An Improved Method for Preparing Very High Molecular Weight Polyimides

Abstract

Silylated diamines, as more nucleophilic reactants than diamines, were first reported as very suitable monomers for the synthesis of aromatic polyamides by Oishi et al. 1,2 Shortly after, polyimides from dianhydrides and silylated diamines were reported by the same authors. 3,4 Since then, the "silylation method" has successfully been extended to a variety of polyimides and to the preparation of other condensation polymers, such as poly(ether sulfone)s, 5 poly(ether ketone)s, 6 polyesters, 7 or polysulfides. 8 The initial disadvantages of this method, regarding water sensitivity and rigorous control of the reaction conditions, can be overcome with the use of in situ silylated diamines. This is performed by adding trimethylchlorosilane (TMSCl) or other silylating agents to the diamine solutions before adding the electrophilic monomer. The in situ silylation of the diamines presents some advantages; i.e., the TMSCl ensures that small amounts of water do not destroy the moisture-sensitive dianhydrides and, moreover, the experimental procedure is much easier as handling of silylated diamines is avoided.Not many examples have been reported in the literature, apart from silylation, regarding the in situ activation of amino groups. Although there are references dealing with the use of condensing agents 9 or the use of silylated monomers plus a catalyst, for example (NH 4 ) 2 SO 4 , acids, etc., 10 in the synthesis of wholly aromatic polymers, very few examples refer to the role of an organic base as an activating agent. 11,12 In previous papers, 13,14 we reported the synthesis of aromatic polyamides and polyimides by in situ silylation of diamines, and it was studied the role of a tertiary base such as pyridine as an activating agent when added to the silylated diamines. 15,16 This recently explored activation method has proven to be very efficient, yielding high molecular weight polymers even for diamines with steric hindrance. However, only medium-low molecular weight polymers were obtained from diamines with electron-withdrawing groups. Thus, polyimides with inherent viscosity of only 0.4 dL/g could be obtained when 4,4 0 -diaminodiphenyl sulfone (DDSO) was used.These results prompted us to investigate the effect of several activation agents in order to find out a general method to achieve high molecular weights irrespective of the electronic features of the aromatic diamines. Thus, several bases, with pK a ranging from 5.14 to 11.9, such as pyridine, N,N-dimethylaminopyridine (DMAP), triethylamine (Et 3 N), 1,3-diazabicyclo[5.4.0]undecane (DBU), and isoquinoline (IQ) were investigated as individual activating agents or as a pair (base/cobase) in the synthesis of polyimides. Moreover, other reaction conditions (base/cobase relative amount, reaction temperature, solvent, etc.) were also explored and here included.The results reported herein were obtained from a complete investigation of the best reaction conditions in the synthesis of polyimides from unreactive diamines: 6FDA-DDSO and 6FDA-6FpDA (Scheme 1). The latter...

Effect of polymer structure on gas transport properties of selected aromatic polyimides, polyamides and TR polymers

“…The hexafluoroisopropylidene groups that bridge the APAF diamine are strongly electron withdrawing, which is known to reduce the reactivity needed for forming high molecular weight polyimides. 62 Our results suggest that similar principles apply for thermal rearrangement, whereby thermal rearrangement reactivity is lower for the APAF sample because of its hexafluoroisopropylidene functional group.…”

Gas separation membranes made through thermal rearrangement of ortho-methoxypolyimides

“…High-viscosity poly(amic acid) intermediates, HPAA and MeOPAA, were obtained through a base-assisted in situ silylation method [33], which requires the use of a silylating agent (CMTS) and Py and DMAP as activating reagents to increase the reactivity of the -NH2 groups. In a second stage, the poly(amic acid)s HPAA and MeOPAA were cyclized by different imidization methods depending on the desired final polyimide structure.…”

Thermally rearranged polybenzoxazoles and poly(benzoxazole-co-imide)s from ortho-hydroxyamine monomers for high performance gas separation membranes

“…The intermediate poly(amic acid) was attained through a base-assisted in situ silylation method [28] which involves the use of chlorotrimethylsilane as the silylating agent and pyridine and 4-dimethylaminopyridine as activating reagents. Next, the intermediate poly(amic acid) was chemically imidized by cyclodehydration with a mixture of acetic anhydride and pyridine.…”