13  4,118,377  14  furan  in  Example  1, an  increase  in  molecular  weight  is  noted.  wherein  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:  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:  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  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.  1. A  process  of  increasing  the  molecular  weight  of a  bis-maleimide  of the  formula:  What  is  claimed  is:  EXAMPLE  14  EXAMPLE  15  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,  wherein  Y  is  -0-,  -S-,  or  and  EXAMPLE  13  EXAMPLE  12  EXAMPLE  11  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  0  II  0  0  II  II  0  II  r-C~N  C  C  I  \  I  \  Ar-N  Ar’  N  \  I  \  I  C  C  Ar-<C-r  CH-C  II  0  II  II  0  0  II  0  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.  c 11 H~co 1 \  N-C$_{6}$H,OC$_{6}$ H 4 OCoH4N  CH-CO  co-r  CO-CH 