There is a gazilion ways poly or kevlar strings can influence spin- Including string pattern, tension, stiffness and weight distribution in the racquet.
Some are stiff strings which snap back and create the lift over the ball to generate the high spins.
Some are shaped sharply or textured to do the same job with less emphasis on the snap back and more on comfort and power.
And some will have a combination of both. (Not always good to go for the supposed allrounder!)
http://tt.tennis-warehouse.com/showthread.php?t=309803 - Check this link - racquets that are polarized in their weight distribution also greatly help with that loop in topspin you're after.
Higher tenions and/or a dense string pattern will need more effort in the swing to get that action generally. The strings don't move as much either - so if your racquet is a dense pattern I wouldn't recommend stiff snap back string. Id go with something textured or shaped.
Lower tensions with an open string pattern can get away with a stiff - low powered snap back poly as you'll have more dwell time to work with. As with a flexible racquet.
Hope this helps!