We present the synthesis of vertically aligned carbon nanotube arrays with controlled diameter and density distributions. A polystyrene-polyacetic acid diblock copolymer is dissolved in toluene to form well defined spherical reverse micelles. Addition of nickel or iron salt in solution leads to a metal rich phase inside the reverse micelle that can then be patterned onto a substrate by spin-coating or dip-coating. After removal of the polymeric material by oxidation, a well ordered array of monodisperse metal nanoparticles remains. These particles are then activated by standard thermal chemical vapor deposition process to grow multiwalled carbon nanotube forests. Through this approach, we have demonstrated 6.4 ± 1.3 nm catalyst nanoparticles which lead to nanotubes of comparable diameters. TEM images demonstrate these tubes to be 3-walled nanotubes. In contrast, a substrate grown under identical conditions with standard sputter deposited metal catalyst yielded nanotubes ranging from 10-20 nm in diameter. We have demonstrated tunable densities by altering the catalyst deposition procedures, and diameters may be tuned by altering block copolymer molecular weights. Applications of this product to thermal interface materials will also be discussed.