4-O-methylglucuronoxylan, isolated by ultrafiltration from a birch kraft cooking liquor, was delignified and subsequently modified by grafting lactide-chains onto the hydroxyl groups via ring opening polymerization with L-lactide. The structures and average molar masses of the lactide-grafted xylan polymers were characterised by NMR and SEC respectively. By varying the lactide-to-xylan feed ratio, graft polymers with different graft levels were synthesized. The degree of substitution of the hydroxyls in the xylan back-bone ranged from 0.7 to 1.7 with lactide side-chains having an average length of 1.2 to 2.5 lactide units. The side-chain length and degree of substitution influenced the hydrophobicity, thermal properties and tensile strength properties. A glass transition could be detected for lactide-grafted xylan polymers with lactide chains longer than 1.8 lactide units. Solution-cast thin films prepared from lactide-grafted xylans exhibited strong tensile strength and high modulus with decreasing strength and increasing elongation at break as the lactide chain length was increased.

Birch xylan (4-O-methylglucuronoxylan) isolated from a kraft cooking liquor was delignified and grafted with polylactide of predictable branch length. This graft copolymerization resulted in very high total yields, greater than 90%, and with less than 10% polylactide homopolymer byproducts. Mild reaction conditions (40 degrees C, 5 to 120 minutes) were used, which was believed to limit transesterification reactions and thus make it possible to reach good predictability of the polylactide branch length. The thermal properties of the polylactide-grafted xylan depended on the branch length. Short branches resulted in fully amorphous materials with a glass transition temperature of about 48 to 55 degrees C, whereas long polylactide branches resulted in semi-crystalline materials with melting points of about 130 degrees C. Using mixtures of L-lactide and D/L-lactide in the monomer feed further altered the thermal properties. The degradation temperatures of the polylactide-grafted xylans were higher than that of the unmodified xylan, with degradation temperatures of about 300 degrees C and 250 degrees C, respectively. Tensile testing showed increased elongation at break with increasing branch length. The proposed method thus enables tailor-making of copolymers with specific thermal and mechanical properties.