1. FIELD OF THE INVENTION
The present invention relates to bulk polymerization processes for making vinyl aromatic/vinyl cyanide copolymers, and more particularly relates to bulk polymerization processes having a vapor phase additive which inhibits popcorn formation.
2. DESCRIPTION OF THE RELATED ART
Mass or bulk polymerization techniques for making copolymers of monoethelenically unsaturated polar monomers and monovinylidene aromatic monomers are known, see U.S. Pat. Nos. 3509237; 3660535; 3243481; 4221833 and 4239863, all of which are incorporated herein by reference. Such copolymers may be rubber modified graft copolymers or may be rubber-free rigid copolymer. Bulk processes, such as those involving a boiling reactor, typically involve a liquid phase held under a nitrogen (N2) atmosphere. In boiling reactors, heat of reaction is the source of heat to the reactor to cause boiling of the liquid monomeric composition. Boiled monomer then enters the nitrogen vapor phase, contacts the reactor dome, which is typically air or water cooled, condenses and returns to the liquid phase. Condensed monomer on the reactor dome will generate undesired, crosslinked popcorn polymer. Popcorn generation at the dome surface may be due in part to the absence of typical polymerization inhibitors in the gas phase because of low volatility of the inhibitors with respect to the monomers, thereby necessitating that an inhibitor be present in the vapor phase if popcorn formation is to be inhibited. In the past, inhibitors such as oxygen have been incorporated into the vapor phase to prevent popcorn formation. It is believed, however, that oxygen may be incorporated into polymer and may contribute to the formation of black carbonaceous material on the reactor walls. Analysis of the black material has indicated that the material has a high oxygen content which tends to support the proposition that the oxygen inhibitor is part of the cause of the formation thereof. Additionally, oxygen (O.sub.2) has a high solubility in many liquid organic monomers which tends to support the proposition that the oxygen is present in the liquid monomer phase during the bulk polymerization process.
Accordingly, there is a need for vapor phase additives which will inhibit popcorn formation in bulk polymerization processes.