No abstract

High-strength, surface-eroding polymers were synthesized from methacrylated anhydride monomers of sebacic acid (MSA) and 1,6-bis(carboxyphenoxy) hexane (MCPH). These multifunctional monomers were photopolymerized using ultraviolet light to produce highly crosslinked polyanhydride networks. Through this approach, the crosslinking density of the resulting polymer network was used to control the final mechanical properties, while the degradation time scale was controlled by the chemical composition of the network. The combined hydrophobicity of the polymer backbone with the hydrolytically labile anhydride linkages led to surface-eroding networks, as confirmed by linear cumulative mass loss profiles as a function of degradation time for crosslinked polymer disks. By copolymerizing varying amounts of MSA and MCPH, the degradation rate of the final network was controlled from 2 days to 1 year. The tensile modulus of crosslinked poly(MSA) (1.4 GPa) was nearly an order of magnitude larger than that of linear poly(sebacic acid). In general, the mechanical properties of the crosslinked polyanhydrides networks were within ranges of those reported for cortical and trabecular bone. However, unlike bulk degrading polyesters such as poly(lactic acid), these surface eroding networks maintained >70% of their tensile modulus with 50% mass degradation.

The polymerization behavior of a new class of dimethacrylated anhydride monomers that react to form highly cross-linked degradable networks was investigated using various photoinitiation schemes. Polymerizations occurred in seconds to minutes depending on the initiating conditions, and conversions in excess of 0.95 were achievable. A photobleaching visible light initiating system was used to improve the depth of cure for the production of polymers with appreciable dimensions. One potential application for the proposed multifunctional monomers is in vivo curing of high-strength, degradable polymers for fracture fixation or filling of trabecular bone defects.

A unique class of surface-eroding polyanhydrides was developed and explored for use in medical applications requiring high-strength biomaterials (e.g., orthopedics). In particular, dimethacrylated anhydride monomers were synthesized that photopolymerize quickly to render densely crosslinked polymer networks that degrade from the surface only by hydrolysis of labile anhydride linkages. Previous research on these materials has shown that the rate of hydrolysis of the degradable linkages is dependent on the hydrophobicity of the network composition. This article demonstrates the versatility in controlling the degradation process and resulting cellular response in these materials through the incorporation of new chemistries and the formation of polymer-polymer composite structures. Specifically, the rate of mass loss was controlled by the addition of hydrophobic linear polymers [e.g., poly(methyl methacrylate)] or monovinyl monomers based on hydrophobic natural components (e.g., cholesterol, steric acid). In addition, a newly established photografting method was used to modify the network surface chemistry with cholesterol- and stearic acid-based polymer grafts to control the degradation front and cellular interactions at the polymer-tissue interface. Finally, a porogen leaching method was used to form porous polyanhydride constructs, which can be subsequently filled with osteoblasts photoencapsulated in a hydrogel, as potential synthetic allograft materials for tissue engineering bone.

Multifunctional anhydride monomers were synthesized and photopolymerized to form highly cross-linked, degradable networks. The networks were synthesized from monomers and oligomers of dimethacrylated sebacic acid of varying molecular weight, as well as under varying reaction conditions. The cross-linked polymers were subsequently degraded in phosphate buffered saline, and the degradation products, sebacic acid and poly(methacrylic acid), were isolated. Matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) was used to characterize the absolute molecular weight distribution of the linear poly(methacrylic acid) degradation product, especially as a function of the network evolution (i.e., double-bond conversion), rate of initiation, and monomer size. MALDI-TOF results, supported by 1H NMR, showed that the distribution of kinetic chain lengths was relatively narrow, with average lengths shorter than calculated from experimentally measured rate data, indicating the influence of diffusion-controlled kinetics as well as chain transfer. Furthermore, the average kinetic chain length shifted to lower values with increasing initiation rate and double-bond conversion. Since multifunctional monomer polymerizations are extremely complex and notoriously difficult to characterize due to the insoluble nature of the resulting cross-linked polymer structure, this work demonstrates, for the first time, how these degradable monomers can provide further insight and characterization of multifunctional monomer polymerizations.