A review of chain transfer catalysis and catalytic homopolymerization of acrylates and styrene

Low molecular weight polymers are used in industry for the production of hi gh-solids automotive finishes, adhesives, coatings, polymer blends, and oth er similar applications. These polymers are usually obtained by adding thio ls or mercaptans as molecular weight regulators during polymerization of th e respective monomers. Because of the low reactivity of thiols or mercaptan es rather high concentrations are required which in turn impart undesirable properties, such as toxicity, color, and odor; to the finished product. Th is limitation on the use of thiols or mercaptans justifies a need for findi ng alternatives for use as regulators in the free radical initiated polymer ization. although a very rapidly growing interest is evident from literatur e, no review on this subject has so far appeared nor have the trends emergi ng from the growing research been identified. The present paper is an effort to consolidate the existing literature on ch ain transfer catalysis. The paper is divided into two parts: (a) a review o f all chain transfer catalysts reported so far in the literature is given, and (b) the work involving application of a new catalyst for bulk homopolym erization of acrylates and styrene is reported. The basis of the undergoing research on Catalytic Chain Transfer Catalysts lies in successful use of metal-chelates such as cobalttetra(aryl)porphyrin ce (TAP(Co-II)(.) (the dot indicating the radical nature of cobalt(II)), an d cobalt(II)dimethylglyoximates [Co(dmg-2H)(2)(BF2)(2)], at ppm levels, as very effective Catalytic Chain Transfer Agents (CCTA) for regulating the mo lecular weight of the polymeric acrylates. The [Co(dmg-2H)(2)(BF2)(2)] comp lex has been patented as a CCTA by DuPont. The experimental section involves: (a) the synthesis and characterization of new metal chelates used as CCTAs, (b) bulk homopolymerization of acrylates and styrene in the presence of the CCTA, and (c) polymerization as a function of concentration of the CCTA. Our studies using [Co(afdo-2H)(2)(BF2)(2)] (afdo = alpha-furildioxime) as a CCTA for AIBN-initiated (AIBN = azobis(isobutyronitrile) bulk homopolymeri zation of MMA revealed that its catalytic activity is similar to the DuPont catalyst [Co(dmg-2H)(2)(BF2)(2)], observed for homopolymerization of MMA a nd styrene. An approximately 10-fold decrease in the molecular weight of PM MA was observed with concentration of [Co(afdo-2H)(2)(BF2)(2)] at PPM level s. The molecular weight of the PMMA as well as of polystyrene depends upon factors such as the time of addition of the CCTA relative to the initiator, concentration of the CCTA, and the reaction temperature. Similar retardati on of molecular weight was observed in AIBN-initiated bulk co-polymerizatio n of MMA and styrene. A large volume of plastics is based upon polystyrene and/or polyacrylates. Low molecular weight polyacrylates, polystyrene, or styrene/acrylates macro -monomers are useful materials for several industrial applications. The use of metal chelate as CCTA will afford the synthesis of new polymeric materi als avoiding the problems of toxicity, odor and color, associated with the use of thiols or mercaptans as polymer chain growth interceptors. The co-po lymerization of styrene with polyacrylates in the presence of a CCTA will o ffer an entirely new class of polymeric materials. In addition to its commercial importance, catalysis of chain transfer remai ns a growing area of research because beside its industrial application, it provides a unique opportunity for obtaining fundamental knowledge on eleme ntary reactions of both organometallic and free radical polymerization chem istry.