Process for preparing styrene polymers

The invention relates to a process for preparing polymers by bulk polymerizing a monomer feedstock composed of styrene or styrene-like monomers and/or anionically copolymerizable comonomers employing an anionic initiator and a conventional spray-tower as reactor in the absence of a quenching solvent that fills the lower portion of the spray-tower, and the polymers so produced.

FIELD OF THE INVENTION 
The invention relates to a process for preparing styrene polymers (i.e., 
polystyrene and copolymers of styrene and/or vinylarenes with 
copolymerizable comonomers), and the polymers so produced. 
BACKGROUND OF THE INVENTION 
Ionic bulk polymerization of styrene is not widespread. Typically, styrene 
polymers are prepared by thermal or radical-initiated (batch or 
continuous) bulk polymerization; solution polymerization; emulsion 
polymerization; or suspension polymerization. 
The bulk polymerization in a spray-tower of conjugated diolefins for the 
production of rubbers and of thermoplastic polymers is known from U.S. 
Pat. No. 3,350,377. The patent discloses the anionic polymerization of, 
e.g., 1,3-butadiene, optionally with other polymerizable monomeric 
materials such as, among others, styrene and methylstyrene. Additionally, 
the patent mentions the Ziegler-Natta catalysed polymerization of 
1-olefins (propylene, butylene, isobutylene, styrene, methylstyrene, 
etc.). This process comprises: (a) maintaining the monomeric liquid 
material at a first temperature at which polymerization will not occur; 
(b) admixing a suitable polymerization catalyst (such as butyllithium in 
the case of (co)polymerizing conjugated diolefins and Ziegler-Natta 
catalysts in case of 1-olefins) with said monomeric material at said first 
temperature; (c) spraying the resulting admixture downwards in the upper 
portion of a polymerization zone; (d) passing the vapor from a volatile 
hydrocarbon quench liquid heated to a second temperature at least above 
the polymerization initiation temperature of said admixture upwardly in 
said polymerization zone, thereby heating the downwards flowing spray of 
said admixture, and thus initiating polymerization; (e) permitting the 
polymer to quench and settle in the lower portion of said polymerization 
zone containing the relatively cooler quench liquid thereby vaporizing at 
least a portion of said quench liquid to produce said vapors that pass 
upwardly in said polymerization zone; and (f) withdrawing polymer solids 
from said lower portion. 
The disadvantage of this process concerns the removal of the quenching 
solvent, that is essential to initiate the polymerization and prevent 
run-away reactions, from the polymerized droplets in a subsequent drying 
zone. Moreover, the organometal compounds mentioned as (part of the) 
catalyst for polymerizing the 1-olefins are simply too hazardous to handle 
in such process (note that at the reaction conditions the alkylaluminium 
compounds are relatively volatile and extremely air and moisture 
sensitive). Finally, the patent specification merely mentions the 
possibility of polymerizing 1-olefins, without any reference to the 
molecular weight of the so produced thermoplastic polymer or its residual 
monomer content. This is not entirely surprising as the patent is 
primarily concerned with the production of rubbers whereas the remaining 
monomers are anyway removed in the drying zone. 
U.S. Pat. No. 3,644,305 discloses a process that is coined 
"spray-polymerization". It is similar to the process in U.S. Pat. No. 
3,350,377, however, concerning radical-initiated, solution polymerization 
of vinyl monomers, in particular water-soluble, ethylenically-unsaturated 
monomers. The disadvantages of this process, if applied in respect of the 
production of styrene polymers, concern the presence of the solvent and 
the lesser stability of the radical polymerized polymer. Moreover, 
according to the patent specification, the time of contact is relatively 
short and therefore spray polymerization provides only relatively low 
molecular weight polymers. 
The inventors set out to develop a process for anionically preparing 
styrene polymers, as anion-initiated polymerization more readily allows 
the production of copolymers, and provides more stable polymers (avoiding 
or at least reducing the need for light stabilizers). The ideal process 
should moreover be able to afford a wide range of different grades, 
produced in high yield and preferably in continuous mode at low fixed and 
variable costs. The process should also be highly flexible, avoiding the 
production of twilight materials. Naturally, problems associated with 
residual styrene monomer or viscosity of the styrene polymer should be 
evaded. 
SUMMARY OF THE INVENTION 
The inventors surprisingly arrived at the present process whereby styrene 
polymers of sufficient molecular weight can be prepared by anion-initiated 
bulk polymerization in a spray-tower without risk of detrimental run-away 
reactions and thus without requiring the presence of a quench liquid in 
the lower portion of the spray-tower. The anionic initiator is 
conveniently admixed with the monomer feedstock prior to the same being 
sprayed into the spray-tower. Accordingly, the anionic initiator is 
preferably soluble in the monomer feedstock.

DETAILED DESCRIPTION OF THE DRAWINGS 
The inventors surprisingly arrived at the present process whereby styrene 
polymers of sufficient molecular weight can be prepared by anion-initiated 
bulk polymerization in a spray-tower without risk of detrimental run-away 
reactions and thus without requiring the presence of a quench liquid in 
the lower portion of the spray-tower. 
The selection of initiator species, i.e., anionic initiator, is fairly 
crucial for proper operation of the present invention as the other 
initiators (radical, cationic, etc.) are too slow or not thorough enough 
to yield the desired styrene polymer. The anionic initiator is 
conveniently admixed with the monomer feedstock prior to the same being 
sprayed into the spray-tower. Accordingly, the anionic initiator is 
preferably soluble in the monomer feedstock. 
Suitable initiators are selected from alkyllithium, aryllithium, and, to a 
lesser extent (being less soluble and less reactive), from alkylsodium, 
arylsodium (for instance sodium naphthalene), alkylpotassium, 
arylpotassium or similar compounds known in the art. Very suitable anionic 
initiators are n-butyllithium and sec-butyllithium, the first for its 
stability, the latter for being more basic and hence more reactive. 
Sec-butyllithium is the preferred initiator. Other initiators that may be 
used advantageously are so-called capped initiators, i.e., initiators that 
are dormant below their activation temperature (K. Yagi et al, Journal of 
polymer science, vol. 14, 1976, pp. 1097-1105), but upon dissociation 
above that temperature turn into initiators that are as effective as 
regular initiators. Use of capped initiators, such as lithiumdiethylamide 
capped by pyridine, alleviates the cooling requirement at the 
prepolymerization stage discussed in detail herein after. 
The selected anionic initiators, when used to prepare styrene polymers in 
accordance with the invention, will provide high initiation and 
propagation rates. The advantage thereof is twofold: (1) the time to 
complete the polymerization reaction is short, allowing for the production 
of high molecular weight polymers within the available time, and (2) the 
reaction proceeds essentially to completeness. The residual monomer 
content is thus very low, in particular as the present anionic 
polymerization reaction is devoid of termination reactions (provided the 
monomer feedstock is sufficiently pure). 
To avoid premature polymerization, the anionic initiator is preferably 
admixed with the monomer feedstock just prior to being sprayed into the 
spray-tower. Nonetheless, it is also within the scope of the present 
invention to admix the anionic initiator and the monomer feedstock well 
before being sprayed into the reactor. In that case, the admixture should 
preferably be cooled sufficiently to essentially inhibit polymerization. 
If necessary, the admixture of monomer feedstock and anionic initiator may 
be brought to polymerization initiation temperature just prior to or upon 
spraying into the spray-tower. For the preferred embodiment, wherein a 
homopolymer is made of styrene using sec-butyl lithium as anionic 
initiator, that temperature suitably is about 0.degree. C., and heating 
occurs inside the spray-tower. 
To achieve high molecular weight styrene polymers, the anionic initiator is 
preferably employed in an amount relative to the monomer feedstock that is 
in the range of 0.1 to 30 mmole/mole. To produce styrene polymers having a 
number average molecular weight of about 150,000 to 300,000 the amount is 
preferably in the range of 0.2 to 1.0 mmole/mole. 
It is within the scope of the invention to enhance the polymerization rate 
and/or protect the living polymer chains against termination by die-out 
through the addition of small amounts of polar solvents such as THF or 
other ethers. In particular, use of crownethers may be beneficial (F. Cook 
et al, ACS paper Org. Coat. Plast. Chem., vol. 44, 1981, pp. 139-144). 
The monomer feedstock is selected from styrene; from styrene-like monomers; 
from mixtures of styrene with styrene-like monomers; from mixtures of 
styrene with anionically copolymerizable comonomers; from mixtures of 
styrene-like monomers with anionically copolymerizable comonomers; or from 
mixtures of styrene with styrene-like monomers and anionically 
copolymerizable comonomers. The mole/mole ratio of styrene and/or 
styrene-like monomers versus the anionically copolymerizable comonomers 
may vary broadly depending on the kind of polymer that is desired. A 
suitable mole/mole ratio is at least 1, preferably at least 10. Most 
preferably, the monomer feedstock is solely composed of styrene (i.e., the 
preferred embodiment mentioned above). 
The styrene-like monomers are suitably selected from vinylarenes 
(vinylnaphthalene, etc.) and vinyl-substituted or aryl-substituted 
vinylarenes (e.g., para-phenylstyrene, vinyltoluene, alpha-methylstyrene, 
divinylbenzene, etc.). The anionically copolymerizable comonomers are 
suitably selected from vinyl monomers having electron-withdrawing 
substituents (such as cyano or ester groups, etc.) or having groups 
providing resonance stabilization (especially butadiene, 
2-methyl-1,3-butadiene or 1,3-pentadiene). Naturally, the person skilled 
in the art will select the initiator, the composition of the monomer 
feedstock and the polymerization conditions (pressure and temperature) in 
such a manner that the respective initiation and propagation rates of the 
individual monomers allow for the polymerization to run smoothly and 
completely. 
The choice of pressure in the spray-tower used as reactor depends amongst 
others on the selection of the monomer feedstock. In one embodiment, the 
total monomer feedstock is in liquid form. In another embodiment, the 
spray-tower is pressurized to the level whereby part of the monomer 
feedstock is in vapor form. To minimize the risk of incomplete conversion, 
resulting in a too high residual monomer content, elevated pressures are 
preferred. As the pressure controls the residence time of the droplet by 
turbulent drag, elevated pressures further provide longer residence time, 
higher convective heat- and mass transfer coefficients and higher reaction 
rates. On the other hand, having part (of the most volatile comonomer) of 
the monomer feedstock in vapor form, provides opportunities for 
evaporative cooling (and thus for changing the temperature of the droplet 
where the exothermic reaction takes place) as well as for varying the 
properties of the copolymer. Although this latter embodiment may require a 
degassing step, due to the pulverulent or bead-like form (higher available 
polymer surface area), degassing at or above its glass transition 
temperature of the so-produced styrene polymer will be less difficult than 
degassing polymers in an extruder as carried out in for instance the 
continuous mass polymerization process. Foaming, which may occur when the 
pressure in the spray-tower is too low, resulting in too vigorous 
evaporation, should preferably be avoided. 
In the preparation of the homopolymer of styrene (i.e., polystyrene), the 
pressure is suitably in the range of 1 to 50 bar g, typically about 40 bar 
g. Styrene-like monomers usually have a higher boiling point, therefore 
generally requiring a similar pressure range. On the other hand, some of 
the comonomers mentioned above are more volatile and hence require higher 
pressures. Optimal conditions will be easily found by routine optimization 
test-runs. 
The spray-tower is preferably pressurized with an essentially inert gas. 
The person skilled in the art may select from a wide range of inert 
gasses, although nitrogen is preferred for being the least expensive. 
In an alternative embodiment, the spray-tower is pressurized with an inert, 
evaporated hydrocarbon and/or similar polymer solvent. This hydrocarbon 
and/or polymer solvent need not be removed, but is rather retained in the 
polymer particle to allow subsequent expansion of the particle when 
heated. The process therefore appears eminently suitable for preparing 
expandable polystyrene beads, in particular since it provides far better 
control of both the average particle bead size and the particle bead size 
distribution. 
Preferably the inert gas is introduced in counter current flow to the 
monomer feedstock, thereby prolonging the free-fall of the polymerizing 
droplets of monomer feedstock. In relatively high spray-towers the time of 
free-fall is already sufficient to ensure full conversion of the monomer 
feedstock upon reaching the bottom of the spray-tower (approximately 10 
seconds). In such spray-towers, the inert gas may be introduced at any 
level of the spray-tower, i.e., in counter current flow (upward), in 
co-current flow (downward) or in a more complex manner of flow. 
The temperature inside the spray-tower need not be elevated, but should 
rather be able to cool the polymerizing, free-falling droplets 
sufficiently to solidify the same upon reaching the bottom of the 
spray-tower. In case of the preferred embodiment (polystyrene), the exit 
temperature of the droplets should therefore not exceed 100.degree. C. 
This is achieved, for instance, by introducing the gas stream in the 
bottom part of the spray-tower, flowing counter currently upwards, which 
gas stream is introduced at a temperature in the range of 20.degree. to 
100.degree. C., typically about 25.degree. C. 
In the spray-tower, the nozzle and the size of the orifices of the nozzle 
should preferably be designed to create droplets of very uniform size, 
that are large enough to enable direct polymer processing of the resulting 
polymer spheres (typically 0.5 to 3 mm diameter) without being too large. 
Droplets that are too large will otherwise shatter by shear forces and 
also have a shorter residence time (risking incomplete conversion and 
higher terminal velocity). Droplets that are too small risk falling in a 
non-ballistic manner. Through collision with other droplets a variety of 
particle size polymer particles may result, having complex molecular 
weight distributions. A nozzle suitably used is that known by the skilled 
man as the cooled rotating bucket. 
In a further embodiment of the invention, the monomer feedstock comprises 
prepolymers, such as for instance high molecular weight polystyrene 
produced by thermal polymerization. Addition of such prepolymers provides 
control of the viscosity of the monomer feedstock and consequently 
additional control of the size of the droplets of polymerizing monomer 
feedstock formed in the spray-tower. Moreover, plasticity of the polymer 
particle (relevant for, e.g., EPS-like purposes) may so be differentiated. 
To ensure sufficient residence time to allow the monomer feedstock to fully 
convert into the styrene polymer, the spray-tower preferably has an 
internal height in the range of 2 to 50 m. An internal height shorter than 
5 m will require the inert gas-stream to be introduced at very high 
velocities, making the process more difficult to control. Spray-towers 
having an internal height larger than 50 m are very expensive, whereas the 
expense is not compensated by any advantage. 
DESCRIPTION OF PREFERRED EMBODIMENTS 
The process of the invention and the spray-tower suitably applied is 
illustrated by the following example of the anionic preparation of 
polystyrene particles that will have an average particle diameter of 2 mm, 
a number average molecular weight of approximately 100,000 and a residual 
styrene content of less than 500 ppm. A scheme of a counter current cooled 
spray-tower reactor is given in the FIG. 1. 
A cooled admixture 3 (0.degree. C.) of monomer feedstock (styrene) and 
anionic initiator (sec-butyl lithium 1.0 mmole/mole based on the styrene) 
is sprayed by a cooled rotating bucket 2 from the top of the spray-tower 1 
(effective height of 20 m) into the reaction compartment with a counter 
current pressurized gas-stream 4 of nitrogen/styrene gas (at 0.1 m/s, 20 
bar g, and 40.degree. C.). The big droplets 5 are quickly heated up by 
monomer condensation from the rising hot gas-stream and convective 
heating. The exothermic reaction is thereby initiated. The temperature is 
determined by convective and evaporative cooling, stripping the condensed 
monomer from the droplet. The droplet is converted to solid styrene 
polymer before it reaches the bottom of the spray-tower 1. The gas stream 
flowing upwards and exiting the spray-tower at the top is cooled down in a 
heat exchanger 6, whereupon the vaporized styrene is separated from the 
nitrogen in an ordinary condenser 7. The cooled gas is recycled to the 
bottom of the spray-tower 1. The liquid from the condenser 7 is monomer 
which is sent to a mixer 8 which mixes the recycled monomer, fresh 
monomer, and anionic initiator.