Polyamic acid precursors and methods for preparing higher molecular weight polyamic acids and polyimidebenzoxazole

DETAILED DESCRIPTION OF THE INVENTION In the present invention, polyimide precursors are prepared by reacting an aromatic dianhydride, an aromatic diaminobenzoxazole and a bifunctional chain terminator.
The aromatic dianhydride and an aromatic diaminobenzoxazole suitably employed in the practice of the present invention are those dianhydrides and diamines which are capable of reacting to form a polyamic acid.
Representative aromatic dianhydride include compounds of the following structural formula: ##STR4## in which Ar can be an aromatic, including polyaromatic and fused aromatic, or inertly substituted aromatic, wherein "inertly substituted aromatic" means an aromatic having one or more substituent groups such as a halogen which is essentially inert, preferably inert, to reaction with the aromatic diaminobenzoxazole.
For example, Ar can be: ##STR5## where T is --O--, alkylene, --S--, --CF.
sub.
2 --, --SO.
sub.
2 --, --CH.
sub.
2 --, ##STR6## The most preferred dianhydrides are pyromellitic dianhydride ("PMDA"): ##STR7## Representative diaminobenzoxazoles useful in the practice of the present invention include compounds of the following structural formula: ##STR8## where Ar.
sup.
1, Ar.
sup.
2, Ar.
sup.
3, and Ar.
sup.
4 are an aromatic group, inertly substituted aromatic group, or pyridine group.
Preferred diaminobenzoxazole monomers include: ##STR9## when Ar.
sup.
1 is: ##STR10## and T.
sup.
2 is --O--, --SO.
sub.
2 --, --S--, --S(=O)--, --CH.
sub.
2 --, --Si(R).
sub.
2 --, ##STR11## and T.
sup.
4 is: --S--, --O--, SO.
sub.
2 --, --CH.
sub.
2 --, --HC=CH-- ##STR12## The most preferred diaminobenzoxazole monomers are 2,6-(4,4'-diaminodiphenyl)benzo[1,2-d:5,4-d']bisoxazole ("DABO"): ##STR13## 5-amino-2-(p-aminophenyl)benzoxazole ("p-DAMBO"), ##STR14## 5-amino-2-(m-aminophenyl)benzoxazole ("m-DAMBO"), ##STR15## 4,4'-diphenylether-2,2'-bis(5-aminobenzoxazole) ("OBA(DAMBO).
sub.
2 "), ##STR16## 2,2'-p-phenylenebis(5-aminobenzoxazole) ("TA(DAMBO).
sub.
2 "), ##STR17## and 2,2-bis(4-phenyl)hexafluoropropane-2,2'-bis(5-aminobenzoxazole) ("6FA(DAMBO)