Synthesis of phosphate monomers and bonding to dentin: Esterification methods and use of phosphorus pentoxide

Objectives The aim of this study was to synthesize an acidic monomer using an alternative synthetic pathway and to evaluate the influence of the acidic monomer concentration on the microtensile bond strength to dentin. Methods The intermediary 5-hydroxypentyl methacrylate (HPMA) was synthesized through methacrylic acid esterification with 1,5-pentanediol, catalyzed by p-toluenesulfonic acid. To displace the reaction balance, the water generated by esterification was removed by three different methods: anhydrous sodium sulfate; molecular sieves or azeotropic distillation. In the next step, a phosphorus pentoxide (4.82 mmol) slurry was formed in cold acetone and 29 mmol of HPMA was slowly added by funnel addition. After the reaction ended, solvent was evaporated and the product was characterized by 1HNMR and FTIR. The phosphate monomer was introduced in a self-etch primer at concentrations of 0, 15, 30, 50, 70 and 100 wt%. Clearfil SE Bond was used as commercial reference. Microtensile bond strength to dentin was evaluated 24 h after the bonding procedures, followed by fracture analysis (n = 20). Data was submitted to ANOVA and Tukeyʹs post hoc test. Results The highest yield was obtained (62%) when azeotropic distillation was used, while the reaction with molecular sieves was not feasible. The phosphoric moiety attachment to the monomer was successfully performed with a quantitative yield that reached around 100%. The acidic monomer concentration significantly affected the bond strength and the highest mean (55.1 ± 12.8 MPa) was obtained when 50% of acidic monomer was used. Conclusion The synthesis pathways described in the present study appear to be a viable alternative for developing phosphate monomers.