Optical nonlinear susceptibilities were measured in Poly(phenylene ethynyl silane) (PPES1) and Poly(phenylene ethynyl disilane) (PPES2) at 590nm and 1064nm. Both the real and imaginary parts of chi((3)) were evaluated with the Z scan technique using closed and open apertures, respectively. Closed aperture scans of PPES1 exhibited the classic negative Re chi((3)) peak valley signature, whereas, closed aperture scans of PPES2 displayed interesting additional features. These features are present at both 590nm and 1064nm, and indicate the presence of alternating sign higher order nonlinearities with negative chi((3)), positive chi((5)), and negative chi((7)).