MXPA98009092A - Procedure for preparing sinumotactic vinylaromatic polymers semicristali - Google Patents

Abstract

Semi-crystalline copolymers of styrene and one or more alkyl-substituted styrene compounds in the ring having a crystalline melting point of 180 ° C to 254 ° C and a styrene content of 50 to 94 mol% are described which can only be prepared in a solution polymerization procedure

Description

PROCEDURE FOR PREPARING SEMI-RYSTALLINE SINUMOTACTIC VIN1LAROMATIC POLYMERS DESCRIPTION OF THE INVENTION The present invention relates to a process for preparing polymers of vinylaromatic monomers that are semi-crystalline due to the presence of intentional chain defects, which reduce the rate of crystal formation. In addition, the present invention relates to a solution process for preparing said polymers. Previously it has been known in the art to prepare polymers including copolymers of vinylaromatic monomers that are highly syndiotactic. Typically, said polymers are prepared through the use of certain catalysts containing a Group 4 metal complex. Such polymers rapidly crystallize from the melt due to their uniform polymer structure, resulting in highly crystalline solids. Due to said high crystallinity, these polymers are generally insoluble in commonly available solvents. Although solvent resistance is desirable in a molded article under certain conditions of use, this same property makes such polymers difficult to prepare. In particular, when a solution polymerization process is used to prepare said polymers, a gel phase generally occurs and then a biphasic mixture. This results in poor heat dissipation from the reaction mixture, difficulty in mixing and inefficient mass transport during the polymerization process. The incrustation of the surfaces of the reactor is a constant and significant problem. Attempts to overcome the problems associated with said overall polymerization process or solution for preparing syndiotactic vinyl aromatic polymers include the use of clean surface reactors, the use of powder bed reactors wherein the liquid phase is maintained on the surface of the product. in particles, and the use of suspension polymerization procedures. There are techniques that have been previously described in EP-A-389,939, in US-A-5,484,862 and US-A-4,950,724 and elsewhere. Despite the advances achieved in the polymerization of vinylaromatic monomers to prepare highly crystalline syndiotactic polymers, the difficulties mentioned previously continue to limit the ease of manufacture of syndiotactic vinyl aromatic polymers. It could be desirable if polymers having a sufficient degree of syndiotactic capacity were provided to possess a crystalline melting point thereby imparting improved heat resistance to such polymers, compared to atactic polymers, but where the degree of crystallinity thereof is sufficiently low and the crystallization rate is sufficiently slow in order to make said polymers soluble in aromatic solvents, especially at elevated temperatures. Such desirable polymers are referred to herein as "semicrystalline". In addition, it may be desirable to provide a process for preparing said semicrystalline syndiotactic vinyl aromatic polymers under solution polymerization conditions.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 indicates the physical state (solution, gel or solid) of various styrene / p-methylstyrene polymerization mixtures defined by molar percentage of paramethylstyrene in the copolymer and volume fraction copolymer in the polymerization mixture at 70 ° C with toluene diluent. The operating conditions to ensure that the solution polymerization conditions are maintained (determined by setting the data to a theoretical model) are also provided. Figure 2 gives crystalline melting points of various styrene / p-methylstyrene copolymer films as a function of the p-methylstyrene content. Figure 3 provides a graph of crystallinity as a function of the comonomer content for styrene / p-methylstyrene copolymers. Both the crystallinity as measured from the melt bath and the crystallinity measured after stretching and heat setting are provided. In accordance with the present invention, a solution procedure for the preparation of semicrystalline polymers of vinylaromatic monomers having a syndiotactic structure is provided. The inventors of the present invention have discovered that certain syndiotactic copolymers of styrene and alkyl substituted styrene compounds in the ring possess a crystalline melting point of 180 ° C to 254 ° C (thereby imparting increased distortion temperature properties). by heat compared with atactic polyvinylaromatic polymers) and a styrene content of 50 to 94 mol% are also soluble in aromatic solvents under polymerization conditions due to a low degree of crystallization and also at a slow crystallization rate. In addition, the crystallinity of these same polymers can be increased in articles or molded objects through the use of molding conditions without extinction through the use of stress induced crystallization techniques, thus imparting an acceptable solvent resistance in articles and film. as formed from said copolymers. Specifically, articles such as films having a high degree of crystallinity can be obtained both through laying and sputtering film techniques after a fixing step or through appropriate control of the cooling rate of the polymer. Accordingly, the invention lies in the discovery of the above single species of semicrystallized vinyl aromatic symotactic polymers and under solution polymerization conditions suitable for use in the preparation thereof.

In addition, certain of the above semicrystalline copolymers of styrene and one or more alkyl-substituted styrene compounds having a crystalline melting point of 180 ° C to 234 ° C and a styrene content of 50 to 85 mol% are particularly suitable for preparation through the above polymerization process and, therefore, are highly desirable as compositions of matter. Semi-crystalline polymers suitable for the preparation and use of the present invention are interpolymers of styrene and one or more alkyl-substituted styrene monomers in the ring, containing 50 to 94 mol% styrene, preferably 55 to 85 mol% styrene As used herein, the term "interpolymer" is used interchangeably with the term "copolymer" and refers to a polymer comprising one or more comonomers. Styrene contents near the scale of crystallinity specified herein can not be introduced into the solid polymer through ordinary techniques. Said non-crystalline polymers are not sufficiently resistant to solvents, especially aromatic liquids, as desired, nor do they possess a crystalline melting point, which imparts a higher usage temperature. At styrene contents greater than 85%, especially greater than 94%, the comonomer content is insufficient to introduce a significant number of chain defects in the polymer, so that the polymers have a high crystallization rate and, consequently, are they become insoluble in the polymerization mixture or liquid diluent at a low concentration of polymer thus limiting the total conversion in a polymerization process. Such copoiomers with high styrene content are not desirable according to the present invention, since they are not suitable for the manufacture, according to the present invention, in the solution polymerization technique. In accordance with the present invention, the above semicrystalline syndiotactic vinylaromatic copolymers are prepared by contacting the styrene and one or more alkyl-substituted vinylaromatic monomers in the polymerizable ring with a catalyst comprising a Group 4 metal complex at a low temperature and polymerization conditions, so that the resulting polymer is retained in solution. Optionally, an inert liquid diluent, preferably an aromatic solvent, may also be present during the polymerization. Through the use of said liquid diluents, the polymerization can generally be conducted to a higher degree of conversion than is possible under conditions that overall polymerization, while retaining the polymer in solution. By the term "overall" polymerization conditions is meant that the polymerization is conducted in the substantial absence of liquid diluent other than vinylaromatic monomers. The alkyl-substituted vinylaromatic monomers in the ring preferred for use herein correspond to the formula: wherein R is C 1-10 alkyl- The alkyl-substituted vinylaromatic monomers in the ring highly referred to are the various isomers of vinyltoluene, especially p-vinyltoluene. A most preferred interpolymer is a copolymer of styrene and p-vinyltoluene. Suitable inert liquid diluents for use in the polymerization include aromatic compounds, especially toluene, xylene, ethylbenzene, and mixtures thereof. A preferred diluent is toluene. Typically, the amount of diluent used is from 30 wt% to 90 wt% of the total polymerization mixture. The preferred amounts of diluent used are from 35 to 75% by weight of the total polymerization mixture. Suitable polymerization temperatures are from 40 to 90 ° C, preferably from 50 to 85 ° C. The polymerization is preferably conducted under conditions to provide a total conversion of vinylaromatic monomers that is determined by the formula: (1 -f?) (Pcnc + Pest (1 -np M S) where: X is the total monomer conversion; fi is the volume fraction of the solvent; f2 is the volume fraction of the copolymer; p2 is the density of the copolymer; pc is the density of the comonomer; pEst is the density of styrene; and nc is the mole fraction of the comonomers in the copolymer. The maximum volume fraction of the copolymer that is soluble in the reaction mixture can be contained by solving for f2 in the second degree equation: where: R is the universal gas constant; T is the temperature of the polymerization; VT is the molar volume of diluent; You see the molar volume of the monomer unit; Xi is the Flory-Huggins interaction parameter between the diluent and the copolymer; f2 is the volume fraction of copolymer; ? HU is the heat of fusion for the repeating unit; and T ° m is the melting temperature of the copolymer Al carrying out the polymerization so that the conversion does not exceed the maximum volume fraction of the copolymer determined by this formula, preferably so that the conversion does not exceed 0.95 of said fraction of maximum volume, the polymerization will not depart from the solution polymerization conditions. The polymers desirably have a crystallinity which is measured by differential scanning calorimetry (DSC) from annealing the melt from 2 to 42% by weight, most preferably from 10 to 40% by weight. In the prior art for determining the crystallinity, the melting endotherm is divided by 54 Joules / g to provide the measurement without percent unit of crystallinity. Those skilled in the art will appreciate that the invention described herein may be practiced in the absence of any component, which has not been specifically described. The following examples are provided as further illustration of the invention and are not constructed as limiting. Unless stated otherwise, all parts and percentages are expressed on a weight basis.

EXAMPLE A 20 ml ampule was dried and purged with a nitrogen atmosphere. All monomers and solvent were purified by passing through a 3A molecular sieve (0.3 nm), an alumina bed, and finally contacting with hydrogen in the presence of a palladium catalyst to hydrogenate the acetylenic impurities. 2.34 g of purified styrene and 0.89 g of purified p-vinyltoluene were added under an inert nitrogen atmosphere. Then, 5.58 g of dry toluene was also added under a nitrogen atmosphere. The ampule was sealed and placed in a water bath at 70 ° C for 10 minutes. At that time, 363 μL of a toluene solution of 0.003 pentamethylcyclopentadienyltitanium trimethoxide catalyst was added. (The catalyst solution was prepared by combining 3.0 x 10"s moles of titanium complex, 2.25 x 10" 3 moles of methylalumoxane co-catalyst and 7.5 x 10"4 moles of aluminum triisobutyl in 10 ml of dry toluene). The reaction was allowed to proceed until the catalyst was no longer active (approximately 16 hours) .After this time, the copolymer remained completely soluble in the reaction mixture.No crystallization or precipitation was observed.The polymer was recovered through precipitation With methanol, it was filtered through a filter paper and washed with fresh methanol.The solvent and residual monomers were removed through reduced pressure at 50 ° C for 12 hours.The final weight of the polymer was 2.08 g. of total monomers was 64% by weight The copolymer composition was 75 mol% styrene and 25 mol% p-vinyltoluene The number average molecular weight and the weight average molecular weight of the polymer stratum, as determined through size exclusion chromatography at high temperature using atactic polystyrene standards that were: Mn = 191,000, Mw = 780,000, Mw / Mn = 4.1. The copolymer samples were extruded to a 100 mm wide band with a thickness of 0.13-0.25 mm. The sample was extinguished using a cooling roller at the exit of the die. The strip was punctured in a 50 mm x 50 mm square and stretched over a stretcher of biaxial film of the T. M. Long mark at various lengths at a deformation rate of 10 mm / sec. at 120 ° C. The degree of crystallization of the sample depends on the amount of orientation or stretching. This was quantified through a DSC analysis of both unstretched, stretched and stretched and heat fixed material. The percentage of crystallinity was calculated by determining the integrated energy from the melting point and subtracting the energy of the cold crystallization peak and dividing by 54 Joules per gram. The undrawn film had a crystallinity of 0%. The stretched film had a crystallinity of 23% by weight, thus indicating that stress-induced crystallization occurred. The film stretched after heat fixation at 180 ° C for 40 seconds to relax the amorphous regions and allow the aligned molecules to fully crystallize had a crystallinity of 28% by weight. Accordingly, the polymer was semicrystallized. Additional copolymer samples were prepared by substantially repeating the above procedure using different concentrations of comonomer in the reactor and different degrees of monomer conversion. The polymerization conditions and the physical state of the copolymer thus formed were observed. The crystalline melting points and the crystallinity of several representative samples of the copolymers thus formed were measured by DSC before and after stretching. The results of the polymerization are plotted in Figure 1. The results of the melting point and crystallinity measurements are contained in Figures 2 and 3, respectively.

Claims (5)

1. A semicrystalline copolymer comprising styrene and one or more alkyl-substituted styrene compounds in the ring having a crystalline melting point of 180 ° C to 234 ° C and a styrene content of 50 to 85 mol%.

2. A semicrystalline copolymer according to claim 1, wherein the alkyl-substituted styrene compound is p-vinyltoluene.

3. A semicrystalline copolymer according to claim 1, which has a crystallinity of 2 to 42% by weight.

4. - A process for preparing a copolymer of styrene and one or more alkyl-substituted styrene compounds in the ring having a crystalline melting point of 180 ° C to 254 ° C and a styrene content of 50 to 94 mol%, which it comprises contacting the styrene and one more vinyl-substituted vinyl monomers in the ring polymerizable with a catalyst comprising a Group 4 metal complex under polymerization conditions, so that the resulting polymer is retained in solution.

5. A method according to claim 1, wherein the degree of conversion of said method is maintained at a level less than or equal to that determined by the formula: X < AZ & - (1 -f?) (P nc + p £ st (1 -ripMs) where: X is the total monomer conversion, f is the volume fraction of the solvent, f2 is the volume fraction of the copolymer, p2 is the density of the copolymer, pc is the density of the comonomer, pESt is the density of the styrene, and nc is the mole fraction of the comonomers in the copolymer.