Sequential Coupling in Formation of Resinous Block Copolymers

This invention relates to resinous copolymers. In one aspect, this invention relates to a novel polymerization process for the production of polymodal resinous block copolymers with improved melt strength properties.

Resinous block copolymers have been produced by methods employing various sequential polymerization steps. Among the pioneer inventions in the field of resinous block copolymers are such patents as U.S. Pat. No. 3,639,517 to Kitchen and Szalla, U.S. Pat. No. 4,080,407 to Fodor, and U.S. Pat. No. 4,091,053 to Kitchen.

Much effort has been directed to the preparation of substantially transparent block copolymer resins with a variety of block structures produced by a variety of monomer addition sequences and a variety of coupling agents.

One of the more important applications for substantially transparent block copolymer resins is in the packaging and related industries where there is a need for thermoplastic polymers suitable for use in conventional injection and blow molding and in other methods of forming plastics into containers, tubes, film and the like. For applications which require blow molding of the thermoplastic resin it is important that the parison have sufficient melt strength to support itself when blow molding larger parts. Better melt strength is normally associated with low melt flow; however, melt flow must be high enough to maintain processability.

It is an object of this invention to provide resinous copolymers which have improved melt strength properties. It is another object of this invention to provide resinous copolymers which have good melt strength while maintaining processability. An object of this invention is to provide a substantially transparent copolymer that has sufficient melt flow to maintain processability in blow molding operations and also has sufficient melt strength to support the parison during blow molding of larger objects. It is yet another object of this invention to provide methods for producing these resinous copolymers.

In accordance with this invention copolymers are prepared by a process of sequential charge copolymerization comprising the solution polymerization of at least two charges, at least one of these being a conjugated diene and at least one of these being a monovinylarene; followed by a charge of a polyfunctional coupling agent; and the final charge being essentially a difunctional coupling agent.

In accordance with one embodiment of this invention a method for preparing a polymer comprises:

solution polymerization of at least one monovinylarene monomer and at least one conjugated diene monomer in a ratio of about 55 to 95 weight percent monovinylarene and about 45 to 5 weight percent conjugated diene, by a sequential charge polymerization process to produce growing polymer chains,

said sequential charge polymerization process comprising:

charging with a monovinylarene and a monoalkali metal initiator;

charging with a monovinylarene and a monoalkali metal initiator;

charging with a conjugated diene monomer;

charging with a monovinylarene and a monoalkali initiator;

charging with a conjugated diene monomer;

charging with an amount of a polyfunctional coupling agent insufficient for complete coupling of said growing polymer chains;

after substantially complete reaction of said polyfunctional coupling agent, charging with a difunctional coupling agent.

In the drawings which form a part hereof:

FIG. 1 is a graph of strain rates versus polymer melt flow when tetraethoxysilane coupling and difunctional coupling were done;

FIG. 2 is a graph of strain rates versus polymer melt flow when tetramethoxysilane coupling and difunctional coupling were done;

FIG. 3 shows molecular weight distribution curves of a control polymer and of an experimental polymer at several points during polymerization;

FIG. 4 is a graph of strain rates versus polymer melt flow when dimethyl isophthalate coupling and difunctional coupling were done; and

FIG. 5 is a graph of results of sag tests of controls and of the experimental polymer.

The polymers of this invention are characterized as resinous, polymodal block copolymers of at least one conjugated diene with at least one monovinylarene, and are prepared so that at least a portion of the final product is of a branched coupled character.

The copolymers contain about 55 to 95, preferably 60 to 90, more preferably 65 to 85, percent by weight of copolymerized monovinyl aromatic compound (monovinylarene), and correspondingly about 45 to 5, 40 to 10, or 35 to 15 percent by weight of copolymerized conjugated diene. The coupled portions of the resinous, polymodal block copolymers have terminal polymonovinylarene blocks on the extending arms of each linear or radial copolymer molecule, and further contain a central internal block of polyconjugated diene, ignoring any interruption of the internal block by a coupling agent residue. The resinous copolymeric polymodal products also contain portions of linear uncoupled block copolymers of poly(monovinylarene)-poly(conjugated diene); the linear uncoupled block copolymer content is considered to be an important portion of the resinous product with respect to its overall properties.

It has been found that similar copolymers previously known in the art with sufficient melt flow to maintain processability in blow molding operations lacked the necessary melt strength to support the parison during blow molding of larger objects. The present invention has solved that problem by first coupling fewer than all the growing polymer chains with a polyfunctional coupling agent, then completing the coupling in a separate step with essentially no polyfunctional coupling agent and substantially only difunctional coupling agents. This particular sequence and relative amounts of coupling agents are necessary to produce the unique polymodal structure which results from a first, incomplete branched coupling followed by essentially linear coupling.

The detailed description and tables following show the preferred charging sequences and the results of tests of the properties of the inventive copolymer compared with those of copolymers not prepared in accordance with this invention. The differences in results are significant, with the inventive copolymers exhibiting improved melt strength properties.

The solution polymerization process is carried out as is known in the art in a hydrocarbon diluent at any suitable temperature such as in the range of about -10.degree. to 150.degree. C., more usually about 0.degree. to 110.degree. C., at a pressure sufficient to maintain the reaction mixture substantially as a liquid. Preferred are cycloparaffins, alone or in admixture with such as pentane or isooctane. Presently preferred is cyclohexane. As is known, small amounts of polar compounds, such as tetrahydrofuran, can be included in the diluent for vinyl control of the diene polymer blocks, and/or to improve effectiveness of some initiators such as the primary alkyllithium initiators for monovinylarene polymerizations. U.S. Pat. No. 3,639,517 (Kitchen and Szalla), U.S. Pat. No. 4,080,407 (Fodor) and U.S. Pat. No. 4,091,053 (Kitchen) which disclose polymerization procedures are incorporated herein by reference.

The conjugated diene monomers which can be used contain 4 to 6 carbon atoms and include 1,3-butadiene, 2-methyl-1,3-butadiene, 2-ethyl-1,3-butadiene, 2,3-dimethyl-1,3-butadiene and 1,3-pentadiene and mixtures thereof. Presently preferred is 1,3.butadiene.

The monovinylaromatic monomers which can be used contain 8 to 12 carbon atoms and include styrene, alpha-methylstyrene, p-vinyltoluene, m-vinyltoluene, o-vinyltoluene, 4-ethylstyrene, 3-ethylstyrene, 2-ethylstyrene, 4-tert-butylstyrene and 2,4-dimethylstyrene and mixtures thereof. Presently preferred is styrene.

The initators can be any of the organomonoalkali metal compounds known for such purposes. Preferably employed are the hydrocarbylmonoalkali metal compounds which correspond to the formula RM in which R is a hydrocarbyl aliphatic, cycloaliphatic, or aromatic radical, preferably alkyl, and M is an alkali metal, preferably lithium. Presently preferred are alkylmonolithium initators such as sec- and n-butyllithium. The amounts of monoalkali metal-based initiator employed depend upon the desired polymer or incremental block molecular weight, as is known in the art, and are readily determinable from the ranges of molecular weight desired, making due allowance for traces of poisons in the feed streams.

The polymerization is conducted in the substantial absence of air or moisture, preferably under an inert atmosphere. The resulting polymers contain a very high percentage of molecules in which an alkali metal atom is positioned at an end of the polymer chains. Of course, traces of impurities present in the feeds, such as water or alcohol, tend to reduce the amount of monoalkali metal-terminated polymer formed. Thereafter, the coupling steps are performed.

The term "coupling" as used in this application means the bringing together and joining, by means of one or more central coupling atoms or coupling moieties, two or more of the living monoalkali metal-terminated polymer chains.

Typically, the total amount of coupling agent is in the range of about 0.1 to 10 phm (parts per 100 parts by weight of total monomers employed in the polymerization), presently preferably about 0.2 to 1 phm.

For the purposes of this application, polyfunctional (or multifunctional) coupling agents are defined as those having three or more active sites for reaction with polymer lithiums. Polyfunctional coupling agents considered useful in the first coupling step of the inventive process include alkoxysilanes, such as Si(OR).sub.4, HSi(OR).sub.3, RSi(OR).sub.3 ; cyclic anhydrides such as ##STR1## and their esters and diesters, such as ##STR2## in which R can be alkyl groups containing from 1 to about 18 carbon atoms, with those containing 1 to 4 carbon atoms preferred. Examples of suitable polyfunctional coupling agents include tetramethoxysilane, tetraethoxysilane, tetrabutoxysilane, tetradodecyloxysilane, trimethyoxysilane, triethoxysilane, trihexyloxysilane, phthalic anhydride, 1,2-cyclohexanedicarboxylic anhydride, dimethyl phthalate, dibutyl phthalate, diethyl 1,2-cycyclohexanedicarboxylate, dimethyl isophthalate, dihexyl isophthalate, dimethyl terephthalate, diethyl terephthalate, didodecyl terephthalate, and the like. Each acyl group of the esters and anhydrides is considered to be difunctional for coupling since two molecules of polymer lithium can react with the acyl group. Other acceptable polyfunctional coupling agents are multiisocyanates, multiimines, multianhydrides, multihalides particularly halosilanes, multiesters multiketones tri alkylarylphosphites and alkoxytin compounds. Presently preferred are alkoxysilanes such as tetraethoxysilane, tetramethoxysilane, trimethyoxysilane and triethoxysilane and esters such as dimethyl isophthalate. Most preferred are tetraethoxysilane and tetramethoxysilane. Mixtures of two or more polyfunctional coupling agents can be used.

The first coupling step should be substantially completely reacted before adding the second coupling agent. If both types of coupling agents are added together, difunctional coupling will dominate. The progress of the first coupling step can be determined by gel permeation chromatography (gpc) which will show multifunctional branching by the appearance of several new peaks compared with the gpc curves of products made with essentially difunctional coupling agents or with mixtures of multifunctional and difunctional coupling agents.

Coupling agents considered useful in the second coupling step of the inventive process include a wide range of essentially difunctional (or bifunctional) coupling agents as are known in the art, such as dialdehydes, diketones, dihalosilanes, diepoxides, monoesters, lactones, vegetable oils, epoxidized vegetable oils, and the like. Examples include hexanedial, phthalaldehyde, 2,5-hexanedione, dichlorodimethylsilane, dichlorodiphenylsilane, butadiene dimer diepoxide, methyl benzoate, butyrolactone, refined soybean oil, epoxidized soybean oil, and the like. Presently preferred as the essentially difunctional agent for the second coupling step of this invention is epoxidized vegetable oil. Most preferred is epoxidized soybean oil such as Vikoflex 7170 from Viking Chemical.

Combinations of two or more essentially difunctional coupling agents, including combinations from different functional groups, can be used.

Any effective amount of the coupling agent can be employed in either of the two coupling steps. While the amount is not believed to be particularly critical, an amount sufficient to give multifunctional coupling as determined by gel permeation chromatography is presently preferred in the first coupling step. It is essential that in the first coupling step, the amount of polyfunctional coupling agent added be insufficient for complete coupling of all of the growing polymer chains. In the second coupling step the amount of difunctional coupling agent used is equal to, or in excess of, the amount needed to complete the coupling.

The amount of multifunctional coupling agent in the first step is about 5 to 80 weight percent of the total amount of coupling agents, preferably 10 to 50 weight percent, more preferably 15 to 30 weight percent. The amount of essentially difunctional coupling agent in the second step is about 95 to 20 weight percent of the total coupling agents, preferably 90 to 50 weight percent, more preferably 85 to 70 weight percent.

The reactions are carried out under conditions sufficient to cause multifunctional coupling in the first coupling step and difunctional coupling in the second coupling step. The temperature of the coupling steps can be any effective temperature and is generally between about 30.degree. and about 150.degree. C. Preferably the temperatures is between 50.degree. and 130.degree. C., and more preferably 80.degree. to 110.degree. C. The temperature of the first and second coupling steps can be the same or different. Excess physical conditions, such as higher reaction temperatures, should be avoided in the second coupling step to minimize deleterious reactions.

At the conclusion of the two-step coupling process, the system is treated with an active hydrogen compound such as water, alcohol, phenols or linear saturated aliphatic mono" and dicarboxylic acids to remove the lithium from the polymer. Preferably, the polymer cement, i.e. the polymer in the polymerization solvent, is treated with terminating agents such as water and carbon dioxide.

The resins are then stabilized with suitable stabilizers, for example, a combination of a hindered phenol and an organophosphite, specifically, octadecyl 3-(3',5'-di-t-butyl-4'-hydroxyphenyl) propionate and tris-nonylphenylphosphite. After stabilization, the hydrocarbon diluent is then flashed off the polymer solution to increase the solids content.

The resulting copolymers have a melt flow according to ASTM D 1238-86 at 200.degree. C. with a 4.9 kg weight from about 1 to about 15 g/10 min.; more preferably, from about 5 to about 10 g/10 min.; most preferably, from about 6 to about 8 g/10 min.

The polymodal resinous block copolymers produced by the methods of this invention contain mixture of two or more different length butadiene-styrene copolymers coupled with a multifunctional coupling agent and an essentially difunctional coupling agent. The fraction of polymer coupled with the multifunctional coupling agent is substantially branched; the fraction of polymer coupled with the difunctional coupling agent is substantially linear.

The following examples will describe in more detail the experimental process used and the polymodal block copolymers obtained as a result of the process.

In the following examples, dried cyclohexane (85%), butadiene (Texas El Paso), and styrene (Stirling Chemical) were used. The dried cyclohexane and styrene were given a nitrogen sparge and the solvent and monomers were stored in charge tanks. Dry, uninhibited monomers were stored in a deep freezer. n-Butyllithium was purchased from Lithium Corporation of America and was charged as a 2 weight percent solution in cyclohexane. Coupling agents used are listed below:

Polymerizations were carried out in an 8.8 liter stainless steel reactor. The monomer, solvent, and initiator charge tanks were connected with direct lines to the reactor. Weights of solvent, monomer, and initiator charged to the reactor were determined by measuring differences in weight on an electronic balance. Pressure was maintained by nitrogen and polymerizations were adiabatic once initiated at 50.degree. C. When required, warming was controlled by automatically-regulated, steam addition to the water jacket.

The following polymerization recipe was used in the examples with changes in coupling agents and n-butyllithium levels. phm is the quantity in parts per hundred parts by weight of total monomer charge.

Cyclohexane was flashed from the resin leaving about 14 weight percent cyclohexane in the polymer. The polymer was dried in a vacuum oven (92.degree. C.) for one hour, chopped, and dried another hour in the vacuum oven.

Polymer molecular weight was determined by gel permeation chromatography using a Waters Ultrastyragel column with tetrahydrofuran as solvent and a UV detector. Melt flows were measured according to ASTM D 1238-86 using an extrusion plastometer at 200.degree. C. with a 4.9 kg weight.

"Sag tests" of the inventive copolymer were done using extruded sheet. The test was done with a Comet Labmaster Model L-5 Thermoformer with the bottom heater on full and the top heater off. The dwell time was 60 seconds and sag was measured. The sheet thickness was measured with a TMI Model 549 Micrometer. Samples about 35.5 cm by about 15.5 cm were used. The samples were clamped at the ends of the longer dimension and the distance between clamps was 33 cm.

A laboratory test was developed to give an indication of the polymer melt strength. A Rheometrics Mechanical Spectrometer (RHS) (Rheometrics, Inc., Piscataway, N.J.) measures strain verses time at a constant stress. A low strain rate indicates more chain entanglements and a higher melt strength than a high strain rate. The strain rate tests were carried out with a cone and plate configuration to measure strain versus time at 190.degree. C. with a constant stress of 1.times.10.sup.4 dynes/cm.sup.2. A minimum of for hours warm-up time was used to bring the RMS to equilibrium temperature before starting the test runs. Strain rates should be compared between samples with the same melt flow since the polymer melt strength is also a function of molecular weight.

This example illustrates the preparation of butadiene-styrene copolymers with improved melt strength by partially coupling the living polymer with a multifunctional coupling agent [tetraethoxysilane - (OEt).degree. Si]and completion of the coupling with a conventional, essentially bifunctional coupling agent (Vikoflex 7170).

The polymers are summarized in Table 1. Polymers 1 and 2 are control resins made to different melt flows without a multifunctional coupling agent and with the Vikoflex coupling agent. Polymers 3 through 7 are invention polymers made with different levels of n-butyllithium, tetraethoxysilane, and Vikoflex.

The Rheometrics Mechanical Spectrometer (RMS) strain rates in %/min at 190.degree. C. for these polymers (Table 1) show that the invention polymers have lower values (higher melt strengths) than control polymers of the same viscosity. In order to clearly show the differences in values, the strain rate values from Table I are plotted in FIG. 1 versus the polymer melt flow. A line drawn through the two control polymer points shows the normal relationship between the melt flow and strain rate. Samples falling below the line, low strain rate, would be expected to show improved melt strengths over control polymers with the same melt flow. Invention samples 3 through 7 are clearly below the line, demonstrating improved melt strengths from the two-step coupling.

Gel permeation chromatography provides information about the polymer structures. Control Polymer 1 has a weight average molecular weight (Mw) of 138,000 atomic mass units (amu) and a number average molecular weight (Mn) of 89,000 amu for a Mw/Hn ratio of 1.55, while Control Polymer 2 has a Mw/Mn ration of 1.62. Invention Polymer 4 has a Mw of 156,000 amu and a Mn of 92,000 amu for a Mw/Mn ratio of 1.69, while Polymer 6 has a Mw/Mn ratio of 1.72.

Tetramethoxysilane [(OMe).sub.4 Si] was used as the multifunctional coupling agent in this example. The polymers recipes and strain rates are shown in Table II. Plots of the polymer strain rates against melt flows are shown in FIG. 2. Polymers 8 through 12 have strain rates well below the line drawn through the points for control Polymers 1 and 2 indicating higher melt strengths for invention polymers 8 through 12.

A gel permeation chromatography examination of Polymer 9 shows that it has a Mw of 166,000 amu and Mn of 95,000 amu for a Hw/Hn ratio of 1.75.

The difference in structure between polymers made according to the present invention compared with polymers made with only bifunctional coupling agents is readily seen in the molecular weight distribution (MWD) curves produced by gel permeation chromatography. The ordinate (vertical) scale represents the magnitude of the peaks and the abscissa (horizontal) scale represents the gpc elution count in arbitrary units for comparison of figures. In FIG. 3, curve 3a is a HWD curve of a typical control polymer made with a bifunctional coupling agent. Curves 3b, 3c, and 3d are MWD curves of the polymerization mixture at several points during the polymerization of a polymer made in a manner similar to that used for the preparation of Polymer 8. Curve 3b shows the MWD after all three n-butyllithium charges and clearly shows the three different polymer segments before any coupling. The multifunctional coupling (curve 3c) changes the gpc trace, especially the shape of the larger (highest molecular weight) peak. After the Vikoflex coupling step (curve 3d), further changes in the curve occur. A comparison of curve 3 d with 3a clearly shows the changes in MWD by the process of this invention.

Several polymers were prepared using dimethyl isophthalate (DMIP) as the multifunctional coupling agent. Polymers 13, 14, and 15 are described in Table III with their melt flows and strain rates. Plots in FIG. 4 of the polymer melt flow against strain rate show that Polymers 13 and 14 are below the line through the two control polymers. Polymers made with DMIP have unusually low Mw/Mn ratios. Polymer 14 has a Mw of 139,000 amu and a Mn of 125,000 amu for a Mw/Mn ratio of 1.11. Polymer 15 has a Mw/Hn ratio of only 1.06, indicating relatively low multifunctional coupling under these conditions. The DMIP has a relatively low solubility in cyclohexane. At the lower level of DMIP (0.06 phm) used in Polymer 15, a longer reaction time is believed to be needed to obtain enough multifunctional coupling for good melt strength.

A sag test was used to further demonstrate the advantages of polymers prepared according to the present invention. A sheet about 0.57 mm thick was extruded from the polymers and held in a hot thermoformer. After the sheet had been heated for 60 seconds, the amount of sag was measured. As shown in Table IV, polymers prepared with both a multifunctional coupling agent and a bifunctional coupling agent had less sag than control polymers made without the multifunctional coupling agent. Higher melt strength polymers would be expected to sag less than polymers with low melt strength.

The sag test results are plotted in FIG. 5 against polymer melt flow values. All invention sample points are located below the control line, indicating better melt strength for polymers made with the two-step coupling.

Physical properties of several of the polymers from the above examples were determined by injection molding test specimens for evaluation. The results are described in Table V. Polymers 16 and 17 are control polymers prepared in a manner similar to Polymers 1 and 2, but with larger reactor volumes. Polymer 18 was prepared using tetraethoxysilane in a manner similar to that used for the preparation of Polymer 5, but with a larger reactor volume. Polymers 16, 17, and 18 have melt flow values in g/10 min of 6.1, 8.1 and 8.5, respectively. The strain rates in %/min. of polymers 16, 17 and 18 are 100, 143 and 85, respectively.

The results in Table V show that the invention polymers have physical properties similar to the control polymers. Variations in physical properties are also observed as a result of differences in melt flow. These results show that the two-step coupling technique of this invention does not cause a reduction in molded product properties.

Runs were made to compare invention polymers (made with sequential charges of a multifunctional coupling agent followed by a bifunctional coupling agent) with polymers coupled with a mixture of multifunctional and bifunctional coupling agents. Polymer 19 was an invention polymer prepared in a manner similar to Polymers 3, 4, 5, 6 and 7, with sequential coupling charges of first tetraethoxysilane (multifunctional) and then Vikoflex 7170 (bifunctional). Polymer 20 was a control polymer prepared in a manner similar to Polymer 19 with the exception that coupling was accomplished by charging with a mixture of multifunctional and bifunctional coupling agents (tetraethoxysilane and Vikoflex 7170). Preparation of polymers 19 and 20 is summarized in Table VIa.

Comparison of Polymers 19 and 20 showed that there are significant important differences between polymers made with a mixture of multifunctional and difunctional coupling agents rather than sequential charges of the same coupling agents. The strain rates of Polymer 19 was measured in %/min at 190.degree. C. using a Rheometrics Mechanical Spectrometer. The strain rate of Polymer 20 was not measured because at the time of the research Polymer 20 was so different from Polymer 19 that it was considered a "failure". Melt flows were measured using an extrusion plastometer at 200.degree. C. with 4.9 Kg weight. Gel permeation chromatography was used to investigate the polymer molecular weight distributions. Some of the properties and characteristics of these two polymers are shown in Table VIb.

The molecular weight and the Mw/Mn ratio of Polymer 19 are greater than those of Polymer 20. These differences are both indicative of multiple coupling. The larger heterogenity index (Mw/Mn ratio) of Polymer 19 indicates a broader molecular distribution. The final melt flows of the two polymers were dramatically different. This difference in final melt flows is even more striking when considered in view of the similar molecular weights and is indicative of completely different molecular structures. Gel permeation chromatography results were consistent with this conclusion.

While the methods and polymers of this invention have been described in detail for the purpose of illustration, the inventive methods and polymers are not to be construed as limited thereby. This patent is intended to cover all changes and modifications within the spirit and scope of this patent.