Branching by Reactive End Groups. Synthesis and Thermal Branching of 4-Hydroxybenzocyclobutene/p-tert-Butylphenol Coterminated Bisphenol A Polycarbonates

Abstract

Long chain branched bisphenol A polycarbonates (BA PC's) were prepared by use of a combination of thermally reactive 4-hydroxybenzocyclobutene (BCB-OH) and the nonreactive p-tert-butylphenol (PTBP) chain terminators. These two monophenols react under interfacial conditions at similar rates to provide a statistical distribution of coterminated PC's having molecular weights controlled by the total amount of coterminators. Heating these materials to 300 °C causes branching and/or cross-linking depending on the value of XBCB, thereby separating the polycondensation and branching processes and allowing greater degrees of branching than possible by random branching during polycondensation. At XBCB < 0.55 the BCB-OH/PTBP BA PC's branch without cross-linking. The poly-BCB products formed upon the thermal branching of these PC's are the same as those observed in cross-linked BCB terminated BA PC's. The nongel value of XBCB is that expected for a thermally reacted BCB functionality of 2.6, which is the same as that estimated for cross-linked BCB PC's. Based on the statistical distribution of the chain ends and the functionality of the poly-BCB products, BCB-OH/PTBP BA PC's having XBCB values of 0.50 have a minimum of 0.45 number fraction branched chains. The Mw and polydispersity of branched BCB-OH/PTBP BA PC's depend on their initial molecular weight and increase with XBCB. The Mw's for each composition converge to large values (∼100 000 Da) near the gel point, after which the Mw's of the soluble fractions decrease. The Tg's of these polymers scale inversely with Mn rather than Mw. Film toughness also trends with Mn. Films of branched BCB-OH/PTBP BA PC's having Mn > ∼2Me are relatively tough and flexible, while those having lower Mn's, regardless of Mw, are relatively brittle. The melt viscosities of branched BCB-OH/PTBP PC's are very high at low shear rates, as expected from their high Mw's, but decrease dramatically with increasing shear rates to values approaching those of conventional linear and randomly branched PC's. Such high zero-shear viscosity, which is indicative of high melt strength, and large shear sensitivity suggests that these new materials could display significant improvements in melt processing by techniques such as blow molding, thermoforming, injection molding, and extrusion.