Palladium(II) Complexes of 1‐Phosphaisoprene Molecules and Polymers: Reversible Cross‐Linking of a Phosphorus‐Containing Polymer

The anionic polymerization of 1‐phosphaisoprene [Mes*P=C(Me)−CH=CH2 (E‐1)] affords poly(1‐phosphaisoprene) 2 in high yield (75 %). Concentrated solutions of polymer 2 (Mn=21,800 g mol−1; Đ=1.02) a P‐analogue of natural rubber, undergo gelation upon treatment with [Pd(cod)Cl2] (0.15 P equiv). Evidence for P‐coordination of 2 to PdII was obtained by 31P and 1H NMR spectroscopy. The gelation is reversed by the addition of PMe3 and the reformation of recoverable 2 along with [PdII−PMe3] complexes were confirmed by 31P NMR spectroscopy. The use of labile metal‐ligand bonds to reversibly form gels is unprecedented and has relevance to self‐healing materials. In contrast, coordination of 2 to [Pd(η3‐C3H5)(μ‐Cl)]2 affords the well‐defined complex 2 ⋅ [Pd(η3‐C3H5)Cl] which was characterized by 31P, 1H, 13C{1H} NMR spectroscopy and GPC. This polymer chemistry was complemented by detailed molecular model studies of the coordination chemistry of monomer 1‐phosphaisoprene E‐1 with [Pd(cod)Cl2] and [Pd(η3‐C3H5)(μ‐Cl)]2]. Palladium(II) complexes of phosphorus rubber [poly(1‐phopshaisoprene)] are disclosed. Addition of [Pd(cod)Cl2] to a solution of poly(1‐phosphaisoprene) resulted in gelation of the material which was reversed by the addition of a competing molecular phosphine ligand. By contrast, the macromolecular complex formed from the addition of [Pd(η3‐C3H5)(μ‐Cl)]2 to poly(1‐phospaisoprene) did not exhibit gelation. These results suggest cross‐linking of the polymer facilitates gel formation.