Cationic ring-opening polymerization of benzoxazines

Details Cationic ring-opening polymerization of benzoxazines Cationic ring-opening polymerization of benzoxazines

1998-06-26

2001-05-01

Dawson, Robert (Department: 1712)

Synthetic resins or natural rubbers -- part of the class 520 ser

Synthetic resins

From heterocyclic reactant containing as ring atoms oxygen,...

C544S090000

Reexamination Certificate

active

06225440

ABSTRACT:

FIELD OF INVENTION
Benzoxazine monomers can be formed from the reaction of a phenolic compound, an aldehyde (desirably formaldehyde), and a primary amine. Cationic initiator are disclosed herein for polymerizing benzoxazines. The resulting polymers have higher char yields than conventionally (thermally) polymerized benzoxazines. The polymer structure, as determined by NMR, FTIR and molecular weight distribution of the polymer, is also different than conventionally (thermally) polymerized benzoxazine.
BACKGROUND OF THE INVENTION
Polymers derived from the ring opening polymerization of benzoxazine compounds compete with phenolic, epoxy, and other thermoset or thermoplastic resins in various applications. Benzoxazines can be used as a precursor for chars used in applications where high thermal stability is required such as aircraft disc brakes and thermal insulators.
Benzoxazines have advantages over conventional thermoset resins as the benzoxazine can be readily molded from the melt and then polymerized without releasing substantial amounts of polymerization reaction by-products.
The mechanism of thermally induced ring opening polymerization of benzoxazines is not sufficiently understood to provide control over the molecular weight distribution and branching of the polymer. It is anticipated that the performance of benzoxazine polymers in terms of modulus, processability, and char yield on heating could be improved by controlling the microstructure (branching and molecular weight). Current thermally induced polymerization processes offer little or no opportunity to better understand or control the factors that determine branching, polymer molecular weight, etc. Attempts to use initiators and/or catalysts to polymerize benzoxazine monomers in the past have only slightly modified (shifted) the exotherm temperature of the polymerization.
SUMMARY OF INVENTION
Cationic polymerization initiators have been found to be effective in initiating ring opening polymerizations of benzoxazine monomers. These initiators allow the polymerization of benzoxazine monomers as low as −60° C. rather than the traditional 150-300° C. for thermal polymerization. The polymers from these cationically initiated polymerizations have controllable and different structures as noted by the different infrared spectrum, different NMR spectrum, and different molecular weight distributions as compared to thermally polymerized polymers from benzoxazines. It is anticipated that little or no volatiles will be released from these cationic polymerizations. Further as in thermally initiated ring opening polymerizations of benzoxazines, it is anticipated that little shrinkage of the benzoxazine compositions will occur during polymerization. These cationic polymerizations hold promise for processes where the benzoxazine monomers are molded or shaped into articles and then polymerized into thermoplastics or thermosets or polymerized into thermoplastic polymers and molded or shaped and optionally crosslinked. Although the initial experiments were run on monofunctional and difunctional monomers, it is understood that polyfunctional monomers can be used to yield branched or crosslinked polymers.
The conventional thermally polymerized benzoxazine based on bisphenol-A, formaldehyde, and aniline has char yields by TGA at 800° C. of about 30 weight percent Unexpectedly, the polymers from the cationically initiated polymerization of the same benzoxazine can have char yields at 800° C. by thermogravimetric analysis (TGA) of 43, 49 or 50 weight percent to 62 weight percent or higher.
DETAILED DESCRIPTION OF THE INVENTION
The benzoxazine monomers readily polymerized by the cationic initiator disclosed later are monobenzoxazine and polybenzoxazine compounds generally having the general formula
wherein n can be any integer from 1 to 20 and is preferably an integer from 1 to 4 and is most preferably 1 or 2 and R
1
can be hydrogen, one or more groups selected from hydroxyls, one or more linear or branched alkyls of 1 to 80 and more preferably 1 to 10 carbon atoms, aromatics, alkyl substituted aromatics, aromatic substituted alkyls of 6 or 20 carbon atoms, mono or poly fluorine substituted alkyls of 1 to 20 carbon atoms, mono or poly fluorine substituted compound having at least 1 aromatic ring and 6 to 20 carbon atoms, and phenolic compounds of 6 to 20 carbon atoms (for thc purpose of this specification phenolic compounds may have more than one hydroxyl group as defined in chemical dictionaries) including poly phenolic compounds having on average 6 to 20 carbon atoms per phenol group. When n is 2, 3 or 4, R
1
can be selected from the connecting groups below
Depending on whether phenolic or phenoxy repeat units are desired in said polybenzoxazine, it may be desirable that R
1
be ortho, meta or para to the oxygen atom of the benzoxazine monomer of Formula A. Further R
2
may be an alkyl of 1 to 40 carbon atoms, an aromatic, an alkyl substituted aromatic or aromatic substituted alkyl of 6 to 40 carbon atoms, mono or poly fluorine substituted alkyl of 1 to 20 carbon atoms, a mono or poly fluorine substituted compound having at least one aromatic ring and 6 to 20 carbon atoms or an amine of 1 to 40 carbon atoms including polyamines and aromatic, alkyl substituted aromatic, or aromatics substituted alkyls having 6 to 40 carbon atoms. Furthermore, each benzene ring, as shown by (R
3
), where p is an integer from 0 to 3 and R
3
is as defined later, can have more than one substituent of the same structure or a mixture of the R
3
structures.
The variable m can be an integer from 0 to 5 and R
3
can be H or R
2
. Preferably R
3
is not the amine or polyamine components of R
2
. Preferably R
3
is an alkyl of 1 to 9 carbon atoms such as CH
3
, C
2
H
5
, C
3
H
7
or C
4
H
9
, or a mono or poly fluorinated alkyl of 1 to 9 carbon atoms such as CF
3
, C
2
F
5
, C
3
F
7
. These R
1
compounds are well known to those familiar with phenolic compounds. Generally R
1
can be any of the known connecting groups than interconnect two or more phenols. Known connecting groups refers to those which are present in commercially available phenols, are in experimentally available phenols, and phenols whose synthesis are described in the published literature. Examples of such phenols include
As the cationically initiated ring opening polymerization of monofunctional benzoxazines generally results in linear polymers, it is desirable that at least 25 mole percent, more desirably at least 50 mole percent and preferably at least 75 mole percent or 90 mole percent of the R
1
groups are not an additional hydroxyl groups or a phenolic or polyphenolic compound and n in Formula A is 1. Also desirably at least 25 mole percent, more desirably at least 50 percent, still more desirably at least 75 or 90 mole percent of the R
2
groups are neither polyamines nor include additional benzoxazine compounds. These limitations are desirable for thermoplastic polybenzoxazines but it is understood that if thermoset polybenzoxazines are desired the amount of difunctional or polyfunctional benzoxazine monomers (those where n is 2 to 20 or R
2
is a polyamine or includes additional benzoxazine rings) could be higher. When high molecular weight thermoplastic polybenzoxazines are desired, desirably the number average molecular weight of said polybenzoxazine is at least 5,000 and more desirably at least 10,000.
As is well known, benzoxazine monomers arc made from the reaction of three reactants, aldehydes, phenols, and primary amines by procedures using a solvent or known as solventless systems. U.S. Pat. No. 5,543,516, hereby incorporated by reference, sets forth a generally solventless method of forming benzoxazine monomers. An article by Ning and Ishida in Journal of Polymer Science, Chemistry Edition, vol. 32, page 1121 (1994) sets forth a procedure using a solvent which can be used to prepare benzoxazine monomers. The procedure using solvents is generally common to the literature of benzoxazine monomers.
The preferred phenolic compounds are phenol or cresol but can include diph

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