Cyclization during polyesterification: Isolation of an 18‐member ring compound from reaction of phthalic anhydride with 2,2‐dimethyl‐1,3‐propanediol

Abstract

It is generally accepted that cyclization reactions occur during condensation polymerization.'.' However, direct confirmation of their occurrence is rarely obtained.Here we report isolation of 18-member ring compound 1 as a byproduct of oligomerization of phthalic anhydride ( P A ) and 2,2-dimethyl-1,3-propanediol (neopentyl glycol, NPG). Our findings provide direct evidence that cyclization occurs, and they also demonstrate a convenient method for synthesis of 1, although yields are low (1.7%).Existence of compound 1 has been p~s t u l a t e d ,~ and we recently isolated it during attempts to synthesize monodisperse P A / NPG oligomers using monosilylated NPG.' To our knowledge, these are the first reported instances in which a cyclic byproduct of phthalic anhydride polycondensation has been isolated. EXPERIMENTALComplete experimental details are described in the Ph.D. dissertation of G.-F. Chen? Materials were the best grades available from Aldrich Chemical Company and were used without further purification. Isolation of Compound 1 from Bulk Oligomerization of PA and NPGA melt of NPG (124.8 g, 1.2 mol), P A (118.4 g, 0.8 rnol), andp-toluene sulfonic acid (0.25 g) in a 500-mL round-bottom flask was stirred under a slow N2 purge as I t was heated at the following temperature schedule: 100°C for 2 h, 140°C for 2 h, 160°C for 2 h, 220°C for 3 h, and 260°C for 4 h. The product was a viscous liquid; acid number: 3.2 mg KOH/g resin; GPC: M,, = 860, Mu = 1360.A solution of 12.8 g of the above liquid in 100 mI, of methanol was kept for 2 days at room temperature. The white precipitate that formed was collected (3.84 g). washed with methanol, and dried. Acid number: 3.74 mg KOH/g resin; GPC: M , = 1930; Mu, = 2210.The filtrate was kept at room temperature for 2 more days; more white precipitate appeared.I t was collected (0.40 g) and recrystallized from 60 mL of methanol to yield 0.166 g (1.3 wt A of 12.8 g) of crystalline 1, mp 21 1-213°C. Isolation of Compound 1 from Solution Oligomerization of PA and NPGA solution of NPG (41.6 g, 0.40 rnol), PA (39.4 g, 0.265 rnol), and tris(2-ethylhexyl) phosphite (0.42 g) in xylene ( 189 g ) in a 500 mL round-bottom flask was stirred a t reflux under N2 for 10 h. Xylene was removed by gradually heating to 240°C over 1 h to yield a viscous liquid having an acid number of 66 mg KOH/g resin, corresponding to 83% conversion of -COOH groups to ester groups. GPC: M,, = 560; Mu = 780.