Branching by reactive end groups. Synthesis and thermal branching of 4-hydroxybenzocyclobutene/p-tert-butylphenol coterminated bisphenol A polycarbonates

Long chain branched bisphenol A polycarbonates (BA PC's) were prepared by u se of a combination of thermally reactive 4-hydroxybenzocyclobutene (BCB-OH ) and the nonreactive p-tert-butylphenol (PTBP) chain terminators. These tw o monophenols react under interfacial conditions at similar rates to provid e a statistical distribution of coterminated PC's having molecular weights controlled by the total amount of coterminators. Heating these materials to 300 degrees C causes branching and/or crosslinking depending on the value of X-BCB, thereby separating the polycondensation and branching processes a nd allowing greater degrees of branching than possible by random branching during polycondensation. At X-BCB < 0.55 the BCB-OH/PTBP BA PC's branch wit hout 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 terminate d BA PC's. The nongel value of X-BCB is that expected for a thermally react ed 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 an d the functionality of the poly-BCB products, BCB-OH/PTBP BA PC's having X- BCB values of 0.50 have a minimum of 0.45 number fraction branched chains. The M-w and polydispersity of branched BCB-OH/PTBP BA PC's depend on their initial molecular weight and increase with X-BCB. The M-w's for each compos ition converge to large values (similar to 100 000 Dal near the gel point, after which the M-w's of the soluble fractions decrease. The T-g's of these polymers scale inversely with M-n rather than M-w. Film toughness also tre nds with M-n. Films of branched BCB-OH/PTBP BA PC's having M-n > similar to 2M(e) are relatively tough and flexible, while those having lower M-n's, r egardless of M-w, are relatively brittle. The melt viscosities of branched BCB-OH/PTBP PC's are very high at low shear rates, as expected from their h igh M-w's, but decrease dramatically with increasing shear rates to values approaching those of conventional linear and randomly branched PC's. Such h igh zero-shear viscosity, which is indicative of high melt strength, and la rge shear sensitivity suggests that these new materials could display signi ficant improvements in melt processing by techniques such as blow molding, thermoforming, injection molding, and extrusion.