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furan in Example 1, an increase in molecular weight is noted. 

In a round bottom flask equipped with condenser, stirrer, heating mantle etc., there is placed 4.5 parts (0.01 mole) of APDM (Example 3), 68 parts (1 mole) of furan and the mixture allowed to stand at ambient tem› perature for 24 hours. Then the excess furan is slowly distilled from the reaction mixture over a period of 4 hours. During the course of distillation, an increase in viscosity is observed; and, eventually the pot tempera› ture of the residue increases. The resulting viscous mass becomes converted to an insoluble, fusible state at a pot temperature of about 150° C. When the pot temperature is about 180° C., gelation occurs. 

Example 11 is repeated in the presence of 40 mg. of sodium acetate. Gelation is observed to occur in about I hour. 

Example 6 of my aforesaid parent application is re› peated using, instead of SDA-3,3 an equivalent amount of DAPB-3,3 as prepared in Example 2(a) of my afore› said parent application. There is obtained a dioxane soluble polyimide (BTMl-5) of the formula: 

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Example 13 is repeated using procedure of Example 12 (with sodium acetate). Gelation occurs in I hour. 

Example 1 is repeated using 134 grams of the bis› maleimide, 136 grams of furan, 4 grams of disodium maleate and I liter of DMAC. The viscosity increases similarly to extent or Example 1. However, when 102 grams of furan are eliminated, the mixture is allowed to cool to room temperature and precipitated in either methanol or water. The precipitate is removed by filtra- tion and dried. The product is fusible when heated in excess of 130’ C. When admixed with Bronsted acid salts alone or in the presence of an aprotic solvent acti› vator, a convertible composition is obtained which cures readily at temperatures of 160’ C. or higher. 

What is claimed is: 

1. A process of increasing the molecular weight of a bis-maleimide of the formula: 

## EXAMPLE 11 

## EXAMPLE 12 

## EXAMPLE 13 

## EXAMPLE 14 

## EXAMPLE 15 

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wherein 

Ar is a divalent aromatic organic radical, $^{C-CH }$ 

Ar’ is a tetravalent aromatic organic radical, the four carbonyl groups being attached directly to separate carbon atoms and each pair of carbonyl groups being attached to adjacent carbon atoms and in the Ar’ radical, and 

n is O or a positive integer of I to 20, comprising: reacting at a temperature in the range of about 5°-100° C. one mole of the bis-maleimide with from I to 2 moles of a cyclic diene of the formula: 

wherein 

Y is -0-, -S-, or 

and 

each R$_{1 }$ is hydrogen or an aliphatic hydrocarbon of 1 to 4 carbon atoms with the proviso that the sum of the R$_{1}$ ’s is no greater than 4 carbon atoms, 

and thereafter thermally eliminating substantially all the cyclic diene at a temperature in the range of about 30’ -120° C. to extend the bis-maleimide in molecular weight to at least a dimer of the bis-maleimide. 

2. The process of claim 1 wherein at least 1 mole of cyclic diene is used per mole of bis-maleimide. 

3. The process of claim 2 wherein the molecular weight is increased to a point before gelation occurs. 

4. The process of claim 2 wherein the molecular weight is increased to an infusible state. 

5. The process of claim 2 wherein the reaction is carried out in an aprotic solvent having a dielectric constant in the range of 35 to 45. 

6. The process of claim 2 wherein the reaction is conducted in the presence of a catalytic amount of an alkali metal salt of a Bronsted acid, said alkali metal salt of a Bronsted acid selected from the group consisting of 

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