ATRP grafting of styrene from benzyl chloride functionalized polysiloxanes: An AFM and TGA study of the Cu(0)/bpy catalyst

Various combinations of Cu(0), CuCl, 2,2'-bipyridine (bpy) and 1,10-phenanthroline (phen) were used as catalysts for the grafting polymerizations of styrene from polysiloxane macroinitiators functionalized with benzyl chloride. While Cu(0)/bpy alone promotes the grafting, narrower polydispersities were obtained in the presence of CuCl. Analysis of the Cu(0) surface before and after polymerization by a combination of AFM, TGA and FTIR investigations reveals the formation of bpy or phen films on Cu(0). In the presence of CuCl, the ligand film appears decorated with CuCl particles which increase in size with increasing the CuCl concentration. The initial layer occurs most likely as a result of complexation between the ligands and the Cu(0) surface and acts as a support for the rest of the film. These observations are consistent with the film formation on Cu(0) from related nitrogen donors and indicate that the reactivity of the Cu surface may depend not only on its prior treatment but also on the deposition of ligands from the reaction Mixture. (c) 2005 Elsevier Ltd. All rights reserved

Mild-Temperature Mn₂(CO)₁₀-Photomediated Controlled Radical Polymerization of Vinylidene Fluoride and Synthesis of Well-Defined Poly(vinylidene fluoride) Block Copolymers

By contrast to typical high-temperature (100–250 °C) telo-/polymerizations of gaseous fluorinated monomers, carried out in high-pressure metal reactors, the visible light, Mn₂(CO)₁₀-photomediated initiation of vinylidene fluoride (bp = −83 °C) polymerization occurs readily from a variety of alkyl, semifluorinated, and perfluorinated halides at 40 °C, in low-pressure glass tubes and in a variety of solvents, including water and alkyl carbonates. Perfluorinated alkyl iodide initiators also induce a controlled radical polymerization via iodine degenerative transfer (IDT). While IDT proceeds with accumulation of the less reactive P_<i>m</i>-CF₂-CH₂-I vs the P_<i>n</i>-CH₂-CF₂-I chain ends, Mn₂(CO)₁₀ enables their subsequent quantitative activation toward the synthesis of well-defined poly­(vinylidene fluoride) block copolymers with a variety of other monomers

Metal and Ligand Effects of Photoactive Transition Metal Carbonyls in the Iodine Degenerative Transfer Controlled Radical Polymerization and Block Copolymerization of Vinylidene Fluoride

The metal and ligand effect of a series of transition metal carbonyls in conjunction with alkyl and perfluoroalkyl halides was investigated in the initiation and control of the visible light, radical photopolymerizations of vinylidene fluoride (VDF) and respectively, in the synthesis of PVDF block copolymers. No polymerization was observed for CpMn­(CO)_3, CpCo­(CO)₂, Cp₂Fe₂(CO)_4, Cp*₂Cr₂(CO)_4, Mo­(CO)₆, Fe­(CO)_5, Cr­(CO)₆, Co₂(CO)₈, Co₄(CO)₁₂, Fe₃(CO)₁₂, Ru₃(CO)₁₂, (PPh₃)₂Ni­(CO)₂, Cp₂Ti­(CO)₂, and Au­(CO)­Cl. A free radical polymerization, and respectively an iodine degenerative transfer, controlled radical polymerization was obtained for Mn₂(CO)₁₀ ∼ Re₂(CO)₁₀ ≫ Cp₂Mo₂(CO)₆ ≫ Cp₂W₂(CO)₆ with CH₃(CH₂)₅–Br, CH₃(CH₂)₅–I, CH₃–I, CCl₃–Cl, CCl₃–Br, Br–(CF₂)₆–Br, and respectively with CF₃(CF₂)₃–I and I–(CF₂)_4,6–I. Furthermore, while Fe­(CO)₅, Cp*Cr₂(CO)₄ and Co₄(CO)₁₂ led to ∼ CF₂–I bond insertion, Re₂(CO)₁₀, Mn₂(CO)₁₀, Cp₂W₂(CO)₆, Cp₂Mo₂(CO)₆ and Cp₂Fe₂(CO)₄ provided quantitative radical activation of both PVDF–CH₂–CF₂–I and PVDF–CF₂–CH₂–I chain ends, and were employed in the synthesis of well-defined ABA triblock PVDF copolymers with vinyl acetate, <i>tert</i>-butyl acrylate, methyl methacrylate, isoprene, styrene, and acrylonitrile

In Vitro Kinetic Degradation and In Vivo Biocompatibility Evaluation of Polycaprolactone-Based Growth Factor Delivery Matrices in the Rotator Cuff

The recent years have seen a significant surge in the use of synthetic biodegradable polymers for growth factor delivery in the rotator cuff. While these polymers have been successfully applied in delivery of factors in other tissues, the anatomical complexity, hypovascularity, cellularity, and reduced clearance of degradation by-products in the rotator cuff, creates unique requirements in tailoring the physical dimensions, chemical constituents, drug release and degradation characteristics of biomaterials for implantation. In this study, we investigate poly-lactic acid co-epsilon-caprolactone (30:70 LA:CL ratio) at 35-45kDa range and varying polymeric films casting concentrations (5-20%) as potential growth factor delivery matrices in the rotator cuff. Matrices were fabricated of 300µm thickness and 3x3mm surface area to facilitate model protein encapsulation and controlled release, and smooth translation of the matrix under the bony acromion after implantation in the rotator cuff. The matrix with the highest casting concentration (20wt%) showed unique, highly regular, and controlled release of the protein payload compared to the lower- casting concentrations (15 and 10wt%) and - molecular weights (35kDa) matrices. All films were found to lose molecular weight rapidly during the first 4 weeks due to the preferential hydrolysis of lactide-rich regions within the polymer, and then maintain a relatively stable molecular weight between week 4 and 8 due to the emergence of highly-crystalline caprolactone-rich regions. Nevertheless, the cleaved lactide-chains were not small enough to exit through the polymeric matrix as was evident from the maintenance of bulk matrix weight, form, and polymer dispersity index. This resulted in recrystallization of the cleaved chains in the presence of water molecules increasing the crystallinity of the matrix as was evident from the H-NMR and thermal analysis. Kinetic analysis revealed an inverse-linear relationship between polymer casting concentration and polymer break down. The ‘context-dependent’ biocompatibility evaluation was carried in a clinically-relevant rat model of acute rotator cuff repair model to address the unique features of both the tissue and the biomaterial being investigated. The matrices were found to remodel locally without undergoing catastrophic breakdown or causing excessive inflammatory reaction at the tissue site during the study period and is anticipated to completely degrade within 6 months of implantation. Our study is significant as it provides a systematic assessment of polymer properties that can be modifies to engineer morphogen release, degradation rates and mechanisms for biologic delivery in the rotator cuff. It also provides a pilot assessment of in situ biocompatibility of the polymeric matrix in the complex rotator cuff tissue

Stabilization of Graphene Sheets by a Structured Benzene/Hexafluorobenzene Mixed Solvent

Applications requiring pristine graphene derived from graphite demand a solution stabilization method that utilizes an easily removable media. Using a combination of molecular dynamics simulations and experimental techniques, we investigate the solublization/suspension of pristine graphene sheets by an equimolar mixture of benzene and hexafluorobenzene (C₆H₆/C₆F₆) that is known to form an ordered structure solidifying at 23.7 °C. Our simulations show that the graphene surface templates the self-assembly of the mixture into periodic layers extending up to 30 Å from both sides of the graphene sheet. The solvent structuring is driven by quadrupolar interactions and consists of stacks of alternating C₆H₆/C₆F₆ molecules rising from the surface of the graphene. These stacks result in density oscillations with a period of about 3.4 Å. The high affinity of the 1:1 C₆H₆/C₆F₆ mixture with graphene is consistent with observed hysteresis in Wilhelmy plate measurements using highly ordered pyrolytic graphite (HOPG). AFM, SEM, and TEM techniques verify the state of the suspended material after sonication. As an example of the utility of this mixture, graphene suspensions are freeze-dried at room temperature to produce a sponge-like morphology that reflects the structure of the graphene sheets in solution