7-Acryloyloxy-4-methylcoumarin (AMC) can undergo a [2 + 2] photodimerization when irradiated with an U-V light of 310 nm. Its copolymerization with methyl methaerylate (MMA) resulted in a photo-cross-linkable copolymer P(MMA-co-AMC), which enabled us to study the effect of cross-linking on the dynamics of semidilute solutions of such a copolymer in a controllable fashion. For each semidilute copolymer solution studied before the photoreaction, dynamic laser light scattering showed a fast and a slow relaxation mode respectively with line widths Gamma(f) and Gamma(s). They were scalable to the scattering vector q as Gamma(f) similar to q(alphaf) with alpha(f) approximate to 2.0 +/- 0.1, which is the hallmark of cooperative diffusion relaxation, and Gamma(s) similar to q(alphas) with alpha(s) approximate to 3.0 +/- 0.2, which could be related to internal motions of large transient clusters made of the entangled chains. Two relaxation modes were also observed during the photoreaction. The photo-cross-linking of the chains in less concentrated solutions (<3 x 10(-2) wt %) only resulted in finite clusters, where af and a, essentially remained, indicating that there was no change in solution dynamics. On the other hand, in more concentrated solutions (>5 x 10(-2) wt %), transparent and homogeneous gels were eventually formed after a long time irradiation. As the cross-linking proceeds, the fast relaxation contributes less and less. At the same time, alpha(f) increases, but alpha(s) decreases, revealing that the diffusion relaxation are gradually mixed with the internal motions and finally diminished in the gel state.