Modification Of Poly (D, L-Lactic Acid) With Carboxymethyl Chitosan And Chitosan By Copolymerization Or Blending

The copolymers of poly(D, L-lactic acid) (PLA) and carboxymethylchitosan were synthesized by two approaches, 1) grafting of carboxymethylchitosan with PLA as a side chain at the carboxymethyl groups ofcarboxymethyl chitosan through the ring-opening reaction of lactide withstannous octoate as a catalyst; 2) Schiff reaction of the aldehyde groups ofoligo(L-lactic acid)-aldehyde (OLLA-CHO) to form covalent imine bondswith the amino groups of carboxymethyl chitosan. OLLA-CHO was preparedby the oxidation of OLLA-OH with acetic anhydride in the solvent mixture ofdimethylsulfoxide (DMSO) and chloroform (9:1, v/v). The dihydroxyl oligo(L-lactic acid) (OLLA-OH) was prepared by the esterification ofoligo(L-lactic acid) and 1,6-hexanediol in the presence of strong cationicexchange resin. OLLA was synthesized by the condensation polymerization oflactic acid with stannous chloride as a catalyst. The obtained copolymers werecharacterized by FTIR analysis and the solubility experiments of thecopolymers in water, DMSO, DMF, acetic ether, and toluene, primarilyvalidating that the two copolymerization approaches could be used formodification ofpolylactic acid. The controlled release formulations of imidacloprid were prepared by amelt method using two low viscosity-average-relative-molecular-mass poly(D,L-lactic acid) (PLA) alone or in the combination with chitosan at differentratios (5: 1, 10: 1, 20: 1, 40:1 ) as matrix materials. The release kineticsof the formulations were investigated in vitro, the degradation of matrix in theformulations was estimated by determining the pH values of water samplesobtained during release experiment, and the erosion of matrices was evaluatedby measuring the dry mass of the formulation residues collected at the end ofthe release experiment, and the release mechanisms were also evaluated byfitting the release data to a generalized model. The results indicated that thePLA with relatively low viscosity-average relative molecular mass could beused alone or in the combination with chitosan for the controlled release ofimidacloprid, showing a slow release controlled by diffusion and erosion and afast release controlled by erosion. An increase in the relative molecular massof PLA resulted in a reduction in the release rates of imidacloprid, and atransition from an initial fast release followed by a slow release to an initialslow release followed by a fast constant release. Reduction in the relativemolecular mass of PLA and the incorporation of chitosan in PLA matrix couldalter the degradation and erosion of PLA, and cause an earlier occurrence ofthe fast constant release. Reduction in the viscosity of chitosan incorporated inthe PLA matrix, however, could only cause increase in the erosion rates andthe early occurrence of the fast constant release to a certain extent. The incorporation of chitosan in the matrix of PLA with Mr/of 10306 at differentratios could alter the release water to neutral, the initial time of fast releasefrom 117h to 336h, and the ts0 value from 228h to 445h. The results indicatedthat the incorporation of chitosan in the PLA-based matrix could be used tomanipulate the rates of degradation and erosion of matrix, leading to theregulation of the release rates of imidaclopride. Low-relative-molecular-massPLA-chitosan matrix could thus be useful to produce multifunctionalcontrolled release formulations for seed treatment.