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Radical polymerizations are used in a wide variety of fields ranging from the fabrication of coatings and adhesives, and more recently for medical use as biomaterials (e.g., bone cements, dental implants) and it is important to better understand their networks for better design.  In recent years, for use in the fields of drug delivery and tissue engineering, degradable polymers that form via a radical polymerization are being developed.  In addition to expanding the overall potential of radically polymerized materials, these new polymers open up avenues to further investigate fundamental structure-property relationships between precursor molecules and final network properties.  Specifically, we proposed to utilize the controlled synthesis of a novel macromer that forms a crosslinked polymer via a radical polymerization to better understand how the macromer structure influences the reaction behavior, degradation, and mechanical properties of formed networks while using isolation and analysis of network kinetic chains to gain more insight into this behavior. 

During the first year, macromers with a variety of molecular weights, branching, and functionalization (i.e., number of reactive groups) were synthesized and polymerized into networks and the reaction behavior and final network properties (i.e., mechanics in tension and compression, glass transition temperature, degradation behavior) were characterized.  To accomplish the aims of this work, a tough biodegradable elastomer, poly(glycerol sebacate) (PGS) was modified with acrylates to impart control over the crosslinking process and allow for more processing options. The diversity in final properties was determined by synthesizing several versions of this polymer.  In general, the Young’s modulus increased as the degree of acrylation increased for a given molecular weight.  As expected, the % strain at break increased as the Young’s modulus and % acrylation decreased for a given molecular weight.  Furthermore, the Young’s modulus and % strain at break increased with increasing molecular weight for similar degrees of acrylation.   Relationships between macromer structure and network properties can easily be drawn from this data.  For instance, increases in the % acrylation led to increases in the number of crosslinks formed, which is associated with an increase in the modulus of a resulting material and a decrease in the ability to elongate a material before failure.  Additionally, more elastomeric-like features were obtained as the molecular weight of the prepolymer is increased.

This past year, networks were further assessed by isolating kinetic chains from crosslinked networks and trying to correlate the influence of the initial macromer structure on the resulting network.  Three macromers were synthesized that had the same molecular weight, but variations in the extent of acrylation (~1% up to ~24%).  These macromers were polymerized using a radical photoinitiated polymerization process into 1 mm thick slabs.  Subsequently, the samples were degraded in a basic solution and neutralized.  The resulting solutions were then processed for gel permeation chromatography.  However, there were no differences observed in the length of degradation products, presumed to be primarily small molecules and kinetic chains.  There are several potential issues with this approach since the thick samples will exhibit some light attenuation with depth, which will increase the polydispersity of the chains and that the light used is not necessarily uniform across the samples.  Ongoing work is investigating the polymerization of thin films using collimating adapters to more closely control for light exposure throughout the sample.  Additionally, variations of the initial macromer molecular weight are being investigated.

In addition to these basic studies of network formation and degradation, the materials being investigated are being developed as tunable scaffolds for biomedical applications.  Basically, it is presumed that variations in the properties in these bulk scaffolds can be translated to changes in the fibrous scaffolds.  Three macromers were synthesized that form networks that vary dramatically with respect to their modulus (~30 kPa to 6.6 MPa) and degradation behavior (~20 to 100% mass loss at 12 weeks) based on the extent of acrylation (~1 to 24%).  These macromers were processed into biodegradable fibrous scaffolds using electrospinning, with gelatin as a carrier polymer to facilitate fiber formation and cell adhesion.  The resulting scaffolds were also diverse with respect to their mechanics (Young modulus ranging from ~60 kPa to 1 MPa) and degradation (~45 to 70% mass loss by 12 weeks).  Mesenchymal stem cell adhesion and proliferation on all fibrous scaffolds was indistinguishable from controls. The scaffolds showed similar diversity when implanted on the surface of hearts in a rat model of acute myocardial infarction and demonstrated a dependence on scaffold thickness and chemistry in the host response.  In summary, these diverse scaffolds with tailorable chemical, structural, mechanical and degradation properties are potentially useful for the engineering of a wide range of soft tissues.

Overall, funding from this type G starter grant from the Petroleum Research Foundation has helped advance our fundamental understanding of crosslinked polymers, as well as to further advance our efforts in the design of biodegradable networks for biomedical applications.  This funding has been very important as a young faculty to help advanced my research program.