Consider a massless string going over a massless frictionless pulley with masses M1 and M2 at the end of the string. The tension in the string would be same. The torque equation of the pulley says (T1-T2)=Iα .Now L.H.S is zero since T1=T2 and also I=0 . So,we have a condition 0=(0)(α) which makes α indeterminate .But we know that the pulley rotates with some angular acceleration. So ,how is α determined ? Is α=a/R where R is the radius of the pulley and a=[(M2-M1)g]/(M1+M2) ?