The new monomer 1,2-o-isopropylidene-[d]-xylofuranose-3,5-cyclic carbonate (IPXTC) was prepared. The organometallic catalysts AlR3−H2O (R = ethyl, isobutyl), ZnEt2−H2O, and Sn(Oct)2 were evaluated for the copolymerization of [l]-lactide ([l]-LA) with IPXTC. This work showed that Sn(Oct)2 was preferred for the formation of high molecular weight copolymers. For example, a copolymerization ([l]-LA/IPXTC = 83:17 mol/mol) at 120 °C for 6 h gave poly([l]-LA-co-7 mol % IPXTC) with an M n and polydispersity (M w/M n) of 78 400 and 1.9, respectively. The comonomer reactivity ratios were 4.15 and 0.255, respectively, for [l]-LA and IPXTC copolymerizations conducted at 120 °C, M/C = 200, and Sn(Oct)2 as catalyst. Structural investigations by NMR revealed that [l]-LA/IPXTC copolymers had short average IPXTC repeat unit segment lengths. Increased copolymer IPXTC content resulted in products with lower melting transition temperatures but higher glass transition temperatures. To obtain hydroxyl functionalized P([l]-LA) copolymers, the pendant IPXTC ketal protecting group was removed. The deprotection was performed in CH2Cl2 using CF3COOH/H2O without substantial molecular weight decrease. Hence, an efficient route has been developed to synthesize high molecular weight PLA-based copolymers that consist of [l]-lactic acid and [d]-xylofuranose repeat units. The [d]-xylofuranose repeat units have vicinal diol groups that will facilitate further functionalization and modification of these copolymers. The “tailorability” of the new copolymers is expected to be of great value for the development of important new bioresorbable medical materials.

Polycarbonates were synthesized by ring-opening copolymerizations of trimethylene carbonate (TMC) with 1,2-O-isopropylidene-d-xylofuranose-3,5-cyclic carbonate (IPXTC). Subsequent deprotection of the ketal protecting groups gave controlled quantities of vicinal diol pendant groups. Studies of TMC/IPXTC copolymerization showed that MAO and ZnEt2−0.5H2O were the preferred catalysts. The reactivity ratios measured by the Fineman−Ross method and using ZnEt2−0.5H2O as the catalyst were 0.31 and 0.20 for IPXTC and TMC, respectively. Hence, even though IPXTC has bulky substituents, IPXTC was more reactive than TMC early in the copolymerization. Consistent with the above, the average IPXTC chain segment length was longer early in the copolymerization but decreased with increased conversion. 1H and 13C NMR were used to analyze the repeat unit sequence distribution of copolymers. For copolymers with high IPXTC contents, three types of IPXTC linkages were found:  head−head, tail−tail, and head−tail. The protecting ketal groups were removed by CF3COOH/H2O to give a novel polycarbonate with hydroxyl pendant groups. Longer deprotection times led to higher extents of deprotection but lower molecular weight. Studies by differential scanning calorimeter (DSC) showed that copolymers having from 8 to 83% IPXTC were all amorphous. In addition, a physical aging transition was apparent. The T g of the copolymer increased with increasing IPXTC copolymer content. Furthermore, the experimental T g values were in good agreement with that calculated by the Fox equation. After deprotection, the copolymer T g decreased, which is consistent with the loss of the bulky ketal side group.

2,2-(2-Pentene-1,5-diyl)trimethylene carbonate ( c HTC) was synthesized from cyclohexene-4,4-dimethanol in high yield (>80%). This new carbonate monomer was successfully ring-open polymerized to form P( c HTC) in bulk at 90 °C using various organometallic catalysts including aluminoxanes (methyl and isobutyl), BunSnCl4-n (n ) 1, 2, 3), BunSn(OMe)4-n (n ) 2, 3), ZnEt2, and ZnEt2-H2O. Comparison of these systems showed that the Zn-and Al-based catalysts were preferred for the preparation of high molecular weight polymers in yields g89% and reaction times of e8 h. For the BunSnX4-n catalysts investigated, values of n ) 1 when X is Cl and n ) 2 when X is OMe resulted in relatively greater polymerization rates and higher polymer molecular weights. The effects of reaction time and monomer/ catalyst molar ratio were investigated for the Al and Zn catalysts. An outcome of this study was determining that the ZnEt2-H2O (1/0.5) catalyst for a monomer/catalyst (M/C) molar ratio of 400 and a 2 h reaction time gave a product with Mn ) 276 000 in 98% yield. The P( c HTC) products were characterized by FTIR, 1 H-NMR, 13 C-NMR, DSC, TGA, and GPC. NMR results showed that c HTC decarboxylation did not occur during chain propagation. P( c HTC) has a moderate glass transition temperature (Tg ) 30 °C) with high thermal stability. 13 C NMR at 62.5 MHz did not resolve chain diad sequences although the polymers are likely atactic. Epoxidation of P( c HTC) vinyl side groups was carried out to various extents by using 3-chloroperoxybenzoic acid at room temperature.

[l]-Lactide ([l]-LA)/ethylene oxide (EO) ring-opening copolymerizations were successfully carried out by using various catalysts including isobutylaluminoxane (IBAO), in situ AlR3·0.5H2O systems (R = ethyl, isobutyl) and Sn−Al bimetallic catalysts. Analysis of products by 1H NMR showed that methanol insoluble copolymer fractions had multiblock structures. The multiblock segment length and molecular weight of the copolymers were regulated by a variation in the reaction temperature, reaction time, reaction medium, and the catalyst structure. An increase in the reaction temperature was used to obtain shorter segment block lengths. Bulk reactions at elevated temperatures gave shorter block lengths than those of corresponding polymerizations conducted in solution (xylene). Differential scanning calorimetry (DSC) results showed two melting transitions corresponding to poly(ethylene oxide) (PEO) and [l]-polylactide ([l]-PLA) crystalline phases. The melting temperature and enthalpy of fusion for the phase-separated [l]-PLA crystalline phase was “tailored” by modulating the copolymer composition and the [l]-PLA block length. Blends with PLA were prepared by substituting poly(ethylene glycol) (PEG) with a high EO content [l]-PLA/EO multiblock copolymer. The idea explored was that the multiblock copolymers would be expected to leach into aqueous environments at a slower rate than PEGs. Substitution of the [l]-PLA/EO copolymer in place of PEG resulted in important increases in the film modulus and yield strength without loss in elongation at yield, break stress, and elongation at break. Thus, we demonstrated a versatile route to important new multiblock [l]-PLA/EO copolymers which have excellent potential to be useful for a wide range of biomedical applications including bioresorbable implant materials and tissue engineering. Furthermore, the synthetic methods developed herein provide routes which will be useful in “fine-tuning” product physicomechanical properties and degradation rates.

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