Abstract:We have developed a novel organocatalytic, direct synthesis of aliphatic polyesters. The synthesis was based on a novel direct organic acid-catalyzed living ring-opening polymerization (ROP) of ecaprolactone and d-valerolactone with benzyl alcohol as the initiator. The organic acid-catalyzed ROPs were performed without solvent and the catalyst was recovered by precipitation of the polyester products. In addition, they were operationally simple, inexpensive and environmentally benign. Organic acid catalysis provides a new entry for the synthesis of valuable biodegradable materials as well as polyesters for biomedical applications.

Cationic catalysis using simple carboxylic acids to combine the ring-opening polymerization of ε-caprolactone and the regioselective acylation of carbohydrates has been investigated. l-Lactic acid catalyzed the acylation of methyl β-d-glucopyranoside and sucrose with ε-caprolactone in high yield by bulk polymerization at 120 °C. The main products were regioselectively acylated on the primary hydroxyl groups of the carbohydrate end groups. The overall conversion to methyl β-d-glucopyranoside-functionalized poly(ε-caprolactone) was more than 90%, M w 2000 and polydispersity index 1.5, with one major product methyl 6-O-poly(ε-caprolactone)-β-d-glucopyranoside, in agreement with the corresponding Candida antarctica lipase B-catalyzed acylation.

The copolymerization of epsilon-caprolactone (epsilon-CL) and d,l-lactide catalyzed by Candida antarctica lipase B was studied. Copolymerizations with different epsilon-CL-to-lactide ratios were carried out, and the product was monitored and characterized by MALDI-TOF MS, GPC, and (1)H NMR. The polymerization of epsilon-CL, which is normally promoted by C. antarctica lipase B, is initially slowed by the presence of lactide. During this stage, lactide is consumed more rapidly than epsilon-CL, and the incorporation occurs dimer-wise with regard to the lactic acid (LA) units. As the reaction proceeds, the relative amount of CL units in the copolymer increases. The nonrandom copolymer structure disappears with time, probably due to a lipase-catalyzed transesterification reaction. In the copolymerizations with a low content of lactide, macrocycles of poly(epsilon-caprolactone) and copolymers having up to two LA units in the ring were detected.

The porosity and the available surface area of a lignocellulosic fiber can influence the accessibility and reactivity in derivatization and modification reactions because the porous cell-wall network determines the upper size limit for molecules that can penetrate and react with the interior of the wall. To obtain information concerning the accessibility of the porous cell wall of wood fibers, surfactant-templated sol-gel mineralization has been examined. Wood and kraft pulp samples of Norway spruce were impregnated with a silica sol-gel and subsequently heated (calcined) and transformed into structured mesoporous silica. Microscopy studies (environmental scanning electron microscopy, transmission electron microsopy, TEM) on the silica casts showed that the three-dimensional architecture of the wood and pulp fiber cell wall was revealed down to the nanometer level. Image analysis of TEM micrographs of silica fragments from the never-dried pulp revealed complete infiltration of the cell-wall voids and microcavities (mean pore width 4.7 +/- 2 nm) by the sol-gel and the presence of cellulose fibrils with a width of 3.6 +/- 1 nm. Cellulose fibrils of the same width as that shown by image analysis were also identified by nitrogen adsorption measurements of the pore size distribution in the replicas.

We have developed a direct organocatalytic synthesis of a dendrimer-like star polyester. The synthesis was based on lactic acid-catalyzed bulk ring-opening polymerization (ROP) of ε-caprolactone (ε-CL) with the hexahydroxy-functional dendrimer 2,2-bis(hydroxymethyl)propanoic acid as the initiator. The polymerization was highly chemoselective, and the catalyst was readily recovered by precipitation of the polyester products. The reaction was performed without solvent and without the need of an inert atmosphere. In addition, the ROPs are operationally simple, inexpensive, and environmentally benign. Organic acid catalysis provides a new entry for the synthesis of valuable dendritic materials as well as polyesters for biomedical applications.