PRODUCTION OF COPOLYMERS OF ALPHA-METHYLSTYRENE

Abstract: Copolymers of α-methylstyrene and other α-­ methylalkenyl aromatic monomers, particularly the copoly­ mers of α-methylstyrene with styrene, which exhibit excel­ lent homogeneity are produced with anionic catalyst in solution systems using intensive backmixing, recirculatory reactors by continuously charging a feed stream of the mixed monomers in which the ratio of reactive α-­ methylalkenylaromatic monomer to styrene or other alkenyl­ aromatic monomer is kept in low relative proportions while the reaction mass in which the copolymer is being formed is maintained as an essentially homogeneous mixture.

Description: PRODUCTION OF COPOLYMERS OF ALPHA-METHYLSTYRENE Copolymerized a-methylstyrene/styrene resinous products and analogous copolymerizates of alkenyl aromatic monomer mixtures containing copolymerized a-methylalkenyl aromatic moieties are notoriously difficult to prepare, especially homogeneous products. As disclosed in U.S. Patent No. 3,725,506, homogeneous a-methylstyrene/styrene analogous copolymers generally provide for fabrication purposes improved heat stability characteristics. Such copolymers have been prepared using conventional batch or- continuous plug flow processing techniques usually at polymerization temperatures of 600C or lower. These procedures, however,.have required feeding a-methylstyrene/styrene mixtures in relatively high respective proportional ratios. Nothing in prior art concerns a way to produce extraordinarily homogenous copolymerizates of a-methylstyrene and styrene by anionic polymerization in solution using closely controlled, low a-methylstyrene/styrene ratio monomer mixture input feed and intensive-mixing, recirculatory reactor units. The present invention involves feeding a mixture of alkenylaromatic monomers to an anionically -catalyzed reaction mass, most advantageously as a solution polymerization system, using an intensively and backmixed, continuous-mode accommodating recirculating coil or backmixed continuous stirred tank reactor wherein the reaction mass is maintained in an essentially uniform homogeneous condition until the copolymer product is ready for withdrawal from the reactor unit and subsequent finishing and solid copolymer product recovery operations. More specifically, the process for preparing an improved copolymer of a-methylstyrene containing 10 to 50 weight percent based on copolymer of at least one monomer of Formula IA: EMI2.1 with the balance of the copolymer comprising units of at least one other monomer of Formula IB: CH2=CHAr, (IB) where Ar in both formula (IA) and (IB) is an aromatic group of from 6 to 10 carbon atoms comprising (A) using an intensively and backmixed, continuous-mode-accommodating, recirculating reactor assembly having therein a reaction zone adapted for anionic polymerization of styrene at 70-1200C and capable of keeping a reaction mass thoroughly mixed and transferred omnidirectionally between upper and lower extremities of the reaction zone; (B) employing an effective amount of an organometallic anionic catalyst soluble in styrene for initiating the copolymerization; ; (C) charging to the reaction zone a mixture of Formulae IA and IB monomers components in proportionate quantities adapted to yield a copolymer containing 10 to 50 weight percent of copolymerized IA, said monomer mixture being charged as a solution in a solvent for the monomer mixture with a monomer mixture:solvent ratio on a parts by weight basis of 1:8 to 8:1, respectively; (D) keeping the reaction mass in the reaction zone in a generally homogeneous condition; (E) maintaining the concentration of unreacted Formula (IB) monomer in the reaction mass in the reaction zone at a level not more than 10 weight percent of the Formula IB monomer added to said mass; (F) holding the concentration of completed copolymer in the reaction mass in the reaction zone in the range of 30 to 70 weight percent; and (G) withdrawing a copolymer product solution as effluent from the reactor and recovering a homogeneous, heat stable copolymer product. Very homogeneous a-methylstyrene alkenylaromatic copolymer products are thus produced having very good heat stability with reliable and consistently reproducible results obtained in practice with conventional apparatus and handling techniques. Figure 1 of the accompanying Drawing demonstrates as a general comparison the substantially higher a-methylstyrene/styrene monomer ratio involved in the feed stream input of prior batch or continuous plug flow processes run above 700C to obtain a given a-methylstyrene/styrene copolymer composition product in comparison with the lower monomer mixture ratio used when the copolymerization is anionically initiated in a continuous stirred tank reactor ("CSTR"). The copolymer products produced with excellent homogeneity by practice of the present invention are alkenyl aromatic copolymers containing in copolymerized form 10 to 50 weight percent (based on copolymer) of an a-methylalkenylaromatic monomer of Formula IA: EMI4.1 with the balance of the copolymer being a copolymerized monomer of Formula IB: CH2=CHAr, (IB) wherein Ar is a 6 to 10 carbon aromatic group. Advantageously, a-methylstyrene is the Formula IA monomer that is utilized although it is sometimes of considerable benefit to utilize ring-substituted a-methylstyrenes such as paraisopropenyltoluene. It is frequently preferable to utilize styrene as the Formula IB monomer to be copolymerized although other species of Formula IB monomers may also be suitably employed such as vinyltoluene, the dimethylstyrenes, t-butylstyrene, and vinylnaphthalene. Various mixtures of Formula IA monomers or Formula IB monomers, or both especially including a-methylstyrene or styrene may be copolymerized to provide the advantageous homogeneous copolymer products. It is frequently more desirable to prepare copolymer products having 10 to 40 weight percent of Formula IA monomer copolymerized therein. The polymerization initiators utilized in the polymerization process are best when soluble in the reaction mass undergoing copolymerization. They are well known organometallic anionic initiators including n-butyllithium; sec-butyllithium; and/or equivalent catalysts, used in conventional effective amounts as disclosed in U.S. Patents 3,322,734, 3,404,134, 4,172,100, 4,172,190, 4,182,818, 4,196,153, 4,196,154, 4,200,718, 4,201,729, and 4,205,016. n-Butyllithium is usually a convenient and satisfactory anionic initiator for purposes of present practice. It is not possible in practice of the present invention to produce the homogeneous copolymer products by "mass" or "bulk" polymerization techniques wherein the monomer or incompletely-polymerized monomer constituents provide the fluid vehicle for the reaction mass. However, it is generally more advantageous and desirable to utilize solution polymerization techniques. In such cases, the solvent is fed along with the monomer mixture to the reaction mass in the polymerization apparatus employed in appropriate proportions. For solution polymerization ethylbenzene is a very good solvent for monomer mixture dilution. Many others can also be utilized in conventional amounts normally employed for the anionic polymerization of styrene. Besides prerequisite dissolving power, the solvent employed must not interfere with function of the anionic initiator. Thus, the solvent should not have deleterious proportions of interfering components such as oxygen containing or active-hydrogen constituents. Boiling point is also significant in solvent selection. Fluids too volatile may require overly-expensive and undesirable pressure-handling capability of the process equipment. On the other hand, "high boilers" tend to cause removal problems in product recovery. Thus, benzene, toluene, xylene and cumene or mixtures thereof are suitable alternatives to ethylbenzene. Also excess monomers of Formula IA can be used as solvents in polymerization as the polymerization is conducted at temperatures above the ceiling temperatures of these monomers. The quantity of solvent relative to the monomer mixture being fed and/or present in the reaction mass is often advantageously about an equal amount on a weight basis. However, this can often be acceptably varied from a 2:3 to 3:2 monomer mixture:solvent weight ratio; and even 1:8 to 8:1 parts by weight monomer:solvent ratios may be tolerable. Recirculating coil and equivalent reactors of the type similar to those described in U.S. Patents 3,035,033 and 3,838,139 are generally suitable for use in the process of the present invention. It is of utmost desirability for the apparatus to thoroughly mix together all parts and segmental portions of the reaction mass mixture present in the reaction zone; this being usually accomplished by means of stirrers or agitators capable of readily and efficiently and effectively transferring the reaction mass both from the upper to the lower part of the involved reaction zone in the equipment, and vice versa. Normal operating temperatures of 70 -120 C with reaction zone residence times in the reactor usually being not more than about 3 hours can be utilized in the practice of the present invention. In practice there are three factors of crucial and critical importance to secure desired results in the present invention. These are: (1) the weight ratio of Formula IA monomer to Formula IB monomer in the feed stream being charged into the reaction zone should be in the range of 0.1 to 2.0; (2) the concentration of Formula IB monomer in the reaction mass undergoing copolymerization should be maintained at a level of not more than 10 weight percent of the Formula IB monomer added to said mass; and (3) the concentration of completed copolymer in the reaction mass should be kept in the range of 30 to 70 weight percent. One of the practical consequences of the factor (1) above employing a lower boiling solvent is that it minimizes the amount of less volatile residual Formula IA monomer in the copolymerized reaction mass, therefore, making easier the finishing operations, particularly the important devolatilization step. By way of emphasis, another essential for good results from practice of the invention is the keeping of the reaction mass mixture in a thoroughly homogeneous condition throughout the anionic copolymerization. Polymerization in accordance with the present invention can be optimized to provide maximum amount of polymer per unit of reactor, or, if desired, to provide the maximum quality of polymer per unit of reactor. If crystal clarity and minimum yellowness is desired, polymerization should be conducted using a minimum amount of lithium initiator. For maximum amount of polymer, greater quantities of the lithium initiator are used. One can obtain a polymer of desired molecular weight distribution using smaller amounts of lithium by incorporating a chain transfer agent. Very convenient -chain transfer agents are methylbenzenes such as toluene, all of the xylene isomers, hemimellitene, pseudocumene, mesitylene, and methylethylbenzene. Desirably, the boiling point of such chain transfer agent is below 2000C and preferably below about 1650C. Figure 2 graphically depicts the relationship between the yellowness index as determined by the ASTM Test D1925 and the lithium content of polymer. If prime color-free polymer is desired, low initiator level with chain transfer agent provides optimum product. If maximum reactor efficiency is required without regard to the color of the polymer, higher -levels of lithium initiator can be employed. For maximum quantity in a color sense, conversion of monomer to polymer as low as 90 percent may be utilized, whereas if some color can be tolerated in the polymer, conversions in excess of 99 percent are possible. A recirculating coil reactor unit similar to that shown in U.S. Patent No. 3,035,033 was utilized for the making of three separate Runs to prepare copolymeric product for demonstration of the present invention. The reactor was a. loop of 2.5 cm (1 inch) inside diameter stainless steel tubing having a Northern Gear pump number 4448 operating at 200 revolutions per minute to provide recirculation within the loop. The internal volume of the reactor was 900 milliliters. The tubing of the reactor was wrapped with 0.64 cm (1/4-inch) copper tubing and steam heated. Feed and initiator were pumped into the gear pump. The initiator was n-butyllithium and was handled as a 1.5 weight percent solution in ethylbenzene. The rate of initiator addition was controlled to provide a constant color in the reaction mixture. In each of the Runs, monomer mixtures, solvent and n-butyllithium initiator were fed into the reactor at a rate such that a 1.5 hour residence time was had. The effluent from the reactor was continuously devolatilized to obtain the product copolymer resin in each Run. Operational and materials parameters and the results obtained are set forth in the following tabulation. Run Numnber: 1 2 3 Feed Composition by Gas Chromatographic Analysis: Weight % &alpha;-methylstyrene 28 10 18 Weight % styrene 41 42 33 Weight % ethylbenzene 31 48 49 Copolymer Composition by Calculation: Weight % &alpha;-methylstyrene 32 14 22 Weight % styrene 68 86 78 Volatile Composition by Gas Chromatograph: Weight % &alpha;-methylstyrene 21 6 17 Weight % styrene 0,3 0,3 0,5 Weight % ethylbenzene 78 93 82 Wt. % Solids in Reaction Mass 60 49 42 Polymerization Temperature, C 102 88 75 n-Butyllithium Used in Parts per Million by weight in monomer 260 240 130 Weight Average Molecular Weight of Product (by Gel Permeation 287,000 168,000 345,000 Chromatography) Each of the a-methylstyrene/styrene copolymer products had very good thermal stability characteristics during molding operations and heat exposures testings.

Claims: WHAT IS CLAIMED IS: 1. Method of preparing homogeneous, heat stable thermoplastic resinous copolymers of a-methylstyrene containing 10 to 50 percent by weight based on copolymer of at least one monomer of Formula IA: EMI12.1 with the balance of the copolymer comprising units of at least one other monomer of Formula IB: : CH2=CHAr, (IB) wherein Ar in both formula (IA) and (IB) is an aromatic group of from 6 to 10 carbon atoms comprising (A) using an intensively and backmixed, continuous-mode-accommodating, recirculating reactor assembly having therein a reaction zone adapted for anionic polymerization of styrene at 700 to 1200C and capable of keeping a reaction mass thoroughly mixed ahd transferred omnidirectionally between upper and lower extremities of the reaction zone; (B) employing an effective amount of an organometallic anionic catalyst soluble in styrene for initiating the copolymerization; ; (C) charging to the reaction zone a mixture of Formula IA and IB monomers components in proportionate quantities' adapted to yield a copolymer containing 10 to 50 weight percent of copolymerized IA, said monomer mixture being charged as a solution in a solvent for the monomer mixture with a monomer mixture:solvent ratio on a parts by weight basis of 1:8 to 8:1, respectively; (D) keeping the reaction mass in the reaction zone in a generally homogeneous condition; (E) maintaining the concentration of unreacted Formula IB monomer in the reaction mass in the reaction zone at a level not more than 10 weight percent of the Formula IB monomer added to said mass; (F) holding the concentration of completed copolymer in the reaction mass in the reaction zone in the range of 30 to 70 weight percent; and (G) withdrawing a copolymer product solution as effluent from the reactor and recovering a homogeneous, heatsstable copolymer product. 2. The method of Claim 1 wherein the monomeric mixture is charged into the reactor to provide a copolymer product containing 10 to 40 percent by weight of at least one Formula IA monomer copolymerized therein. 3. The method of Claim 1 wherein the Formula IA monomer is a-methylstyrene. 4. The method of Claim 1 wherein the Formula IB monomer is styrene. 5. The method of Claim 1 wherein the Formula IB monomer is p-isopropenyltoluene. 6. The method of Claim 1 wherein the monomers are a-methylstyrene and styrene. 7. The method of Claim 1 wherein the initiator is n-butyllithium. 8. The method of Claim 6 wherein ethylbenzene is employed as solvent. 9. The method of Claim 6 wherein xylene, and mesitylene and mixtures thereof are employed as solvent. 10. The method of Claim 1 wherein an excess of the Formula IA monomer is used as the solvent.

Inventors: PRIDDY, DUANE BRUCE
Applicants: THE DOW CHEMICAL COMPANY
Application year: 1983
Main classification: C08F212
IPC codes: C08F12, C08F212, C08F2