The molecular weight, melt viscosity and melt strength of nylon 4.6 are increased by a solid phase polymerization process conducted under anhydrous conditions at temperatures below the melting point of the nylon. In a preferred embodiment, the nylon used in the process is free of surface air, has a moisture content less than 0.2 wt %, preferably less than 0.02 wt %, and more preferably less than 0.01 wt %. It is also preferred that the extractable impurities are less than 0.2 wt %, preferably less than 0.02 wt %, and more preferably less than 0.01 wt %. The nylon 4.6 product of this anhydrous process is characterized by molecular weights in excess of 50,000, melt viscosities in excess of 600 Pascal.seconds, relative viscosities in excess of 160 and degradation ratios of 7 or less.
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