Method for preparing imide-containing polymers

Imide-containing polymers of styrene-type monomers are prepared by a process comprising solution polymerizing under free radical polymerization conditions a polymerizable monomer composition comprising at least one vinylaromatic monomer and at least one maleamic monocarboxylic acid monomer.

BACKGROUND OF THE INVENTION 
The present invention relates to imide-modified vinylaromatic polymers and 
copolymers. Vinylaromatic polymers and copolymers, such as polystyrene, 
SAN and ABS are important classes of thermoplastic polymeric materials. 
Their impact resistance, tensile and flexural strength, and elongation 
values make these polymers extremely useful in fields where strength, 
durability and ease of manufacture are desired. The polystyrene 
thermoplastic resins are conventionally molded in sheet form and 
structural form, designed and adaptable as packaging structures, housings, 
support structures, furniture molded articles, toys, architectural trim, 
motor housings, television cabinets, and the like. In many applications, 
such as vehicles, the building trades, electrical appliances and consumer 
goods, heat resistance of polymeric materials is a highly desirable 
property. Like most thermoplastic resins, there is a desire to increase 
the heat resistance of vinylaromatic resins without negatively detracting 
from other desirable properties of these polymers. 
A variety of approaches to improving heat resistance of thermoplastic 
vinylaromatic resins have been explored. For example, organic and 
inorganic fire retardants have been effective but are difficult to 
homogenously disperse in the polymer, which not only can give rise to poor 
surface appearance, but can also negatively affect heat resistance. 
Attempts have also been made to take advantage of the high heat resistance 
of polyimides by interpolymerizing phenylmaleimide with vinylaromatic 
monomer compositions. While the resulting interpolymers do exhibit 
improved heat resistance, the phenylmaleimide monomer significantly raises 
the cost of the polymer product, because it is difficult to prepare and 
purify. In addition, it is the only arylmaleimide monomer currently 
commercially available. 
Another approach to preparing imide-containing polymers is to post-imidize 
maleic anhydride-containing polymers. For example, a styrene-maleic 
anhydride copolymer may be reacted with a primary amine, such as aniline, 
to yield a styrene-phenylmaleimide copolymer. The difficulties associated 
with the use of maleic anhydride in polymerization processes make this 
approach to imide-containing polymers unattractive. 
SUMMARY OF THE INVENTION 
In the present invention imide modified vinylaromatic polymers having 
improved heat resistance are surprisingly prepared by a process comprising 
solution polymerizing at least one vinylaromatic monomer in the presence 
of at least one maleamic acid monomer under free radical polymerization 
and dehydration conditions and separately recovering an imide-containing 
vinylaromatic polymer. The maleamic acid monomers are easily prepared and 
purified. The polymerization and in situ imidization proceeds at moderate 
reaction conditions, even in the substantial absence of a dehydration 
catalyst. The present invention thus surprisingly economically and 
conveniently provides vinylaromatic polymers with high heat distortion 
temperatures. 
DESCRIPTION OF THE PREFERRED EMBODIMENT 
Imide-containing vinylaromatic polymers having improved heat resistance are 
produced in accordance with the invention by solution polymerizing under 
free radical polymerization conditions a polymerizable monomer mixture 
comprising at least one vinylaromatic monomer and at least one maleamic 
monocarboxylic acid monomer. 
Vinylaromatic monomers, which are employed in forming the imide-containing 
thermoplastic polymer compositions of the present invention, have the 
formula 
EQU CH.sub.2 .dbd.CX--Ar; 
wherein Ar is an aromatic radical, including various alkyl and 
halo-ring-substituted aromatic units, of from 6 to 10 carbon atoms and X 
is hydrogen or an alkyl group having from 1 to 4 carbon atoms. 
Representative of suitable vinylaromatic monomers include styrene; 
alpha-alkylmonovinylaromatic compounds such as alpha-methylstyrene, 
alpha-ethylstyrene, alpha-methyl-vinyltoluene; ring substituted 
alkylstyrenes such as vinyltoluene, o-ethylstyrene, p-ethylstyrene, 
2,4-dimethylstyrene; ring-substituted vinylaromatic compounds such as 
o-chlorostyrene, p-chlorostyrene, o-bromostyrene, 2,4-dichlorostyrene; 
ring alkyl/ring halo substituted vinylaromatic compounds such as 
2-chloro-4-methyl styrene, 2,6-dichloro-4-methylstyrene and the like; 
vinylnaphthalene, vinylanthracene, indene, p-cyanostyrene, 
p-methoxystyrene, and the like. The alkyl substituents generally have 1 to 
4 carbon atoms and may include isopropyl and isobutyl groups. If desired, 
mixtures of such vinylaromatic monomers may be employed. 
The vinylaromatic monomers can be copolymerized with up to 30 percent by 
weight non-vinylaromatic monomers (based on total weight of the two types 
of monomers). Non-vinylaromatic monomers which are copolymerizable with 
the vinylaromatic monomers employed in forming the imide-containing 
vinylaromatic thermoplastic resin compositions of the invention are 
characterized by the presence of a single olefinically unsaturated group 
which can be terminally or internally located. Representative 
ethylenically unsaturated non-vinylaromatic monomers include unsaturated 
nitriles such as acrylonitrile, methacrylonitrile and ethacrylonitrile; 
alpha-or beta-unsaturated monobasic acids or derivatives thereof such as 
acrylic acid, methyl acrylate, ethyl acrylate, butyl acrylate, 
2-ethylhexyl acrylate, methacrylic acid and the corresponding esters 
thereof; acrylamide, methacrylamide, vinylidene chloride, vinylidene 
bromide, vinyl esters such as vinyl acetate, vinyl proprionate and the 
like; dialkyl maleates or fumarates such as dimethyl maleate, diethyl 
maleate, dibutyl maleate, and the corresponding fumarates, vinylidene 
cyanide and esters of alpha-cyanoacrylic acid. 
The maleamic monocarboxylic acids which are interpolymerized with 
unsaturated monomers to form imide-containing polymers in accordance with 
the invention are characterized by the formula 
EQU R--NH--CO--CH.dbd.CH--CO--OH; 
wherein R is an alkyl group of 1 to 20 carbon atoms, phenyl, benzyl, 
cyclohexyl, pyridyl or quinolyl, including such groups having at least one 
halogen, carboxyl or nitrile substituent. 
The maleamic acids are readily prepared by reacting at least one primary 
monoamine with maleic anhydride. Examples of representative primary amines 
include, without limitation thereto, methylamine, ethylamine, 
n-propylamine, isopropylamine, n-butylamine, sec-butylamine, 
isobutylamine, tert-butylamine, n-hexylamine, n-dodecylamine, benzylamine, 
cyclohexylamine, aniline and ethylanilines. 
Synthesis of the maleamic acid proceeds virtually stoichiometrically, 
preferably employing from 0.8 to 1.5, most preferably 0.9 to 1.2, moles of 
primary amine per mole of maleic anhydride. The reaction is preferably 
effected in the presence of an organic solvent such as methylene chloride. 
The interpolymerization of vinylaromatic monomers with maleamic 
monocarboxylic acid monomers to prepare the imide-containing vinylaromatic 
polymers in accordance with the present invention may be carried out in 
the presence of suitable solvents under reaction conditions normally 
employed for the solution polymerization of vinylaromatic monomers. 
The imide-containing polymers are preferably formed by mixing from 70 to 95 
weight percent of at least one vinylaromatic monomer and from 5 to 30 
weight percent of at least one maleamic monocarboxylic acid monomer. The 
vinylaromatic monomer can be replaced with up to 30 percent of 
non-vinylaromatic monomers which are copolymerizable with vinylaromatic 
monomers. A currently desirable copolymer comprises from 15 to 30 weight 
percent of acrylonitrile, 55 to 80 percent of styrenic monomer and 5 to 30 
weight percent of maleamic monocarboxylic acid monomer. Styrene is the 
currently preferred vinylaromatic monomer and when copolymerized with one 
or more other vinylaromatic monomers and/or non-vinylaromatic monomers, 
the amount of styrene is preferably at least 50 percent of the 
polymerizable monomer charge. 
The polymerizable monomers may be combined with from 2 to 30 weight percent 
of an inert liquid diluent, including dimethylformamide or aromatic 
hydrocarbons such as toluene, benzene, ethylbenzene, xylene or the like. 
The aromatic liquid diluents are currently preferred, with ethylbenzene 
and xylene being the most preferred. 
Substantially any of the known free radical polymerization catalysts, may 
be employed in the polymerization reactions of the invention if desired. 
The polymerization is effected at solution polymerization conditions which 
are commonly employed in polymerization of styrene-type monomers. Such 
reaction conditions are well known. Preferably the polymerization is 
conducted at temperatures from 25.degree. C. to 200.degree. C. 
Beneficially under such conditions in situ imidization occurs through loss 
of water. Although not necessary for success of the dehydration process a 
suitable dehydration catalyst, especially an acid may be employed to 
assist in the imidization. 
The invention also contemplates the preparation of impact resistant, heat 
resistant polymers obtained by polymerizing at least one vinylaromatic 
monomer and at least one maleamic monocarboxylic acid monomer in the 
presence of 5 to 20 percent by weight of an elastomer phase. A variety of 
elastomers may be employed as the elastomeric substrate in forming impact 
styrenic thermoplastic compositions of the invention including conjugated 
1,3-diene rubbers, ethylene-propylene-diene terpolymer rubbers, 
acrylate-diene interpolymer rubbers, nitrile rubbers, and mixtures 
thereof. The preferred rubbers are diene rubbers or mixtures of diene 
rubbers of one or more conjugated 1,3-dienes such as butadiene, isoprene, 
piperylene and chloroprene. Such rubbers include homopolymers of 
conjugated 1,3-dienes and interpolymers of such 1,3-dienes with up to an 
equal amount by weight of one or more copolymerizable monoethylenically 
unsaturated monomers, including the vinylaromatic monomers and 
non-vinylaromatic monomers described above. 
The high impact heat resistant vinylaromatic resin compositions of the 
invention are prepared by polymerizing the polymerizable monomer mixtures 
comprising vinylaromatic monomers and, optionally, non-vinylaromatic 
monomers, in combination with the maleamic monocarboxylic acid monomer in 
the presence of preformed rubber substrates, generally in accordance with 
conventional solution polymerization conditions. In such solution 
polymerizations, the preformed rubber substrate is preferably dissolved in 
the monomers and this mixture is polymerized to graft at least a portion 
of a polymer on the rubber substrate. The rubber particles in the high 
impact, heat resistant styrenic resin compositions which result are 
dispersed throughout the rigid phase and will have a rigid phase grafted 
onto the substrate and the rubber particles will contain occlusions of 
grafted or ungrafted monomer. 
The following examples illustrate the present invention and should not be 
construed to limit its scope. In the examples, all parts and percentages 
are by weight unless otherwise indicated.

EXAMPLE 1 
Preparation of the Maleamic Monocaraboxylic Acid Monomer 
A 70 percent solution of ethylamine in water was added to an equal molar 
amount of maleic anhydride dissolved in methylene chloride and cooled to 
approximately 0.degree. C. The desired ethylmaleamic acid product formed 
as a white precipitate in the solution. The N-ethylmaleamic acid was 
isolated by filtration and dried at 60.degree. C. in vacuo. 
EXAMPLE 2 
Preparation of Imide Modified Polymer 
A mixture comprising 60 parts by weight styrene, 25 parts by weight 
acrylonitrile and 15 parts by weight N-ethylmaleamic acid is reacted at a 
temperature of 125.degree. C. under thermal polymerization conditions in 
the substantial absence of a dehydration catalyst. There was separately 
recovered a terpolymer of styrene-acrylonitrile-ethylmaleimide which has a 
glass transition temperature of 122.degree. C. Evidence for the formation 
of the ethylmaleimide terpolymer is confirmed by IR spectroscopy. 
EXAMPLE 3 
Preparation of Imide Modified Polymers 
Following the procedure of the preceding example 2, phenylmaleamic acid and 
methylmaleamic acid were substituted for the ethylmaleamic acid to obtain 
terpolymers of styrene/acrylonitrile/phenylmaleimide, and 
styrene/acrylonitrile/methylmaleimide terpolymers, respectively. 
It is understood that the above are merely preferred embodiments and that 
various changes and alterations can be made without departing from the 
spirit and broader aspects of the invention.