Process for preparing alpha-methylstyrene polymers

Polymers containing greater than 60 wt% alpha methylstyrene, from 28 to 35 wt% acrylonitrile and from 0 to 7 wt% styrene having excellent color and usefully high molecular weights result from suspension polymerization of the monomer mixture above 90.degree. C. and employing monomer phase to aqueous phase ratios greater than 1:1.5.

BACKGROUND OF THE INVENTION 
This invention relates to a process for preparing polymers of alpha 
methylstyrene. More particularly this invention relates to a suspension 
polymerization process for producing polymers of alpha methylstyrene, 
acrylonitrile and styrene in bead form at a high level of monomer 
conversion, having high molecular weight and low residual monomer content. 
Conventional suspension or "bead" polymerization processes are known for 
the preparation of a variety of polymers such as polystyrene and 
styrene-acrylonitrile copolymers. These methods generally are 
characterized by incomplete conversion of the monomers, and consequently 
the resulting bead polymers are recovered containing varying high levels 
of residual monomer present. A variety of techniques have been developed 
to avoid the presence of unreacted monomer in the bead product, such as 
for example that shown in U.S. Pat. No. 3,288,731 wherein the 
polymerization reaction mixture is treated with a strong base at a 
monomer-polymer conversion point of about 75-85 percent to remove the 
unreacted acrylonitrile prior to isolating the beads. This method produced 
bead products relatively free of residual acrylonitrile but the low 
conversions result in lowered yields, a substantial loss of monomers and a 
concommitant increase in production costs. Alternate approaches include 
the use of carefully programmed process temperatures to increase the level 
of conversion followed by steam distillation to remove unreacted monomers 
such as is taught in U.S. Pat. No. 3,491,071. This latter process, 
employing styrene, alpha methylstyrene and acrylonitrile, requires the use 
of programmed process temperatures followed by steam distillation of 
unreacted monomer (acrylonitrile) prior to 98% conversion. The use of 
programmed process temperatures and steam distillation of unreacted 
monomers adds substantially to the costs, and additionally does not avoid 
the loss of some monomer through incomplete conversion. Further, when 
applied to terpolymers wherein alpha methylstyrene comprises the largest 
weight fraction of the monomer mixture, conversion is low and the 
molecular weight of the resulting product is significantly decreased. 
An efficient process for producing high molecular weight polymers from 
mixtures of alpha methylstyrene, acrylonitrile and styrene having high 
percentages of alpha-methylstyrene is clearly needed. 
SUMMARY OF THE INVENTION 
It has now been found that the suspension polymerization of mixtures of 
alpha methylstyrene, acrylonitrile and styrene having greater than 50 
weight percent alpha methylstyrene, produces high molecular weight bead 
polymers in greater than 96% conversion, when carried out at an elevated 
temperature and employing a high ratio of monomer-to-aqueous phase. More 
particularly, the polymerization of monomer mixtures of alpha 
methylstyrene, acrylonitrile and styrene containing greater than 60 wt. 
percent alpha methylstyrene carried out at a temperature above 90.degree. 
C. under suspension polymerization conditions with a monomer-to-aqueous 
phase ratio greater than about 1:1.5 results in monomer conversions of 
generally greater than 96% to polymers having a molecular weight greater 
than about 40,000 (Mn) without requiring the use of programmed reaction 
temperatures. The product bead polymers are of good color and low residual 
monomer content even though no steam distillation or vacuum removal of 
monomers is employed. 
The polymers are prepared from monomer mixtures containing from 60 to 70 
wt. percent alpha methylstyrene, from 35 to 28 wt. percent acrylonitrile 
and from 0 to 7 wt. percent styrene. The particular range of monomers 
employed is selected to provide an advantageous balance of the high heat 
distortion properties of alpha methylstyrene and the excellent chemical 
and solvent resistance imparted by acrylonitrile. These particular 
compositions and their desirable property characteristics have long been 
known in the art, and the compositions themselves are not regarded as part 
of this invention. 
As is well known in the art, homopolymerization of alpha-methylstyrene 
proceeds at a very slow rate. However, the presence of even a small amount 
of styrene greatly accelerates the rate of polymerization. Accordingly, it 
is advantageous to employ a mixture of alpha methylstyrene and styrene in 
order to accelerate the polymerization reaction. However, preferably the 
styrene content will be kept to less than about 1 part styrene to about 9 
parts alpha methylstyrene, since greater amounts do not significantly 
increase the polymerization rate, while the heat distortion temperature of 
the resulting polymeric product decreased as the proportion of styrene 
increases. 
The polymerization process of this invention is carried out under 
suspension polymerization conditions, wherein the monomer mixture together 
with the requisite polymerization initiator is fed into a stirred 
polymerization reactor containing as the suspending medium water and a 
minor proportion suspension stabilizer or suspending agent. 
The suspending agents useful are those conventionally employed for 
suspension polymerization including finely-divided inorganic salts such as 
calcium phosphate, talc or a variety of protective colloids including 
polyvinyl alcohol and the like. Inasmuch as the polymerization is run at 
elevated temperatures, the suspending aids which are the more effective at 
these temperatures will be preferred, including calcium phosphate. The 
suspension stability is further improved by the use of dispersing aids in 
the form of surfactants such as salts of alkyl aryl sulfonates and the 
like. 
The amount of water employed is such that the ratio of monomer phase to 
water phase is greater than about 1:1.5, and preferably greater than about 
1:1. As will be shown herein below, both the degree of conversion and the 
molecular weight of the final product increase with increasing 
monomer/water ratio, and below a ratio of about 1:1.5, the molecular 
weight of the product will be unacceptably low. At higher ratios, 
particularly at ratios at and above about 1:1, the molecular weight of the 
product increases to a useful level, and the degree of monomer conversion 
is increased to greater than 96%. As a practical matter, even though 
further increases result in further molecular weight improvements, 
monomer/water ratios in excess of about 1:0.5 become difficult to handle 
in conventional equipment due primarily to decreased suspension stability. 
The organic phase of the reaction mixture will contain catalytic quantities 
of a free radical polymerization initiator such as a peroxy or azo 
compound, e.g. t-butyl perbenzoate, 2,5-dimethyl-2,5-bis(benzoyl peroxy) 
hexane, 1-cyano-1-(t-butyl azo) cyclohexane, 
2-t-butylazo-2-thiophenoxy-4-methyl pentane and the like. 
The polymerization temperature employed will be greater than 90.degree. C., 
and more particularly from about 100.degree. C. to about 125.degree. C. 
Inasmuch as at these elevated temperatures both the monomers and the 
aqueous phase are volitile, the reaction will be carried out in a sealed 
reactor under autogenous or, more preferably under slightly elevated 
pressures by the addition of an inert gas such as nitrogen. 
The light colored, high molecular weight products of this invention result 
by carrying out the reaction under the described high solids and elevated 
temperature conditions, and neither controlled reagent addition nor 
programmed temperatures are required. Further, unlike prior art processes, 
the very high levels of conversion that result obviate the need for steam 
distillation to remove unreacted monomers and low molecular weight 
reaction by-products.

The invention will be further understood by consideration of the following 
examples. 
EXAMPLE 1 
Polymerization at 1:1 monomer phase to aqueous phase. A 20 gallon pressure 
reactor was charged with a solution of 0.9 lbs of Na.sub.3 PO.sub.4 
--12H.sub.2 O in 37.5 lbs of water. The solution was stirred, heated to 
65.degree. C., and a solution of 0.683 lbs of CaCl.sub.2.2H.sub.2 O in 
15.0 lbs of water was added, followed by a solution of 0.005 lbs of 
CaCO.sub.3 and 0.003 lbs of Naccona.sup.1 NRSF alkylaryl sulfonate 
surfactant in 15 lbs of water. After stirring 30 minutes, a mixture of 
49.5 lbs of alpha-methylstyrene, 23.25 lbs of acrylonitrile, 2.25 lbs of 
styrene and 0.174 lbs of 2,5-dimethyl-2,5-bis(benzoylperoxy)hexane was 
added. The reactor was purged with nitrogen, and sealed under 50 psi 
nitrogen pressure. The stirred mixture was heated to 104.degree. C. and 
held with stirring at that temperature for 24 hours. The mixture was then 
cooled to 48.degree. C., and sufficient HCl was added to give a pH of 
2.0. The bead product was isolated by centrifugation washed with water and 
air-dried at 71.degree. C. 
The product, obtained in 99% conversion, had the following non-aqueous 
volatile content: 
Alpha methylstyrene: 0.50 wt% 
Acrylonitrile: 0.23 wt% 
Styrene: 0.04 wt% 
The number average molecular weight of the product was 53,000, by Gel 
permeation chromatographic analysis. 
EXAMPLE 2 
The preparation of Example I was repeated. The product, after centrifuging 
and rinsing, was re-slurried in 200 parts demineralized water, then 
steam-sparged at atmospheric pressure for 8 hours, centrifuged and 
air-dried at 71.degree. F. 
The product, obtained in greater than 99% conversion, had a number average 
molecular weight of 53,000 and the following non-aqueous volatile content: 
Alpha methylstyrene: 0.47 wt% 
Acrylonitrile: 0.13 wt% 
Styrene: 0.01 wt% 
The products of Examples 1 and 2 were compression molded at 177.degree. C., 
giving light yellow moldings essentially identical in color and hue. 
It is thus apparent from the results of Examples 1 and 2 that high 
molecular weight products are obtainable from the high solids suspension 
polymerization process of this invention. Further, the products are 
produced in surprisingly high conversion, and have very low residual 
monomer content without resort to steam distillation or sparging. 
EXAMPLE 3 
The process of Example I was repeated, except that at the end of the 24 
hour heating period, the temperature of the mixture was elevated to 
130.degree. C. and held for 4 hours before cooling and isolating the 
product. The bead product, again obtained in 99% conversion, had 
essentially the same residual monomer (non-aqueous volatiles) content as 
the product of Example I, demonstrating that extended reaction times at 
elevated temperatures do not further improve conversion or further lower 
the residual monomer content. 
EXAMPLES 4-9 
A series of runs were carried out using different ratios of monomer phase 
to aqueous phase. The formulations and results are summarized in Table I. 
The procedures employed were essentially those of Example 1, the 
conditions modified as given in Table I. The materials are given in parts 
by weight. 
Table I 
__________________________________________________________________________ 
Example No: 4 5 6 7 8 9 
__________________________________________________________________________ 
Materials 
H.sub.2 O 300 180 122 122 82 55 
PVA.sup.(1) 0.67 
0.40 
0.37 
-- -- -- 
NaCl 2.5 1.5 1.39 
-- -- -- 
Talc 0.42 
0.25 
0.23 
-- -- -- 
Na.sub.3 PO.sub.4 . 12H.sub.2 O 
-- -- -- 1.2 1.2 1.2 
CaCl.sub.2 . 2H.sub.2 O 
-- -- -- 0.69 
0.69 
0.69 
CaCO.sub.3 -- -- -- 0.067 
0.067 
0.067 
Naccanol NSRF.sup.(1) 
-- -- -- 0.004 
0.004 
0.004 
A-Methylstyrene 
63 63 63 63 63 63 
Styrene 5.8 5.8 5.8 5.8 5.8 5.8 
Acrylonitrile 
31.2 
31.2 
31.2 
31.2 
31.2 
31.2 
Peroxide.sup.(1) 
0.232 
0.232 
0.232 
0.232 
0.232 
0.232 
T, .degree. C. 
100 100 100 100 100 100 
t, hrs. 24 24 18 24 20 24 
Monomer/water ratios 
1.00 
1.00 
1.00 
1.00 
1.00 
1.00 
3.00 
1.80 
1.22 
1.22 
0.82 
0.55 
Conversion % 94 96 97 96 96 97 
Mn 43000 
48000 
52000 
53000 
56000 
60000 
__________________________________________________________________________ 
Notes: 
.sup.(1) PVA is polyvinyl alcohol; Naccanol NSRF is alkyl aryl sulfonate 
surfactant; Peroxide is 2,5-dimethyl-2,5-bis (benzoylperoxy) hexane. 
It will be apparent from these data that the molecular weight of the 
product is markedly affected by the ratio of monomer phase to aqueous 
phase employed, with useful molecular weight products resulting at a 
monomer phase/aqueous phase ratio of about 1:1.5 or above. 
EXAMPLES 10-13 
A series of runs were carried out using the procedure of Example 3, but 
varying the ratio of monomers. The results are tabulated in Table II. The 
monomer amounts are in wt% of monomer mixture. The mixtures were run at 
monomer to water ratio of 1.00/0.67 (60 wt%). 
Table II 
______________________________________ 
Example No: 10 11 12 13 
______________________________________ 
Monomers: 
.alpha.-Methylstyrene 
63 65 66 69 
Acrylonitrile 
31.2 29 31.1 31 
Styrene 5.8 6 2.9 -- 
Conversion % 97 99 98 97 
Molecular wt 61,000 54,000 55,000 50,000 
______________________________________ 
It will be seen from the data for Examples 10-13 that at a given ratio of 
monomer phase to aqueous phase, changes in monomer ratio effect minor 
changes in conversion and in molecular weight. From the data for Example 
13, it will be apparent that useful polymeric products will also result 
when only .alpha.-methylstyrene and acrylonitrile are employed, provided 
that a high ratio of monomer phase to aqueous phase is used. 
EXAMPLES 14-18 
A series of runs were carried out using the following formulation: 
______________________________________ 
Demineralized water 
100 parts by weight 
Na.sub.3 PO.sub.4 . 12H.sub.2 O 
1.20 
CaCl.sub.2 . 2H.sub.2 O 
0.91 
CaCO.sub.3 0.067 
Wetting Agent 0.004 
.alpha.-Methyl Styrene 
66.0 
Styrene 3.0 
Acrylonitrile 31.0 
2,5-diemethyl-2,5- 
bis (benzoylperoxy) hexane 
0.232 
______________________________________ 
These runs were carried out at the polymerization temperatures shown in 
Table III. 
Table III 
______________________________________ 
Example No: 14 15 16 17 18 
______________________________________ 
Polymerization T,.degree. C. 
100 104 110 115 125 
Polymerization t, hrs.sup.(1) 
24 24 24 24 24 
Conversion % &gt;98 99 &gt;98 &gt;98 &gt;98 
Mn 60,000 53,000 43,000 
45,000 
39,000 
Melt Index.sup.(2) 
1.04 1.89 2.34 (5.1) (21.5) 
______________________________________ 
Notes: 
.sup.(1) time at the polymerization temperature. 
.sup.(2) 410.degree. F. condition A.sub.3, g./3 min.; value for Example 1 
calculated from condition A.sub.1 data; value for Example 18 calculated 
from condition B.sub.1 data. 
It will be apparent from the data for Examples 14-18 that products with 
useful molecular weights, i.e. at or above about 40,000 (Mn) result only 
if the polymerization temperature is maintained in the range 
100.degree.-125.degree. C. At lower temperatures reaction rates become too 
slow, and the final molecular weight becomes too high to be useful. At 
even higher temperatures, the trend to lower molecular weights results in 
products having molecular weights too low to be useful. 
COMATIVE EXAMPLE 
In U.S. Pat. No. 3,491,071, a process employing carefully controlled 
temperature staging during the polymerization step is disclosed. The 
patented process was employed with substantially the formulation of 
Examples 14-18, but employing 0.15 parts by weight ditertiary butyl 
peroxide as the initiator. The procedure followed was that set forth in 
Example III of the patent. 
The suspension was prepared, stirred and heated to 112.degree. C. as 
described in the patent, then gradually over a 9.5 hr. period to 
150.degree. C. Upon cooling, the reaction mass gelled, freezing the 
agitator. The mass was vacuum-dried for 16 hrs. at 100.degree. C., to give 
a polymer having a molecular weight (Mn) of 28,500 and a melt index at 
410.degree. F., condition A.sub.3 of too rapid to measure. The conversion 
based on recovery, was 85%. 
It is thus clear that the controlled staging technique of the patent is not 
applicable to the preparation of polymers from monomer mixtures having a 
high level of alpha-methylstyrene. Only the process of this invention, 
wherein longer polymerization times at a single temperature in the range 
100.degree.-125.degree. C. can be employed for the preparation of high 
molecular weight polymers in high conversion from such monomer mixtures by 
suspension polymerization. 
Generally, the process of the instant invention will be seen to be 
characterized as a suspension polymerization process for the preparation 
of polymers of from 60 to 70 wt percent alpha methylstyrene, from 32 to 
about 29 wt percent acrylonitrile and from 0 to 7 wt percent styrene, 
wherein the polymerization is carried out at a temperature of from about 
100.degree. to about 125.degree. C., for at least several hours and the 
ratio of monomer phase to aqueous phase is greater than about 1:1. Unlike 
prior art processes, the instant process produces high molecular weight 
polymers of good color and in high conversion without resorting to the use 
of expensive and time-consuming temperature programming, and the use of 
steam distillation and/or similar process steps to the reduce unreacted 
monomer content of the final product is unnecessary.