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A series of amido-oxazolinate zinc complexes have been employed for the ring-opening polymerization of rac-β-butyrolactone (BBL). Experimental results show that these complexes efficiently catalyze the reactions, yielding cyclic poly­(3-hydroxybutyrate)­s (PHBs) with high molecular weights (M n up to 197 kg/mol) and relatively low dispersity. In comparison, in the presence of alcohol cocatalysts, the zinc-catalyzed ROP reactions lead to the formation of linear PHBs end-capped by the alcohol initiator and hydroxylbutyrate. A possible mechanism for cyclic PHBs is proposed, in which the zinc catalysts function as a loose Lewis pair at elevated temperature, followed by a fast propagation through a zwitterionic intermediate. Use of diols such as 1,4-cyclohexanediol and 1,4-benzenedimethanol results in the formation of telechelic polyester diols. The thermal analysis by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) shows higher thermal resistance of cyclic PHBs than linear ones.

overnight. After washing with water and drying, the crude product was distilled, yielding 19.4 g. (33%) of X a t 240-242' and 0.05 mm., n% 1.4318.Anal. Calcd. for C~oH~~o0~~Sia: Si, 9.58; MRD, 325.3; mol. wt., 1172. Found: Si, 9.84, 10.01; MRD, 321.7; mol. wt., 1143. Heptakis-( 1,3-dimethylbutoxy)-trisiloxanol (XI) .-Eightyfive grams (0.143 mole) of the disiloxanol (IX) was added dropwise with stirring to a cold mixture of 42 g.(0.140 mole) of IT and 30 g. of dry pyridine. After an hour 70 ml. of water was added rapidly and stirring was continued for an additional hour. The organic layer was extracted with ether, washed with water, and dried over magnesium sulfate. After removal of ether the crude product was distilled, yielding 75 g. (65%) of XI at 185' and 0.1 mm., %' OD 1.4255. mol. wt., 841. Found: Si, 10.5, 10.9; MRD, 228.2; mol. wt., 840, 843. (XV).-Efforts to isolate the trialkoxysilanol derived from 2-ethylhexanol were unsuccessful and this silanol was prepared in situ by addition with stirring of 66.7 g. (0.148 mole) of the chlorosilane (XII), over a period of 2 hours, to 2.66 g. (0.148 mole) of water in 100 ml. of dry pyridine. During the next half-hour 24.6 g. (0.074 mole) of bis-( 2-ethylhexoxy)-dichlorosilane (XIII) was added, and stirring was continued for an additional half-hour. The reaction mixture was washed with water, dried and distilled. Thirty-five grams (41%) of XV was collected a t 260-269' and 0.05 mm., n% 1.4428. Octakis-(2-ethy1hexoxy)-trisiloxaneSince the polymer is insoluble and infusible, attempts were made to introduce and determine radiosulfur-containing end groups derived from persulfate initiator. Polymers initiated by persulfate activated thermally or by reaction with Fe++ or bisulfite did not contain sulfur from the persulfate. When bisuEte containing radiosulfur was employed in oxidation-reduction initiation reactions with persulfate or Fe+++, polymers with non-hydrolyzable sulfur-containing end groups from the bisulfite were obtained. These results are explained on the basis that the following are the most probable radical-producing reactions in decreasing order of effectiveness for initiating tetrafluoroethylene polymerization.The objective of this work was to determine the molecular weight of polytetrafluoroethylene.The translation of radioactive end group determinations to molecular weights is based upon the foregoing view of the initiator reactions and the assumptions (1) that there is no chain transfer or disproportionation in the polymerization, and (2) that termination is by combination of radicals. The number average molecular weights of some relatively low molecular weight polytetrafluoroethylenes are in the range 142,000-534,000.

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