers of similar monomer content, the polymers produced by alternate substrate feeding displayed no improvement in mechanical properties and possessed similar aging characteristics.

Synthesis of N-phenylnorbornene-5,6-dicarboximide (NDI) with substituents at different positions in the aromatic ring and their polymerization by ring-opening metathesis polymerization (ROMP) leading to high-temperature polymers has been previously described. It was shown that the properties of these polymer are significantly influenced by the size and position of the substituent groups on the ring. This paper describes ROMP of NDI's with methyl substituents at ortho, meta, and para positions. These methyl-substituted polymers show similar trends in mechanical and thermal properties, with ortho substitution raising the temperature of the transition in dynamic modulus and the glass transition temperature, Tg, (stiffening) and meta substitution lowering these transition temperatures (plasticizing effect) relative to unsubstituted PNDI. Solid-state NMR relaxation experiments provide insight into these trends in mechanical and thermal properties. Experiments were performed to measure the carbon laboratory-frame relaxation, T1(C), and both proton and carbon rotating-frame relaxation, T1ρ(H) and T1ρ(C). The observed trends in these relaxation values are as follows:  (a) T1(C), o-CH3, m-CH3 > PNDI, p-CH3; (b) T1ρ(H), o-CH3 > PNDI, p-CH3, m-CH3; (c) T1ρ(C), o-CH3 > PNDI, p-CH3, m-CH3. We also performed dipolar rotational spin−echo (DRSE) NMR experiments to demonstrate the phenyl rings are flipping in PNDI and p-methyl PNDI, but not in either the ortho or the meta isomer. These relaxation results are interpreted in terms of a model which provides a molecular basis for the observed trends in the thermal and mechanical properties of these polymers.