Clear impact resistant thermoplastic star-block copolymers
CROSS REFERENCE TO RELATED APPLICATION 
     This application is a continuation-in-part of copending application Ser. No. 9,708, filed Feb. 5, 1979. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to resinous star-block copolymers having improved impact properties and good clarity. 
     Highly branched block copolymers, sometimes called star-block copolymers, are old in the art of anionic polymerization. These star-block copolymers are prepared by first forming linear block polymers having active lithium atom on one end of the polymer chain. These active, linear polymer chains are then coupled by the addition of a polyfunctional compound having at least three reactive sites capable of reacting with the carbon to lithium bond on the polymer chains to add the polymer chain onto the functional groups of the compound. 
     Zelinski, U.S. Pat. No. 3,280,084, polymerized butadiene with butyllithium initiator to form B-Li blocks (where B is polybutadiene) which when coupled with divinylbenzene gave highly branched copolymers having polydivinylbenzene nuclei and several identical arms of polybutadiene branching therefrom. The arms can also be either random or block copolymers of styrene and butadiene (from A-B-Li blocks, where A is polystyrene segment) where the diene is the major component. 
     Childers, U.S. Pat. No. 3,637,554, prepares rubbery star-block copolymers having nuclei formed from polyepoxides, polyisocyanates, polyimines, etc., and identical arms from B-Li and A-B-Li. 
     Kitchen et al, U.S. Pat. No. 3,639,517, teaches that star-block copolymer may be formed having different molecular weight arms attached to the same nucleus. These arms are formed by using multiple additions of styrene monomer and initiator to form A-Li; A&#39;-Li; and A&#34;-Li, where A, A&#39; and A&#34; are polystyrene blocks of different molecular weights, then by a single addition of butadiene, and finally coupling these arms to form star-block copolymers having bi-, tri-, and polymodal molecular weight distribution in the arms. 
     Fetters et al, U.S. Pat. No. 3,985,830, discloses a product having at least two star-block copolymers linked together by a linking agent, such as phosgene, silicon tetrachloride or dihalosilanes. 
     Fodor, U.S. Pat. No. 4,051,197, discloses blends of two star-block copolymers, each having molecular weight distribution in its arms outside a given range, to form a blend having a distribution within the given range. 
     Fahrback et al, U.S. Pat. No. 4,086,298, discloses star-block copolymers having a mixture of arms wherein some are formed by first polymerizing styrene with alkyllithium to form A-Li blocks, and then adding a mixture of styrene and butadiene to form a graded copolymer represented by A-B→A&#39;, where the arrow represents a graded segment. Other arms are made up on only the butadiene-styrene graded copolymer segment. 
     Kitchen, U.S. Pat. No. 4,091,053, teaches that star-block copolymers having long integral hinge flex life and high hardness can be made by the method of U.S. Pat. No. 3,639,517 wherein the ratio of molecular weight of the styrene blocks in a longer (A&#39;) block to that of a shorter (A) block is maintained within the range of 3 to 7. 
     Vreugdenhil, U.S. Pat. No. 4,092,375, makes the bimodal star-block copolymers of Kitchen, U.S. Pat. No. 3,639,517, by separately preparing blocks of A-B-Li and B-Li, and then coupling mixtures thereof. 
     SUMMARY OF THE INVENTION 
     We have now found that transparent impact resistant thermoplastic star-block copolymers can be made which have good impact resistance at low temperature (-20 to -40° C.) as well as at room temperature. 
     The copolymers contain 60 to 90 percent by weight of a monovinyl aromatic compound and 40 to 10 percent by weight of a conjugated diene having 4 to 8 carbon atoms. The copolymers have the general formula (A-A&#39;/B-B&#39;) m  X--B&#39;-B/A&#39;) n  where A is a non-elastomeric polymer segment based on 80 to 95 percent by weight of the total monovinyl aromatic compound, A&#39;/B or B/A&#39; is an elastomeric copolymer segment based on a random copolymer of the monovinyl aromatic compound and the conjugated diene wherein the weight ratio of A&#39; to B must be less than 1.0, B&#39; is an elastomeric segment of poly (conjugated diene) containing 20-40 percent by weight of the total conjugated diene, m and n are integers whose sum is between 3 and 20, and X is the radical of a polyfunctional coupling agent forming the nucleus of the star-block copolymer. 
     DETAILED DESCRIPTION OF THE INVENTION 
     The thermoplastic, star-block copolymers of this invention are prepared by a four-stage process of anionic polymerization. 
     In the first stage, 80 to 95 percent by weight of the total amount of monovinyl aromatic compound is polymerized in an inert solvent using a hydrocarbyllithium initiator to form linear segments (A-Li), of the monovinyl aromatic compound having lithium ions at the ends. The polymerization is allowed to proceed to essential completion. 
     In the second stage, an additional amount of the hydrocarbyllithium initiator, equal to or greater than the amount added in the first stage, is added to the mixture from stage one, the remainder of the monovinyl aromatic compound is added, and 60 to 80 percent by weight of the total amount of the conjugated diene is continuously added at a rate, determined by the amounts of total monomer and the relative copolymerization rates of the monovinyl aromatic compound and the conjugated diene, such that an essentially random copolymer segment, B/A&#39;-Li, is formed. Simultaneously with the formation of the B/A&#39;-Li segments, the A-Li segments from stage two will form A-B/A&#39;-Li segments. 
     The second stage may also be carried out by other known means of preparing styrene/butadiene type random copolymers. For example, after the additional amount of initiator is added to the mixture from stage one, a mixture of the remaining monovinyl aromatic compound and 60 to 80 percent by weight of the total amount of conjugated diene can be added continuously to the reaction mixture at a rate such that an essentially random copolymer block is formed. Methods are also known for producing random copolymer blocks using as randomizer a polar compound, such as the tertiary amines, thioethers, or cyclic ethers. 
     In the third stage, the remaining 20-40 percent by weight of conjugated diene is added to the segments from stage two and allowed to polymerize to essentially complete conversion to form a polydiene block segment on each of the previously formed segments. Thus, the mixture now contains A-B/A&#39;-B&#39;-Li and B/A&#39;-B&#39;-Li segments. 
     In the fourth stage, a polyfunctional coupling agent is added to the solution of A-B/A&#39;-B&#39;-Li and B/A&#39;-B&#39;-Li segments and allowed to couple the segments into the radial block copolymer of general formula (A-B/A&#39;-B&#39;) m  X--B&#39;-A&#39;/B) n , where m and n are integers whose sum is between 3 and 20, A is the nonelastomeric polymer segments whose sum is based on 80 to 95 percent by weight of the total monovinyl aromatic compound, B/A&#39; is an elastomeric copolymer segment based on a random copolymer of the monovinyl aromatic compound and the conjugated diene wherein the weight ratio of A&#39; to B must be less than 1.0, B&#39; is an elastomeric segment based on 20-40 percent by weight of the total conjugated diene, and X is the radical of the polyfunctional coupling agent. 
     Preferred products will have elastomeric segments, B/A&#39;, whose weight ratio of A&#39; to B is between 0.25 and 0.67, more preferably between 0.33 and 0.54. 
     Similarly, preferred products will have a weight ratio of non-elastomer segment, A, to elastomeric segments, B/A&#39;, of greater than 1.0, such as between 1.2 and 6.0, more preferably between 1.5 and 3.0. 
     The monovinyl aromatic compound useful in the invention is preferably styrene, but may be alkyl substituted styrenes which have similar copolymerization characteristics, such as the ring substituted methylstyrenes, ethylstyrenes, and t-butylstyrene. 
     The amount of monovinyl aromatic compound useful in the invention is between 60 and 90 percent by weight based on the total weight of momomers utilized. 
     The hydrocarbyllithium initiators useful in the invention are the known alkyllithium compounds, such as methyllithium, N-butyllithium, sec-butyllithium; the cyclo-alkyllithium compounds, such as cyclohexyl-lithium; and the aryllithium compounds, such as phenyl-lithium, p-tolyllithium, and naphthyllithium. 
     The amounts of hydrocarbyllithium added should be between 0.2 and 10.0 millimoles per mole of monomer in each stage of the instant process. Each stage should use the same or greater amount of initiator than the previous stage. The total amount of initiator used depends on the molecular weight and number of polymer chains of each type desired. 
     The conjugated dienes useful in the invention are those having from 4 to 8 carbon atoms in the molecule, such as 1,3-butadiene, isoprene, 2,3-dimethyl-1,3-butadiene, piperylene and mixtures thereof. 
     The polymerization is conducted in an inert hydrocarbon solvent such as isobutane, pentane, cyclo-hexane, benzene, toluene, xylene and the like. The polymerization is carried out in the absence of air, moisture, or any other impurity which is known to be detrimental to anionic catalyst systems. The temperature of polymerization may be conventionally from 0° to 120° C., and preferably between 40° and 80° C. 
     The polyfunctional coupling agents suitable for the invention may be any of the materials known to have functional groups which can react with carbon to lithium bonds to add the carbon chain to the functional group. Typical examples of the suitable coupling agents are the polyepoxides, such as epoxidized linseed oil; the polyesters such as diethyl adipate; the polyhalides, such as silicon tetrahalide; the polyisocyanates, such as benzene-1,2,4-triisocyanate; the polyimines, such as tri(1-aziridinyl) phosphine oxide; the polyaldehydes, such as 1,4,7-naphthalene tricarboxaldehyde; the polyketones, such as 2,4,6-heptanetrione; the polyanhydrides, such as pyromellitic dianhydride; and the polyacid chlorides, such as mellitic acid chloride. Especially useful, and preferred herein, are the polyvinyl aromatic compounds such as divinylbenzene, which although only difunctional as monomers, can polymerize to form polyfunctional agents in situ and serve as coupling agents. Suitable are the ortho-, meta-, or para-divinylbenzenes, or mixtures thereof. 
     The amount and type of coupling agent used is dependent upon the number of polymer chains having lithium terminated ends and the number of arms desired per star-block molecule. Thus, for agents having a fixed number of functional groups such as silicon tetrachloride, an equivalent of agent per equivalent of lithium terminated polymer chains, gives a four armed star-block copolymer. In the case of difunctional agents which polymerize during the coupling reaction, such as divinylbenzene, the amounts of agent to be used must be determined for the conditions of reaction, since the number of equivalent functional sites is variable. 
    
    
     The following example is given to illustrate the invention, but not to limit the claims. All parts and percentages are by weight unless otherwise specified. 
     EXAMPLE I 
     A one gallon stirred reactor was charged with 2,000 g. of purified cyclohexane and heated to 70° C. A trace of diphenylethylene (0.2 g.) was added to the cyclohexane by means of a hypodermic needle. A solution of sec-butyllithium in cyclohexane was added to the reactor portionwise until a permanent orange-yellow color was obtained. The solution was then backtitrated with cyclohexane until the color just disappeared. The solvent and reactor were now ready for the polymerization of monomer. Into the closed reactor was charged 11.73 m. moles of n-butyllithium and 584 g. of styrene and the reactor held at 70° C. for 25 minutes. Analysis of the solution by U.V. analysis showed that less than 0.01% by weight of the styrene monomer remained. At this point, an additional 11.73 m. moles of n-butyllithium was added and a continuous feed of 144 g. of butadiene was added over 90 minutes using a controlled volume mini-pump. About 2 minutes after the start of the butadiene feed, a charge of 91 g. of styrene was added. When the last of the continuous feed was added, an additional 81 g. of butadiene was added and the whole mixture held for 30 minutes. There was then added 10.5 g. of divinylbenzene of 53% purity and the whole held for 1-2 hours at 70° C. to complete the linking reaction. The system was then terminated by the addition of 1 g. of methanol. The polymer solution was transferred to a 5 gallon polyethylene liner, diluted further with acetone and the polymer was precipitated by adding isopropanol under high speed stirring. The polymer was then treated with 1.0 part Polygard HR, a commercial antioxidant, and 0.5 part 2,6-ditertbutyl-4-methylphenol per 100 parts by weight of polymer. The wet polymer was dried by a hot air stream in a fluid bed dryer, then extruded and pelletized. 
     The product was analyzed and found to have about 10 linear arms which had a bimodal distribution in molecular weight prior to coupling. The longer arms had a number average molecular weight of about 84,000 consisting of a polystyrene block of number average molecular weight 67,000 a block of random styrene/butadiene copolymer of number average molecular weight 13,000 and a polybutadiene block of number average molecular weight 4,000. The shorter arms had a number average molecular weight of 17,000 and consisted only of a random styrene/butadiene copolymer segment and a polybutadiene segment identical to those in the longer arms. Number average molecular weights were determined by Gel Permeation chromatograph using polystyrene standards. The random nature of the rubbery styrene/butadiene segments was shown by plotting Tan δ vs. Temperature (from torsion pendulum measurements) and noting the single glass transition temperature at -45° C. Injection molded tensile bars of the star-block copolymer were very transparent and the physical properties are shown in Table I. The melt index was determined by ASTM D-1238 at 200° C. under 5,000  g. load. The Gardner falling weight impact test was performed on a Gardner Variable Impact Tester manufactured by Gardner Laboratory, Inc., of Bethesda, Md. Izod unnotched impact was measured by ASTM D-256 and the Flex modulus by ASTM D-790. The overall composition of the radial block copolymer was 75% by weight styrene and 25% by weight butadiene, with 86.5% of the total styrene being in the polystyrene block segment of the long arms and 13.5% of the total styrene being in the styrene/butadiene random segment. This material was free of orientation during the processing and non-stress whitening on bending. 
     
                       Table I                                                     
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             Ex. I      Comp. A                                           
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Melt Index (g./10 min)                                                    
               2.0          1.2                                           
Gardner Falling Weight                                                    
(in.-lb.) - at Room Temperature                                           
               320          20                                            
 at -20° C.                                                        
               320          6                                             
 at -30° C.                                                        
               320          0                                             
 at -40° C.                                                        
               100          0                                             
Izod Impact, Unnotched                                                    
(ft.-lb./in.)  23.2         3.8                                           
Flex Modulus (psi)                                                        
               154,000      229,000                                       
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     COMPARATIVE EXAMPLE A 
     A one gallon stirred reactor was charged with 2,000 g. of purified cyclohexane, heated to 70° C., and titrated with sec-butyllithium, as in Example I, to sterilize solvent and reactor. Into the closed reactor was charged 4.93 millimoles of n-butyllithium and 473 g. of styrene and the reactor held at 70° C. for 20 minutes to ensure complete polymerization of all the styrene to polystyryl chains terminated by active lithium ions. Then, an additional 18.53 m. moles of n-butyllithium and 203 g. of styrene were added and allowed to polymerize for 25 minutes. The mixture consisted of two different number average molecular weight chains of polystyryl lithium with essentially no residual styrene monomer. To this mixture was added 225 g. of butadiene and the polymerization allowed to proceed for 45 minutes to form identical polybutadiene blocks at the ends of both polystyrene blocks. To the resulting bimodal mixture of linear block copolymers was added 10.5 g. of 55% divinylbenzene and the whole held for 1-2 hours at 70° C. to complete the linking reaction. The highly branched copolymers having about 10 arms, having bimodal distribution were separated as in Example I and the physical properties measured. The results are also shown in Table I. Torsion pendulum measurements showed a Tg of -75° C., indicating that the butadiene was in the form of pure polybutadiene blocks, rather than the random styrene/butadiene blocks of Example I. 
     The unexpected superiority of the impact strength of the star-block copolymer of Example I over the known copolymer of the Comparative Example A is obvious from the Table I. Especially notable is the improved low temperature falling weight impact values. 
     EXAMPLE II 
     The procedure of Example I is repeated up to the point where the first addition of styrene has polymerized. At this point, an additional 11.73 m. moles of n-butyllithium is added and a continuous feed of a mixture of 91 g. of styrene and 144 g. of butadiene is added over 130 minutes using a controlled volume mini-pump. When the last of the continuous feed is added, an additional 81 g. of butadiene is added and the whole mixture is held for 30 minutes. The coupling agent is then added and the product worked up as in Example I. Analysis of the product will show that the product has the same structure and properties as the product from Example I.