Origin of catalytic activity differences between phosphine and phosphine oxide-based structures in the water-crosslinkable polyalkoxysilane composition

Organocatalysts have attracted enormous interest in the water-crosslinking reaction in silane-grafted polyolefins (SGPOs) system owing to their simplicity, low toxicity and environmentally benign nature compared to the organotin catalysts most commonly used in the SGPOs system. We focus on organophosphorus compounds including four structure types as organocatalysts; phosphoric acids, phosphoric esters, phosphine oxides and phosphine. Their catalytic activities for the water-crosslinking reaction in the 3-methacryloxypropyltrimethoxysilane grafted ethylenepropylene copolymer (EPR-g-MTMS) system were evaluated using the ATR-FTIR technique and the gel-fraction method. Phosphine oxides, phosphoric acids and phosphoric esters possessing an O=PR3 or O=P(OR’)3 unit were found to be an excellent catalyst for the water-crosslinking reaction in the EPR-g-MTMS system, while phosphine (PR3) showed no catalytic activity on the water-crosslinking reaction in this system, indicating that the phosphoryl (P=O) unit plays an important role on the catalytic performance of these compounds. In comparison, phosphine oxides showed considerably higher catalytic activities than phosphoric acids/esters. Density functional theory (DFT) calculations demonstrated that the difference in catalytic activity could be attributed to an electron density at P=O unit making the activation for water through hydrogen-bonding. Finally, a possible catalytic mechanism for phosphoryl compounds in the EPR-g-MTMS system was proposed on the basis of these results and the SN2-Si pathway in silicate sol-gel chemistry.