Problem with dynamic movieclip referencing

I cannot assign _x and _y coordinates in a movie clip dynamically. For example:

for (i=1; i<=5; i++) {
myThumb_mc = “thumb” + i;
myThumb_mc._x = Stage.width/2;
myThumb_mc._y = Stage.height/4;
}

but if I do:

thumb1._x = Stage.width/2;
this works.

what am I doing incorrectly here?