Olefin copolymers and processes for preparing same

Abstract: The present invention provides a propylene random copolymer comprising structural units (a) derived from propylene and structural units (b) derived from an α-olefin of 4 to 20 carbon atoms, and having (i) the structural units (a) in an amount of 90 to 99 mol% and the structural units (b) in an amount of 1 to 10 mol%, (ii) an intrinsic viscosity [η] as measured in decaline at 135°C of 0.5 to 6 dl/g, (iii) a melting point [Tm] as measured by a differential scanning calorimeter falling within the range of 90 < Tm < 155 - 3.5 (100 - P) wherein P is the propylene content (mol%) present in the copolymer, (iv) a ratio ( M w/ M n) of a weight average molecular weight ( M w) to a number average molecular weight ( M n) as measured by gel permeation chromatography of less than 3.5, and (v) a boiling trichloroethylene-insoluble content of less than 5% by weight.

Description: This invention relates to novel propylene random copolymers and process for preparing the same and more particularly to novel propylene random copolymers having low melting points in comparison with conventionally known propylene random copolymers and, in particular, excellent heat-sealing properties and anti-block properties, and to process for preparing the same. Polypropylene has wide applications in the field of plastics because of its excellent physical properties. For example, polypropylene is widely used as packaging film material. In the applications of the type, however, because of its relatively high melting point, polypropylene is generally copolymerized with ethylene or α-olefins of 4 to 20 carbon atoms in order to improve the heat-sealing properties at low temperature, and is used in the form of a propylene/α-olefin copolymer. Packaging films formed from these known propylene/α-olefin copolymers still do not have sufficient heat-sealing properties, though they have excellent transparency and scratch resistance in comparison with those formed from low density polyethylene, and accordingly it is hoped that propylene/α-olefin copolymers having excellent heat-sealing properties even at lower temperatures will be obtained. It is well known that the above-mentioned propylene/α-olefin random copolymers may be improved in heat-sealing properties by increasing the proportion of ethylene or α-olefin of 4 to 20 carbon atoms to propylene in the copolymer. However, if the proportion of ethylene or α-olefin of 4 to 20 carbon atoms is increased in the copolymerization, the resulting propylene/α-olefin copolymer has an increased amount of solvent-soluble component, whereby the resultant copolymer has poor anti-blocking properties and stiffness. Such propylene/α-olefin random copolymers having excellent heat-sealing properties, anti-block properties and stiffness as mentioned above are available only when they have a low melting point in spite of the fact that the proportion of α-olefin in the copolymer is small. Japanese Patent L-O-P Publn. No. 130314/1986 discloses polypropylene having a high isotacticity obtained by polymerization of propylene in the presence of a catalyst system comprising a sterically fixed zirconium-chelate compound and aluminoxane. J. Am. Chem. Soc., 109 , 6544 (1987) discloses the formation of a high molecular weight isotactic polypropylene obtained by polymerization of propylene in the presence of a catalyst system comprising ethylenebis(indenyl)hafnium dichloride or its hydride and aluminoxane, said isotactic polypropylene having a narrow molecular weight distribution ( M w/ M n) of 2.1-2.4. Japanese Patent L-O-P Publn. No. 142005/1987 discloses a stereoblock polypropylene having M w/ M n of 5.0-14.9 obtained by polymerization of propylene in the presence of a catalyst system comprising tetramethylethylenebis(cyclopentadienyl) titanium chloride and aluminoxane. The polypropylene thus obtained has a short isotactic chain length and is a rubbery polymer. GB 2052532 describes a copolymer of propylene and a straight-chain α-olefin of 5 to 12 carbon atoms characterized in that its intrinsic viscosity is as low as 0.3 or as high as 15 dl/g (at 135°C in decalin). The copolymers are said to exhibit excellent heat-resistance and transparency. EP-A-0263718, one of the present applicant's earlier publications, describes a low-crystalline propylene random copolymer composition comprising [I] 60 to 95 % by weight of a propylene random copolymer and [II] 5 to 40 % by weight of a low-crystalline propylene random copolymer. On a molar basis the polymer may contain as little as 75 mole % units derived from propylene. The present inventors have found that when propylene and α-olefins of 4-20 carbon atoms are copolymerized in the presence of an olefin polymerization catalyst composed of specific hafnium compounds and aluminoxane, there are obtained propylene/α-olefin copolymers which have a narrow molecular weight distribution and a small amount of the α-olefin copolymerized therewith, but which have a low melting point in comparison with conventionally known propylene/α-olefin copolymers, on which the present invention has been based. The present invention seeks to provide propylene/α-olefin copolymers which have a narrow molecular weight distribution and small amount of α-olefin copolymerized therewith but which have a low melting point and, moreover, which have excellent heat-sealing properties and excellent anti-block properties and stiffness, and processes for preparing the same. The present invention provides a propylene/α-olefin random copolymer comprising structural units (a) derived from propylene and structural units (b) derived from an α-olefin of 4 to 20 carbon atoms, and having (i) the structural units (a) in an amount of 90 to 99 mol% and the structural units (b) in an amount of 1 to 10 mol%, (ii) an intrinsic viscosity [η] as measured in decalin at 135°C of 0.5 to 6 dl/g, (iii) a melting point [Tm] as measured by a differential scanning calorimeter falling within the range of 90<Tm<155-3.5 (100-P) wherein P is the propylene content (mol%) present in the copolymer, (iv) a ratio ( M w/ M n) of a weight average molecular weight ( M w) to a number average molecular weight ( M n) as measured by GPC of less than 3.5, and (v) a boiling trichloroethylene-insoluble content of less than 5% by weight. The present invention also provides a process for preparing a propylene/α-olefin copolymer as defined above, which process comprises copolymerizing propylene and α-olefin of 4-20 carbon atoms at a temperature of 40 to 100°C in the presence of a catalyst comprising [A] a hafnium compound having as a ligand a multidentate compound in which at least two groups selected from among conjugated cycloalkadienyl groups or substituted groups thereof are linked together via ethylene groups, and [B] an organoaluminum oxy-compound. The present invention additionally provides a heat sealing agent comprising the propylene random copolymers defined above. Fig. 1 is an illustration of process for the preparation of the olefin copolymer of the present invention. Fig. 2 is a graph showing the relationship between the propylene content and melting point of the propylene random copolymer of the present invention. Figs. 3-5 are stepwise illustrative of the method of evaluation of heat-sealing properties of the propylene random copolymer of the present invention. The process for the preparation of the propylene copolymers of the present invention is illustratively shown in Fig. 1 The composition of the copolymer is determined by measuring a spectrum of 13 C-NMR of a specimen obtained by dissolving about 200 mg of the copolymer in 1 ml of hexachlorobutadiene in a test tube of 10 mmø under the conditions of a measurement temperature of 120°C, measurement frequency of 25.05 MHz, spectrum width of 1500 Hz, pulse repetition time of 4.2 sec. and pulse width of 6 sec. The ratio ( M w/ M n) obtained in the invention was determined by the following procedure in accordance with Takeuchi, "Gel Permeation Chromatography," Maruzen, Tokyo. (1) Using a standard polystyrene having a known molecular weight (a monodispersed polystyrene produced and sold by Toyo Soda K.K.), the molecular weight M and GPC (Gel Permeation Chromatography) count of the sample are measured to prepare a correlation diagram calibration curve of the molecular weight M and EV (Elution Volume). The concentration of the sample used is maintained at 0.02% by weight. (2) A GPC chromatograph of the sample is taken by GPC measurement, and a number average molecular weight M n and weight average molecular weight M w, in terms of polystyrene, are calculated from the calibration curve mentioned in the above procedure (1) to obtain a value of M w/ M n. The conditions under which the sample is prepared, and the conditions under which GPC measurement is conducted are as follows: [Preparation of sample] (a) The sample is put in an Erlenmeyer flask together with o-dichlorobenzene so that the same amounts to 0.1% by weight. (b) The Erlenmeyer flask is heated to 140°C and stirred for about 30 minutes to dissolve the sample in the o-dichlorobenzene. (c) The solution is subjected to GPC. [GPC measurement conditions] The measurement was conducted under the following conditions. (a) Apparatus 150C-ALC/GPC manufactured by Waters Co. (b) Column GMH Type manufactured by Toyo Soda K.K. (c) Amount of sample 400 µl (d) Temperature 140°C (e) Flow rate 1 ml/min The catalyst components [A] used in the invention are hafnium compounds having multidentate coordination compounds as ligands in which at least two groups selected from among conjugated cycloalkadienyl groups or substituted groups thereof, e.g. an indenyl group, substituted indenyl group and partially hydrated compounds thereof, are linked together via ethylene groups. The above-mentioned hafnium compounds include, for example, the following compounds. Ethylenebis(indenyl)dimethyl hafnium, Ethylenebis(indenyl)diethyl hafnium, Ethylenebis(indenyl)diphenyl hafnium, Ethylenebis(indenyl)methyl hafnium monochloride, Ethylenebis(indenyl)ethyl hafnium monochloride, Ethylenebis(indenyl)methyl hafnium monobromide, Ethylenebis(indenyl)hafnium dichloride, Ethylenebis(indenyl)hafnium dibromide, Ethylenebis(4,5,6,7-tetrahydro-1-indenyl)dimethyl hafnium, Ethylenebis(4,5,6,7-tetrahydro-1-indenyl)methyl hafnium monochloride, Ethylenebis(4,5,6,7-tetrahydro-1-indenyl)hafnium dichloride, Ethylenebis(4,5,6,7-tetrahydro-1-indenyl)hafnium dibromide, Ethylenebis(4-methyl-1-indenyl)hafnium dichloride, Ethylenebis(5-methyl-1-indenyl)hafnium dichloride, Ethylenebis(6-methyl-1-indenyl)hafnium dichloride, Ethylenebis(7-methyl-1-indenyl)hafnium dichloride, Ethylenebis(5-methoxy-1-indenyl)hafnium dichloride, Ethylenebis(2,3-dimethyl-1-indenyl)hafnium dichloride, Ethylenebis(4,7-dimethyl-1-indenyl)hafnium dichloride, Ethylenebis(4,7-dimethoxy-1-indenyl)hafnium dichloride. The above-mentioned hafnium ccmpounds may contain small amounts of zirconium or titanium. In such a case, the content of zirconium or titanium is less than 1% by weight, preferably less than 0.7% by weight and especially less than 0.5% by weight. The hafnium catalyst components used in the present invention may include compounds obtained by treating the above-mentioned hafnium compounds with alkylsilylated silica gel. More particularly, said hafnium catalyst components may be hafnium compound solutions which are obtained, for example, by passing a solution of the above-mentioned hafnium compound in an organic solvent such as toluene through a column packed with an alkylsilylated silica gel, wherein said hafnium compound is brought into contact with the alkylsilylated silica gel. The organic solvents used in that case are preferably aromatic hydrocarbons such as toluene, benzene and xylene. The alkylsilylated silica gel used may includes those obtained by treating silica gel with dimethyl dichlorosilane, ethylmethyl dichlorosilane, trimethyl chlorosilane, trimethyl bromosilane, divinyl dichlorosilane, diethyl dichlorosilane or methylpropyl dichlorosilane. The hafnium concentration in the hafnium compound solution is usually from 1 x 10 -5 to 5 x 10 -3 mol/l, and the amount of the alkylsilylated silica gel used is usually 20-500 g per 1 mmol of the hafnium compound. The temperature at which the hafnium compound solution is brought into contact with the alkylsilylated silica gel is usually 0-50°C. When the hafnium catalyst components obtained by treating the above-mentioned hafnium compounds with the alkylsilylated silica gel are used as the catalyst components [A], propylene copolymers having excellent transparency are obtained. The catalyst components [B] used in the process of the present invention are organoaluminum oxy-compounds. The organoaluminum oxy-compounds used as the catalyst components may be benzene-soluble aluminoxanes of formula (1) or (II). wherein each R may be the same or different and is a hydrocarbon group such as methyl, ethyl, propyl or butyl, preferably methyl or ethyl and especially methyl, and m is an integer of at least 2, preferably at least 5. The above-mentioned aluminoxanes may be prepared, for example, by the procedures exemplified below. (1) A procedure which comprises reacting a suspension in a hydrocarbon medium of a compound containing water of absorption or a salt containing water of crystallization, for example, magnesium chloride hydrate, copper sulfate hydrate, aluminumsulfate hydrate, nickel sulfate hydrate or serous chloride hydrate, with a trialkylaluminum. (2) A procedure which comprises reacting a trialkylaluminum directly with water, water vapor or ice in a medium such as benzene, toluene, ethyl ether and tetrahydrofuran. The aluminoxanes as illustrated above may contain small amounts of organometallic components. Further, the organoaluminum oxy-compounds used in the present invention may be those which are insoluble in benzene. The benzene-insoluble organoaluminum oxy-compounds are illustrated hereinafter. The benzene-insoluble organoaluminum oxy-compounds used in the invention may be prepared by (i) reaction of organoaluminum compounds with water or (ii) reaction of solutions of aluminoxane, for example, hydrocarbon solutions thereof, with water or active hydrogen-containing compounds. The benzene-insoluble organoaluminum oxy-compounds are considered to have alkyloxyaluminum units of formula wherein R 1 is hydrocarbon of 1 ∼ 12 carbon atoms and the Al component soluble in benzene at 60°C is present in an amount, in terms of Al atoms, of less than 10%, preferably less than 5% and especially less than 2%. Thus they are insoluble or sparingly soluble in benzene. The solubility in benzene of the organoaluminum oxy-compounds is determined by suspending the organoaluminum oxy-compound equivalent to Al of 100 mg atom in 100 ml of benzene, stirring the suspension at 60°C for 6 hours, filtering the thus treated suspension at a temperature elevated to 60°C using a G-5 glass filter equipped with a jacket, washing 4 times the solids portion separated on the filter with 50 ml of benzene kept at 60°C and then measuring the amount of Al atoms (× mmol) present in the total filtrate. In the alkyloxyaluminum units mentioned above, R 1 is, for example, methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, pentyl, hexyl, octyl, decyl, cyclohexyl or cyclooctyl. Among these, preferred are methyl and ethyl, particularly methyl. In addition to the alkyloxyaluminum units of formula the benzene-insoluble organoaluminum oxy-compounds may contain oxyaluminum units of formula In the above-mentioned formulae, R 1 is as defined previously, R 2 is hydrocarbon of 1 ∼ 12 carbon atoms, alkoxyl of 1 ∼ 12 carbon atoms aryloxy of 6 ∼ 20 carbon atoms, hydroxyl, halogen or hydrogen, and R 1 and R 2 are different from each other. In that case, the benzene-insoluble organoaluminum oxy-compounds are preferably those containing the alkyloxyaluminum units in a proportion of at least 30 mol%, preferably at least 50 mol% and especially at least 70 mol%. The organoaluminum compounds (i) used for preparing such benzene-insoluble organoaluminum oxy-compounds as mentioned above are those of formula R 1 n AlX 3-n wherein R 1 is hydrocarbon of 1 to 12 carbon atoms, X is halogen, alkoxyl of 1 to 12 carbon atoms, aryloxy of 6 to 20 carbon atoms or hydrogen, and n is 2 or 3. Such organoaluminum compounds (i) as mentioned above include trialkylaluminum such as trimethylaluminum, triethylaluminum, tripropylaluminum, triisopropylaluminum, tri-n-butylaluminum, tri-sec-butylaluminum, tri-tert-butylaluminum, tripentylaluminum, trihexylaluminum, trioctylaluminum, tridecylaluminum, tricyclohexylaluminum and tricyclooctylaluminum; dialkylaluminum halides such as dimethylaluminum chloride, dimethylaluminum bromide, diethylaluminum chloride, diethylaluminum bromide, and diisobutylaluminum chloride; dialkylaluminum hydrides such as diethylaluminum hydride and diisobutylaluminum hydride; dialkylaluminum alkoxides such as dimethylaluminum methoxide and diethylaluminum ethoxide; and dialkylaluminum aryloxides such as diethylaluminum phenoxide. Of these organoaluminum compounds, preferred are those of the above-mentioned formula in which R 1 is alkyl and X is chlorine, and particularly preferred is trialkylaluminum. In this connection, isoprenylaluminum of the general formula (i-C 4 H 9 ) x Al y (C 5 H 10 ) z wherein x, y and z are each positive integers, and z ≥ 2x may also used as the organoaluminum compound (i). The organoaluminum compounds (i) as illustrated above may be used either singly or in combination. The active hydrogen-containing compounds (ii) used in preparing the benzene-insoluble organoaluminum oxy-compounds include alcohols such as methyl alcohol and ethyl alcohol, and diols such as ethylene glycol and hydroquinone. When water is used in preparing the benzene-insoluble organoaluminum oxy-compounds, the water may be used after dissolving or suspending in hydrocarbon solvents such as benzene, toluene and hexane, ether solvents such as tetrahydrofuran, and amine solvents such as triethylamine, or may be used in the form of water vapor or ice. As the water, moreover, there may also be used water of crystallization of a salt such as magnesium chloride, magnesium sulfate, aluminum sulfate, copper sulfate, nickel sulfate, iron sulfate and cerrous chloride, or water of adsorption adsorbed to inorganic compounds such as silica, alumina and aluminum hydroxide or polymers. As mentioned above, the benzene-insoluble organoaluminum oxy-compounds may be prepared by reaction of the organoaluminum compound (i) with water, or by reaction of a solution of aluminoxane, for example, a hydrocarbon solution thereof, with water or the active hydrogen containing compound. In preparing the benzene-insoluble organoaluminum oxy-compound from the organoaluminum compound and water, the organoaluminum compound is brought into contact with water in a solvent, for example, a hydrocarbon solvent, and in that case, the water is added to the reaction system so that the organoaluminum atoms dissolved in the reaction system become less than 20% based on the total organoaluminum atom. In obtaining the benzene-insoluble organoaluminum oxy-compounds according to the procedure as mentioned above, it is desirable that the water is brought into contact with the organoaluminum compound in the proportion of 1 ∼ 5 moles, preferably 1.5 ∼ 3 moles of the water to 1 mole of the organoaluminum compound. The above-mentioned reaction for forming the benzene-insoluble organoaluminum oxy-compounds is carried out in solvents, for example, hydrocarbon solvents. The solvents used include aromatic hydrocarbons such as benzene, toluene, xylene, cumene and cymene, aliphatic hydrocarbons such as butane, isobutane, pentane, hexane, octane, decane, dodecane, hexadecane and octadecane, alicyclic hydrocarbons such as cyclopentane, cyclooctane, cyclodecane and cyclododecane, such hydrocarbon solvents, for example, petroleum fractions, as gasoline, kerosine and gas oil, halides of the above-mentioned aromatic hydrocarbons, aliphatic hydrocarbons and alicyclic hydrocarbons, especially chlorides and bromides thereof, and ethers such as ethyl ether and tetrahydrofuran. Among these hydrocarbon media as exemplified above, particularly preferred are aromatic hydrocarbons. The concentration in terms of Al atom of the organoaluminum compound in the reaction system is desirably 1 × 10 -3 to 5 gram atom/l preferably 1 × 10 -2 to 3 gram atom/l, and the concentration in the reaction system of water such as water of crystallization is usually 1 × 10 -3 to 20 mol/l, preferably 1 × 10 -2 to 10 mol/l. In that case, it is desirable that the organoaluminum atoms dissolved in the reaction system are less than 20%, preferably less than 10% and especially in the range of from 0 to 5% based on the total organoaluminum atoms. Contact of the organoaluminum compound with water may be carried out, for example, by the following procedures. (1) A procedure wherein a hydrocarbon solution of organoaluminum is brought into contact with a hydrocarbon solvent containing water. (2) A procedure wherein water vapour is blown into a hydrocarbon solution of organoaluminum, thereby bringing the organoaluminum into contact with the water vapour. (3) A procedure wherein a hydrocarbon solution of organoaluminum is mixed with a hydrocarbon suspension of a compound containing water of adsorption or a compound containing water of crystallization, thereby bringing the organoaluminum into contact with the water of adsorption or water of crystallization. (4) A procedure wherein a hydrocarbon solution of organoaluminum is brought into contact with ice. The above-mentioned reaction of the organoaluminum with water is usually carried out at a temperature of from -100 to 150°C, preferably -50 to 100°C and especially -30 to 80°C. The reaction time, though it may vary greatly depending upon the reaction temperature, is usually from 1 to 200 hours, preferably 2 to 100 hours. In preparing the benzene-insoluble organoaluminum oxy-compounds from a solution of aluminoxane and water or a active hydrogen-containing compound, the aluminoxane present in the solution of aluminoxane is brought into contact with water or the active hydrogen-containing compound. The solution of aluminoxane is a solution of aluminoxane in a solvent such as used in forming the above-mentioned benzene-insoluble organoaluminum oxy-compounds, preferably aromatic hydrocarbons such as benzene and toluene. This solution may contain other components so long as they do not affect adversely the reaction between the aluminoxane and water or the active hydrogen-containing compound. The amount of water or the active hydrogen-containing compound used in the above-mentioned reaction is 0.1 to 5 moles, preferably 0.2 to 3 moles, based on 1 gram atoms of aluminum present in the solution of aluminoxane. The concentration in the reaction system of aluminoxane in terms of aluminum atoms is usually 1 × 10 -3 to 5 gram atom/l, preferably 1 × 10 -2 to 3 gram atom/l, and the concentration in the reaction system of water is usually 2 × 10 -4 to 5 mole/l, preferably 2 × 10 -3 to 3 mole/l. Taking, as an example, the reaction of the solution of aluminoxane with water, said solution of aluminoxane is brought into contact with water or the active hydrogen-containing compound, for example, by the following methods. (1) A method which comprises bringing the solution of aluminoxane- into contact with a hydrocarbon solvent containing water. (2) A method which comprises blowing water vapour into the solution of aluminoxane, thereby bringing the aluminoxane present in the solution of aluminoxane into contact with the water vapour. (3) A method which comprises mixing the solution of aluminoxane with a hydrocarbon solution of a compound containing water of adsorption or a compound containing water of crystallization, thereby bringing the aluminoxane present in the solution of aluminoxane into contact with the water of adsorption or water of crystallization. (4) A method which comprises bringing the solution of aluminoxane into contact directly with water or ice. The above-mentioned procedures may also be applied to the case wherein the active hydrogen-containing compound (ii) is used instead of water. The reaction of the solution of aluminoxane with water or the active hydrogen-containing compound as illustrated above may usually be carried out at a temperature of from -50° to 150°C, preferably 0° to 120°C and especially 20 to 100°C. The reaction temperature, though it may vary greatly depending upon the reaction temperature, is usually 0.5 to 300 hours, preferably about 1 ∼ 150 hours. In preparing the propylene copolymers by using the olefin polymerization catalysts as mentioned above, the concentration in the polymerization system of the hafnium compound in terms of hafnium atom is usually 10 -8 to 10 -2 gram atom/l, preferably 10 -7 to 10 -3 gram atom/l. The above-mentioned organoaluminum oxy-compounds are desirably used in an amount in terms of aluminum atoms present in the reaction system of 10 -4 to 10 -1 gram atom/l, preferably 5 × 10 -4 to 5 × 10 -2 gram atom/l. The polymerization temperature employed is from -50° to 150°C, preferably from 0° to 120°C. The olefin polymerization mentioned above is usually carried out in the vapour phase or liquid phase. In liquid phase polymerization, the solvent used may be an inert hydrocarbon, or the olefin itself may also be used as the solvent. The hydrocarbon used in that case includes aliphatic hydrocarbons such as butane, isobutane, pentane, hexane, heptane, octane, decane, dodecane, hexadecane and octadecane, alicyclic hydrocarbons such as cyclopentane, methylcyclopentane, cyclohexane and cyclooctane, aromatic hydrocarbons such as benzene, toluene and xylene, and petroleum fractions such as gasoline, kerosine and gas oil. The polymerization pressure employed is usually from ordinary pressure to 100 kg/cm 2 , preferably from ordinary pressure to 50 kg/cm 2 , and the polymerization may be carried out batchwise, semi-continuously or continuously. The molecular weight of the resulting polymer may be modified by the addition of hydrogen and/or by regulation of the polymerization temperature employed. The propylene/α-olefin random copolymers of the present invention are random copolymers of propylene and α-olefins of 4 to 20 carbon atoms. The propylene/α-olefin random copolymers of the invention desirably contain structural units (a) derived from propylene in an amount of 90 to 99 mol%, preferably 92 to 98 mol%, and structural units (b) derived from α-olefin in an amount of 1 to 10 mol%, preferably 2 to 8 mol%. When the structural units (a) derived from propylene contained in the propylene/α-olefin random copolymer are less than 90 mol%, said copolymer tends to have poor anti-blocking properties and stiffness and, on the other hand, when said units (a) are in excess of 99 mol%, said copolymer has an increased melting point and as poor heat-sealing properties. α-olefins of 4 to 20 carbon atoms used herein include 1-butene, 1-pentene, 1-hexene, 4-methyl-1-pentene, 3-methyl-1-pentene, 1-octane, 1-decene, 1-dodecene, 1-tetradecene, 1-hexadecene, 1-octadecene and 1-eicosene. Among these, particularly preferred is 1-butene. The propylene/olefin random copolymers of the present invention desirably have an intrinsic viscosity [η] of 0.5 to 6 dl/g, preferably 1 to 5 dl/g as measured in decalin at 135°C. If this intrinsic viscosity is less than 0.5 dl/g, the copolymers have poor anti-blocking properties and toughness and, on the other hand if said intrinsic viscosity exceeds 6 dl/g, the copolymers have poor moldability. The propylene/α-olefin random copolymers of the invention have a melting point [Tm] as measured by a differential scanning calorimeter falling within the range of 90 < Tm < 155 - 3.5 (100 - P), preferably 100 < Tm < 150 - 3.5 (100 - P) wherein P is the content (mol%) of propylene in the copolymer. In Fig. 2, there is shown as a straight line A the schematic relationship between the melting point Tm of the above-mentioned propylene/α-olefin random copolymer and the propylene content (mol%) present in said copolymer. There is also shown as a straight line B the relationship between the melting point Tm of the known propylene/α-olefin random copolymer and the propylene content (mol%) present in said copolymer. As is clear from Fig. 2, the melting point of the propylene/α-olefin random copolymers of the present invention is lower by 10 ∼ 20°C than that of the known propylene/α-olefin random copolymers when the amount of α-olefin copolymerized of the former is the same as that of the latter. Accordingly, films obtained from the propylene/α-olefin random copolymers of the invention have particularly excellent heat-sealing properties at low temperatures, and the films exhibit excellent heat-sealing properties even when they have small amounts of the copolymerized α-olefin. Hence they have excellent anti-blocking properties and excellent stiffness. To measure its melting point, the propylene/α-olefin random copolymer was allowed to stand in a differential scanning calorimeter (DSC) at 200°C for 5 minutes, cooled up to 20°C at a rate of 10°C/min, and allowed to stand at 20°C for 5 minutes, and then the temperature was elevated from 20°C to 200°C at a rate of 10°C/min to obtain a temperature (Tm) at a maximum endothermic peak which was then taken as a melting point of said propylene/α-olefin random copolymer. The molecular weight distribution ( M w/ M n) of the propylene/α-olefin random copolymers of the invention as obtained by gel permeation chromatography (GPC) is less than 3.5, preferably less than 3.0 and especially less than 2.5. As stated above, the propylene/α-olefin random copolymers of the invention have a narrow molecular weight distribution and, from this point, they have excellent anti-blocking properties. The determination of melting point was conducted using about 2.5 mg of the specimen and DSC of Perkin Elmer-7 Model at heat-up rate of 10°C/min. The propylene/α-olefin random copolymers of the invention desirably have an amount of soluble portion in boiling n-pentane of less than 3% by weight, preferably less than 2% by weight and especially less than 1% by weight. Further, the propylene/α-olefin random copolymers of the invention desirably have an amount of insoluble portion in boiling trichloroethylene of less than 5% by weight, preferably less than 3% by weight and especially less than 1% by weight. The amounts of the insoluble portion in boiling trichloroethylene and the soluble portion in boiling n-pentane were determined in a manner wherein about 3 g of the finely pulverized specimen was extracted for 5 hours with 180 ml of each of the solvents in a cylindrical filter paper by using a Soxhlet's extractor, and the extraction residue was dried with a vacuum dryer until it reached a constant weight to obtain the weight thereof, whereby a difference in weight between the dried residue and the original specimen is calculated. The propylene/α-olefin random copolymers of the present invention have particularly excellent in heat-sealing properties at low temperatures and hence are used as heat-sealing agents. The propylene copolymers of the invention have a low melting point in comparison with known propylene/α-olefin random copolymers even when said novel propylene copolymers have a low α-olefin content, and hence they have excellent anti-blocking properties and stiffness. In the present invention, there are also provided processes for preparing these novel copolymers readily and efficiently. The present invention is further illustrated below in the following Examples. Example 1 (Preparation of methylaluminoxane) Methylaluminoxane was prepared in accordance with the procedure described in Polymer Commun., 29 , 180 (1988). (Synthesis of ethylenebis(indenyl)hafnium dichloride) A nitrogen-purged 200 ml glass flask was charged with 5.4 g of bis(indenyl)ethane [synthesized on the basis of Bull. Soc. Chim., 2954 (1967)] and 50 ml of THF, and the flask was cooled with stirring to -30° ∼ -40°C. To the flask was added dropwise 31.5 ml of n-Bu Li (1.6M solution), stirred successively at -30°C for 1 hours, and the temperature was elevated spontaneously to room temperature, thereby anionizing the bis(indenyl)ethane. Separately, a nitrogen-purged 200 ml glass flask was charged with 60 ml of THF, and the flask was cooled to below -60°C, followed by gradual addition of 6.7 g of HfCl 4 (which contained 0.78% by weight of zirconium atoms as contaminants). Thereafter, the flask was heated-up to 60°C and stirred for 1 hour. To the flask was added dropwise the anionized ligand, and stirred at 60°C for 2 hours, followed by filtration with a glass filter. The filtrate was concentrated at room temperature to about 1/5 of the original volume. By this operation, solids were separated. The separated solids were filtered with a glass filter, followed by washing with hexane/ethyl ether and vacuum drying to obtain ethylenebis(indenyl)hafnium dichloride. The hafnium compound thus obtained contained 0.40% by weight of zirconium atoms. (Polymerization) A thoroughly nitrogen-purged 2 liter stainless steel autoclave was charged at room temperature with 500 ml of toluene, 3 moles of propylene, 0.1 mole of 1-butene and 5 mg atoms in terms of Al atoms of methylaluminoxane. The temperature in the polymerization system was elevated to 45°C, and 1.25 × 10 -3 mmole of the ethylenebis(indenyl)hafnium dichloride obtained as above was added to the system to carry out polymerization at 50°C for 0.5 hours. The polymerization was stopped by the addition of methanol to the polymerization system. The polymer slurry obtained was poured into large amounts of methanol, the slurry was recovered by filtration and washed with an isobutyl alcohol/hydrochloric acid solution to remove the catalyst components therefrom. The recovered slurry was then vacuum dried overnight at 80°C and 200 to 300 mmHg (26.7 to 40.0 kPa) to obtain 27.5 g of a polymer having a 1-butene content of 2.2 mol%, [η] of 3.02 dl/g as measured in decalin at 135°C, a melting point of 124°C as measured by DSC, a boiling trichloroethylene-insoluble content of 0% by weight, a boiling n-pentane-soluble content of 0.3% by weight and M w/ M n of 2.41 as measured by GPC. Example 2 The polymerization was carried out in the same manner as in Example 1 except that the amount of 1-butene charged was changed to 0.25 mol to obtain 29.1 g of a polymer having a 1-butene content of 5.6 mol%, [η] of 2.95 dl/g, a melting point of 116°C, a boiling trichloroethylene-insoluble content of 0% by weight, a boiling n-pentane-soluble content of 0.4% by weight and M w/ M n of 2.33. Evaluation The heat-sealing properties of the propylene polymer obtained in Example 6 were evaluated in the following manner. Preparation of a film On a press plate were placed an 0.1 mm thick aluminum sheet, a polyester sheet (a product sold under a trade name of Rumiler by Toray) and a polyimide resin sheet (a product sold under a trade name of Capton by Du Pont), a square of 15 cm × 15 cm of the center of which had been cut off, in that order, and 0.8 g of the specimen was placed in this center (the cut-off portion of the polyimide resin sheet), and then Rumiler®, an aluminum sheet and a press plate were superposed thereon in that order (see Fig. 3). The specimen thus interposed between the press plates was placed in a hot press kept at 200°C and preheated for about 5 minutes, followed by subjecting three times to pressure application (20 kg/cm 2 G) and release operation in order to remove air bubbles present in the specimen. Subsequently, the pressure was increased finally to 150 kg/cm 2 G and the specimen was heated for 5 minutes under pressure. After releasing the pressure, the press plate was taken out from a press machine and transferred to separate press machine kept at 30°C in its press in portion and then cooled for 4 minutes at a pressure of 100 kg/cm 2 , followed by releasing the pressure and taking out the specimen therefrom. Of the films thus obtained, those having a uniform thickness of 50 ∼ 70 µm were used as the films for measurement. Measurement of heat-sealing strength The films thus prepared are placed for 2 days in a hygrostat kept at 50°C (aging). In practicing the aging, sheets of paper are applied to both sides of the film so that the films do not come in contact with each other. The films thus aged are cut up into strips of a 15 mm thick, and two sheets of the strip are placed one upon another and then interposed between two sheets of Teflon Film of a 0.1 mm thick, followed by heat sealing. The heat sealing is carried out while maintaining the temperature of a lower portion of a hot plate of heat sealer constant at 70°C and varying only the temperature of an upper portion of hot plate suitably at intervals of 5°C. The heat-sealing pressure employed is 2 kg/cm 2 , the heat-sealing time employed is 1 second, and a width of heat seal is 5 mm (accordingly a sealed area is 15 mm × 5 mm). Heat-sealing strength is determined by obtaining a peeling strength of the heat sealed film subjected to peeling test at a rate of 30 cm/min at each heat-sealing temperature as mentioned above. (See Fig. 4.) Following the above-mentioned procedure, a peeling strength at each of the heat-sealing temperatures preset at interval of 5°C is obtained, and the plots of heat-sealing temperature-peeling strength are connected by means of a curved line. On the basis of the curved line, a heat sealing temperature corresponding to a peeling strength of 800 g /15 mm is taken as a completely heat-sealed temperature (see Fig. 5). The completely heat-sealed temperature of the propylene polymer obtained in Example 1 is 127°C.

Claims: A propylene random copolymer comprising structural units (a) derived from propylene and structural units (b) derived from an α-olefin of 4 to 20 carbon atoms, and having (i) the structural units (a) in an amount of 90 to 99 mol% and the structural units (b) in an amount of 1 to 10 mol%, (ii) an intrinsic viscosity [η] as measured in decalin at 135°C of 0.5 to 6 dl/g, (iii) a melting point [Tm] as measured by a differential scanning calorimeter falling within the range of 90 < Tm < 155 - 3.5 (100 - P) wherein P is the propylene content (mol%) present in the copolymer, (iv) a ratio ( M w/ M n) of a weight average molecular weight ( M w) to a number average molecular weight ( M n) as measured by gel permeation chromatography of less than 3.5, and (v) a boiling trichloroethylene-insoluble content of less than 5% by weight. A copolymer according to claim 1 wherein the α-olefin of 4 to 20 carbon atoms is 1-butene. A copolymer according to claim 1 or 2 wherein the structural units (a) are present in an amount of 92 to 98 mol% and the structural units (b) are present in an amount of 2 to 8 mol%. A copolymer according to any one of claims 1 to 3 which has an intrinsic viscosity (η) as measured in decalin at 135°C of 1 to 5 dl/g. A copolymer according to any one of claims 1 to 4 wherein 100 < Tm < 150 - 3.5 (100 - P) A copolymer according to any one of claims 1 to 5 wherein the ratio ( M w/ M n) is less than 2.5. A copolymer according to any one of claims 1 to 6 which has a boiling n-pentane-soluble content of less than 3% by weight. A process for preparing a propylene random copolymer as claimed in any one of the preceding claims, which process comprises copolymerizing propylene and α-olefin of 4 - 20 carbon atoms at a temperature of 40 to 100°C in the presence of a catalyst comprising [A] a hafnium compound having as a ligand a multidentate compound in which at least two groups selected from among conjugated cycloalkadienyl groups or substituted groups thereof are linked together via ethylene groups, and [B] an organoaluminium oxy-compound. A heat sealing agent comprising the propylene random copolymers as claimed in any one of claims 1 to 7.

Inventors: TOYOTA AKINORI, TSUTSUI TOSHIYUKI, TOYOTA, AKINORI, TSUTSUI, TOSHIYUKI, TOYOTA, AKINORI, C/O MITSUI PETROCHEM. IND, LTD, TSUTSUI, TOSHIYUKI, C/O MITSUI PETROCHEM. IND, LTD
Applicants: MITSUI CHEMICALS INC, MITSUI CHEMICALS, INC.
Application year: 1989
Main classification: C08F210
IPC codes: C08F110, C08F10, C08F210, C08F4