You can build P3HT by using the polymer builder in Materials Studio after drawing the custom repeating unit of 3-hexylthiophene.

Check the 'Using the polymer builder' page in Visualizer Tutorials Section in Help topics.

I have used M7.0 to build a costumed polymer. Check the help file first, there is a detailed instruction to build a costumed polymer. You should build a monomer first, and then define the head and tail. At last, use polymer builder to choose your monomer as the repeated unit, and input the number of degree of polymerization. Then you can build it. You can also define a torsion of each monomer at the second page of polymer builder.