Dynamic mechanical-loss spectra at 1 Hz, and dipolar rotational spin-echo C-13 NMR spectra at 15.1 MHz, have been obtained for a series of copolymers of polycarbonate made from monodisperse oligomers of Bisphenol A polycarbonate (Ex), alternating (via connecting carbonate linkages) with single units of 3,3’,5,5’-tetramethylbisphenol A (T). The mechanical-loss relaxation of BxT suggests cooperative motions in the copolymer glass at T = -100 and -20 degrees C. The NMR results indicate that no phenylene rings undergo pi flips in T units and that most but not all of the B rings flip faster than 10 kHz at 300 K. A necessary but not sufficient condition to constrain a B ring from flipping in BxT is the presence of both interchain and intrachain T-unit nearest neighbors. These results are interpreted in terms of a packing model for polycarbonate-like systems that emphasizes the importance for chain dynamics of the proximity and cooperative motion of nearest-neighbor chains having similar local orientational order.