Mechanisms of Decreased Moisture Uptake in Ortho-Methylated Di(cyanate ester) Networks.

Decreases of up to 50% in the moistureuptake of polycyanuratenetworks based on 2,2-bis(4-cyanatophenyl)propane (BADCy) and 1,1-bis(4-cyanatophenyl)ethane(LECy) were observed when analogous networks containing a single methylgroup ortho- to each aryl–cyanurate linkagewere prepared by reduction and acid-catalyzed coupling of salicylicacid followed by treatment with cyanogen bromide and subsequent cyclotrimerization.The differences in water uptake were observed despite similar decreasesin packing fraction as conversion proceeded in all networks studied.Conversely, the presence or absence of methyl groups at arylene bridges,remote from the cyanurate oxygen, had no influence on water uptake.Vitrification during cure had little effect on either free volumedevelopment or moisture uptake. These results confirm that sterichindrance from ortho-methyl groups inhibits absorptionof water presumably by decreasing the thermodynamic favorability ofsterically permitted interaction with the cyanurate oxygen. A furtherexamination of the effect of two different catalysts, 2 parts perhundred of a 30:1 by weight mixture of nonylphenol and copper(II)acetylacetonate and 500 ppm of dibutyltin dilaurate (DBTDL), comparedto analogous uncatalyzed networks, showed that hydrolytic stabilitywas dramatically affected by catalyst choice, while thermochemicalstability was also impacted. These results provide important insightsinto the mechanisms that determine structure–property relationshipsin polycyanurate networks. [ABSTRACT FROM AUTHOR]