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.