Masahiko Okada

Chemical syntheses of biodegradable polymers

Abstract Commercially available synthetic biodegradable polymers had been limited to aliphatic polyesters until polyesters containing aromatic moieties recently appeared on the market. Currently, several different types of biodegradable polymers have been developed and are being evaluated for practical uses. The present article reviews recent advances in chemical syntheses of biodegradable polymers from the standpoint of molecular design. Thus, synthetic biodegradable polymers are herein classified into three groups: (1) polyesters, (2) polymers containing both ester and other heteroatom-containing linkages in the main chains, and (3) polymers with heteroatom-containing linkages other than ester linkages in the main chains. Progress in the synthesis of the representative polymers in each category is described with emphasis on controlled synthesis, and their biodegradability is discussed in relation to the molecular structure.

Determination of the Degree of Acetylation of Chitin/Chitosan by Pyrolysis-Gas Chromatography in the Presence of Oxalic Acid

A new method to determine directly and rapidly the degree of acetylation of chitin/chitosan was developed based on reactive pyrolysis-gas chromatography in the presence of an oxalic acid aqueous solution. The degree of acetylation was precisely evaluated on the basis of peak intensities of the characteristic products such as acetonitrile, acetic acid, and acetamide originating from the N-acetyl group of N-acetyl-d-glucosamine units of chitin/chitosan. The observed values were in good agreement with those obtained by (1)H NMR and the other methods. Moreover, the proposed technique was applicable to any kinds of chitin/chitosan samples over the whole range of acetylation including insoluble chitin/chitosan and perfectly acetylated artificial chitin having higher crystallinity to which (1)H NMR had been inapplicable.

Biodegradable polymers based on renewable resources. IV. Enzymatic degradation of polyesters composed of 1,4:3.6‐dianhydro‐D‐glucitol and aliphatic dicarboxylic acid moieties

Enzymatic degradation of a series of polyesters prepared from 1,4:3.6-dianhydro-D-glucitol (1) and aliphatic dicarboxylic acids of the methylene chain length ranging from 2 to 10 were examined using seven different enzymes. Enzymatic degradability of these polyesters as estimated by water-soluble total organic carbon (TOC) measurement is dependent on the methylene chain length (m) of the dicarboxylic acid component for most of the enzymes examined. The most remarkable substrate specificity was observed for Rhizopus delemar lipase, which degraded polyester derived from 1 and suberic acid (m = 6) most readily. In contrast, degradation by Porcine liver esterase was nearly independent of the structure of the polyesters. Enzymatic degradability of the polyesters based on three isomeric 1,4:3.6-dianhydrohexitols and sebacic acid was found to decrease in the order of 1, 1,4:3.6-dianhydro-D-mannitol (2), and 1,4:3.6-dianhydro-L-iditol (3). Structural analysis of water-soluble degradation products formed during the enzymatic hydrolysis of polyester 5g derived from 1 and sebacic acid has shown that the preferential ester cleavage occurs at the O(5) position of 1,4:3.6-dianhydro-D-glucitol moiety in the polymer chain by enzymes including Porcine pancreas lipase, Rhizopus delemar lipase, and Pseudomonas sp. lipase.

New chitin‐based polymer hybrids, 3. Miscibility of chitin‐graft‐poly(2‐ethyl‐2‐oxazoline) with poly(vinyl alcohol)

The miscibility of blends of poly(vinyl alcohol) (PVA) with chitin-graft-poly(2-ethyl-2-oxazoline) (1) and poly(2-ethyl-2-oxazoline) homopolymer (PEtOZO) was investigated. Calorimetric results showed a single glass transition temperature (T g ) in the entire range of compositions for both blend systems, which indicated that PVA is miscible with both the graft copolymer 1 and PEtOZO. The T g of PVA is also shifted to lower temperature upon blending with the graft copolymer 1. IR analysis revealed the existence of specific interactions via hydrogen bonding between the hydroxyl groups in PVA and the carbonyl groups in the poly(2-ethyl-2-oxazoline) side chain of graft copolymer 1. The results show that the interaction of graft copolymer 1 with PVA is increased by introduction of longer poly(2-ethyl-2-oxazoline) side chains. Thermal decomposition (TG) measurements supported the compatibility of PVA with graft copolymer 1 and with PEtOZO, and showed that the thermal stability of PVA is improved upon blending with 1 or PEtOZO.

Globular carbohydrate macromolecule “sugar balls” 3. “Radial-growth polymerization” of sugar-substituted α-amino acid N-carboxyanhydrides (glycoNCAs) with a dendritic initiator

Abstract “Radial-Growth Polymerization (RGP)”, polymerization initiated with multivalent dendrimer to afford dendrimer-based star polymer, was proposed as a new class of polymerization systems. Oligoglycopeptide-type sugar balls, i.e., oligo [O-(β- d - glucopyranosyl )- l - serine]-persubstituted poly(amido amine) dendrimer ( 4a ) and oligo [O-(2- acetamido-2-deoxy -β- d - glucopyranosyl )- l - serine]-persubstituted poly(amido amine) dendrimer ( 4b ), were obtained by ring-opening oligomerization of sugar-substituted α-amino acid N -carboxyanhydrides (glycoNCAs), i.e., O- (tetra -O- acetyl -β- d - glucopyranosyl )- l - serine N -carboxyanhydride ( 1a ) and O-(2- acetamido-3,4,6-tri -O- acetyl-2-deoxy -β- d - glucopyranosyl )- l - serine N -carboxyanhydride ( 1b ), respectively, with poly(amido amine) dendrimer ( 2 , generation, 3.0–5.0) as a macroinitiator, followed by quantitative deacetylation with hydrazine monohydrate. Yields and M w M n values of the products of the oligomerization were 97–99% and 1.0 3 –1.1 1 , respectively. Sugar balls 4a and 4b were soluble in water and dimethyl sulfoxide. Molecular recognition ability of 4a and 4b was examined by erythrocyte agglutination inhibition assays using wheat germ agglutinin (WGA).

Synthesis of a novel star‐shaped dendrimer by radial‐growth polymerization of sarcosine N‐carboxyanhydride initiated with poly(trimethyleneimine) dendrimer

A novel ABn-type dendrimer/linear polymer block copolymer, i.e., poly(trimethyleneimine) dendrimer-block-(polysarcosine)64 (1), was synthesized by ring-opening polymerization of sarcosine N-carboxyanhydride initiated with the 64-NH2-terminal poly(trimethyleneimine) dendrimer as a macroinitiator. 1 has narrow molecular weight distributions (Mw/Mn = 1.01–1.03, by size exclusion chromatography) and controlled polysarcosine chain lengths (by varying the monomer/dendrimer feed molar ratios). Small-angle neutron scattering (SANS) data obtained in D2O solution of 1 (DPs of polysarcosine = 2.0 and 24) fitted well with a Guinier plot of a spherical particle, and gave diameters of 44 and 100 A, respectively.

Biodegradable polymers based on renewable resources. III. copolyesters composed of 1,4:3,6-dianhydro-D-glucitol, 1,1-bis(5-carboxy-2-furyl)ethane and aliphatic dicarboxylic acid units

Various copolyesters were synthesized by bulk polycondensation of the respective combinations of 1,4;3,6-dianhydro-D-glucitol (1) as the diol component and 1,1-bis[5-(methoxycarbonyl)-2-furyl]ethane (3b) and seven dimethyl dialkanoates with methylene chain lengths of 4, 5, 6, 7, 8, 10, and 12 (4a–4g) as the dicarboxylic acid components. Most of the copolyesters were amorphous, while a copolyester composed of 1, 3b, and dodecanedioic acid (4g) (3b:4g = 25:75) units as well as homopolyesters derived from 1 and azelaic acid (4d), sebacic acid (4e), and dodecandioic acid (4g), respectively, were partially crystalline. All these homo- and copolyesters were soluble in chloroform, dichloromethane, pyridine, trifluoroacetic acid, and m-cresol. The number-average molecular weights of these polyesters were estimated to be in the range of 10,000–20,000 by SEC using chloroform as an eluent and standard polystyrene as a reference. The biodegradability of these copolyesters was assessed by enzymatic degradation using four different enzymes in a phosphate buffer solution at 37°C and by soil burial degradation tests in composted soil at 27°C. In general, biodegradability of the copolyesters decreased with increase in the difuran dicarboxylate 3b content. Copolyesters containing sebasic acid 4e units showed higher biodegradability. Soil burial degradation in the soil that was treated with antibiotics, together with electron microscopic observation, indicated that actinomycetes are mainly responsible for the degradation of the copolyesters containing 3b units in the present soil burial test.

Synthesis of a Poly(vinyl alcohol)‐Based Graft Copolymer Having Poly(ε‐caprolactone) Side Chains by Solution Polymerization

Poly(vinyl alcohol)-graft-poly(e-caprolactone) (PVA-g-PCL) was synthesized by ring-opening polymerization of e-caprolactone with poly(vinyl alcohol) in the presence of tin(II) 2-ethylhexanoate as a catalyst in dimethyl sulfoxide. The relationship between the reaction conditions of the solution polymerization and the chemical structure of the graft copolymer was investigated. The degree of substitution (DS) and degree of polymerization (DP) of the PCL side chains were roughly controlled by varying the reaction periods and feed molar ratios of the monomer and the catalyst to the backbone. PVA-g-PCL with a PCL content of 97 wt.-% (DP = 22.8, DS = 0.54) was obtained in 56 wt.-% yield. The graft copolymer was soluble in a number of organic solvents, including toluene, tetrahydrofuran, chloroform, and acetonitrile, which are solvents of PCL. The molecular motion of the graft copolymer from 1 H NMR measurements appears to be restricted to some extent at 27-50 °C, however the 1 H NMR signal intensities measured at temperatures higher than ca. 50 °C reflect the actual chemical structure of the graft copolymer as determined by elemental analysis. The graft copolymer having a short PCL side chain (DP = 4.4, DS = 0.15) was amorphous. The melting temperature of a sample with relatively high PCL content (DP = 22.8, DS = 0.54) was observed at 39 °C. Thermogravimetric analysis revealed that the thermal stability of PVA was improved by introducing PCL side chains. The surface free energies of the air-side of a graft copolymer film, as calculated by Owens equation using contact angles, were comparable to that of PCL homopolymer.

A novel evaluation method for biodegradability of poly(butylene succinate-co-butylene adipate) by pyrolysis-gas chromatography

Biodegradation behavior of poly(butylene succinate-co-butylene adipate) (PBSA) film samples during the soil burial degradation test was studied by pyrolysis-gas chromatography (Py-GC). In the pyrograms of the PBSA film sample residues, various ester compounds containing succinate and/or adipate units were observed as the main pyrolysis products along with some minor pyrolyzates such as fatty-acid esters comprising propionates and valerates which might be formed mostly from carboxyl end-groups existing in PBSA molecules. Although the relative yields of the major pyrolyzates were almost unchanged before and after the soil burial test, those of the fatty-acid esters decreased with the soil burial time almost correlating with the decrease in recovery as residue. Thus, the variation of the relative yields of the fatty-acid esters proved to be a good measure to evaluate the degree of PBSA biodegradation. Furthermore, the local structural changes for the biodegraded PBSA film samples were also evaluated from the relative yields of these specific esters observed on the pyrograms for tiny pieces of analyte (ca. 0.1 mg) sampled from local points.

DNA-based polymer hybrids Part 1. Compatibility and physical properties of poly(vinyl alcohol)/DNA sodium salt blend

Transparent blend films of poly(vinyl alcohol) (PVA) and deoxyribonucleic acid (DNA) sodium salt from salmon testes were prepared by the solvent cast method from a homogeneous aqueous solution; as a new class of biopolymer-based hybrid materials. Differential scanning calorimetric (DSC), dynamic mechanical, thermogravimetric, and scanning electron microscopic analyses indicated that PVA and DNA are compatible in a wide range of compositions. As the DNA content increases, a melting peak of PVA reduces in intensity with a lower temperature shift, appearing at 203°C in the PVA/DNA (50 wt%) blend by DSC. Physical properties of the blend films were evaluated by tensile strength and contact angle measurements. The tensile strength values of PVA/DNA (10 wt%) and PVA/DNA (30 wt%) blend films were 56 and 48 MPa, respectively. The surface free energy of PVA/DNA (30 wt%) blend film was 46 dyn/cm, which is identical to that of PVA, while the pure DNA film was revealed to show hydrophobicity (surface free energy 32 dyn/cm; water contact angle 104°).