The adhesion between a polymer melt and substrate is studied in the presence of chemically attached chains on the substrate surface. Extensive molecular dynamics simulations have been carried out to study the effect of temperature, tethered chain areal density (Sigma), tethered chain length (N-t), chain bending energy (k(theta)), and tensile pull velocity (v) on the adhesive failure mechanisms of pullout and/or scission of the tethered chains. We observe a crossover from pure chain pullout to chain scission as N-t is increased. Below the glass transition, the value of N-t for which this crossover begins approaches the bulk entanglement length N-e. For the values of N-t and Sigma used here, no crossover to crazing is observed.