Abstract: Apparatus for the vapor phase polymerization of at least one polymerizable monomer comprising: (a) a horizontal reactor of substantially circular cross-section containing a centrally-located drive shaft extending longitudinally through said reactor to which are attached a plurality of adjacently located paddles, which paddles cause essentially no forward or backward movement of the particulate matter contained in said reactor and extend transversely within and to a short distance from the internal surfaces of said reactor, said reactor being divided into two or more individually gas-composition-controllable and polymerization-temperature-controllable polymerization compartments, by one or more dividing structures so constructed to control gas intermixing and movement of said particulate matter between said compartments; (b) driving means for said drive shaft; (c) one or more reactor off-gas outlets in each of said compartments spaced along the topward part of said reactor; (d) one or more vapor recycle inlets