Polyethylene and method of production thereof

A polyethylene, having 1 to 60 methyl branches and 1 to 60 hexyl or higher branches per 1000 carbon atoms, a g-value of 0.5 to 0.8, and a limiting viscosity [.eta.] of 0.005 to 20.0 dl/g as measured at 140.degree. C. in o-dichlorobenzene, and a method for producing the same by polymerizing ethylene using a catalyst system comprising a coordination nickel compound of zero- or two-valent nickel and an aminobis(imino)phosphorane represented by a general formula (I): ##STR1## where R.sub.1, R.sub.2, R.sub.3 and R.sub.4 may be the same as or different from each other and are respectively n-alkyl, isoalkyl, aryl or trialkylsilyl, in the presence of .alpha.-olefin.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a polyethylene having a novel branching 
structure, and a method of production thereof. More specifically, the 
present invention relates to production of a polyethylene having a novel 
branching structure by polymerizing ethylene by employing a special 
catalyst system with superior polymerization activity. The polyethylene of 
the present invention is useful as inflation films, injection-molded 
articles, blow-molded articles, extrusion-coating materials, polymer 
blending materials, etc. in the same manner as conventional polyethylenes. 
2. Description of the Related Art 
Generally, polymerization of ethylene by a radical initiator at a very high 
temperature under a very high pressure gives a polyethylene having 
branched chains of length comparable with that of the main chain. On the 
other hand, polymerization of ethylene with a Ziegler-Natta catalyst under 
a low pressure gives a polyethylene having almost no branches. 
For the purpose of producing a branched polyethylene by a method using a 
Ziegler-Natta catalyst, ethylene is copolymerized, generally with an 
.alpha.-olefin. The resulting polyethylene, however, does not have as long 
a branch as that of the polyethylene produced by radical polymerization. 
In an attempt to produce a low-pressure polyethylene having such a long 
branched chain, ethylene is oligomerized, for example, by a nickel 
catalyst disclosed in Japanese Laid-Open Patent Application No. 63-12607 
(1988), and the resultant oligomers are copolymerized with ethylene by a 
Cr catalyst. This method, however, does not gives a branch having a length 
comparable with that produced by a radical polymerization. 
SUMMARY OF THE INVENTION 
An object of the present invention is to provide a polyethylene having a 
novel branching structure. 
Another object of the present invention is to provide a method for 
producing a polyethylene having a novel branching structure. 
According to an aspect of the present invention, there is provided a 
polyethylene, having 1 to 60 methyl branches and 1 to 60 hexyl or higher 
branches per 1000 carbon atoms, a g-value of 0.5 to 0.8, and a limiting 
viscosity [.eta.] of 0.005 to 20.0 dl/g as measured at 140.degree. C. in 
o-dichlorobenzene. 
According to another aspect of the present invention, there is provided a 
method for producing a polyethylene, by polymerizing ethylene employing a 
catalyst system comprising a coordination nickel compound of zero- or 
two-valent nickel and an aminobis(imino)phosphorane represented by the 
general formula (I): 
##STR2## 
where R.sub.1, R.sub.2, R.sub.3 and R.sub.4 may be the same as or 
different from each other, and are respectively n-alkyl, isoalkyl, aryl or 
trialkylsilyl, in the presence of an .alpha.-olefin.

DETAILED DESCRIPTION OF THE INVENTION 
The polyethylene of the present invention as mentioned above is produced by 
polymerization of ethylene under a low pressure. A polyethylene having 
such a structure has not been produced under a low pressure until now. 
The polyethylene of the present invention has, as short chain branches, 1 
to 60 methyl branches and 1 to 60 hexyl or higher branches per 1000 
carbons. The existence of such branches is confirmed by .sup.13 C-NMR, and 
the assignment of these branches is made according to, for example, the 
disclosure of J. C. Randall: J.Polym. Sci., Polymn. Phys.Ed., 11, 275 
(1973). 
The existence of long chain branches in the structure of the polyethylene 
is suggested, if the polyethylene has a g-value of less than 1: the 
g-value being defined by g=[.eta.]/[.eta.].sub.1 where [.eta.] denotes a 
limiting viscosity of a linear polyethylene and [.eta.] denotes a limiting 
viscosity of the branched polyethylene having the same melt index. The 
polyethylene of the present invention has a g-value of from 0.5 to 0.8, 
which suggests the existence of long chain branches having a chain length 
comparable with the length of the main chain. The polyethylene of the 
present invention has substantially no ethyl branch or butyl branch, which 
are found in the polyethylenes produced by radical polymerization at a 
high temperature under a high pressure, and the he number of branches 
having 2 to 5 carbon atoms in the polyethylene of the present invention is 
not more than 1 per 1000 carbon atoms, which also evidences the definite 
novelty of the present invention. 
The method for producing the polyethylene is described below. 
In the present invention, specific examples of the coordination nickel 
compounds of zero- or two-valent nickel include biscyclooctadienenickel, 
cyclododecatrienenickel, cyclooctatetraenenickel, bisallylnickel, etc. 
The aminobis(imino)phosphoranes represented by the general formula (I) 
specifically includes 
bis(trimethylsilyl)amino-bis(trimethylsilylimino)phosphorane, etc. These 
compounds can be prepared by the method, for example, described by O. J. 
Scherer, N. Kush: Chem. Ber. 107, 2123 (1974). 
In polymerization of ethylene employing the two component catalyst system, 
the ratio of the nickel compound and the aminobis(imino)phosphorane is 
preferably in the range of from 1:1 to 1:100 (in molar ratio). The 
respective components may be introduced into the polymerization vessel 
either in a form of an undissolved solid of in a form of a solution in a 
solvent, and the order of addition does not affect the structure of the 
polymer of the activity of the catalyst. 
The .alpha.-olefin to be added in the polymerization of ethylene is 
preferably an .alpha.-olefin having from 3 to 20 carbons. The specific 
examples are propylene, 1-butene, 1-pentene, 1-hexene, 1-octene, 1-nonene, 
1-decene, 4-methyl-l-pentene, etc. The amount of the .alpha.-olefin that 
may be used is not limited. However, the use thereof in an amount 
equimolar to nickel or greater improves the catalyst activity. The 
.alpha.-olefin may be used also as a solvent for polymerization. 
The polymerization of ethylene may be practiced either in a liquid phase or 
in a gas phase. The polymerization in a liquid phase is preferably 
conducted in an inert solvent. The inert solvent may be any of the 
solvents which are used in the related technical field, such as aliphatic 
hydrocarbons of 4 to 20 carbons, aromatic hydrocarbons, halogenated 
hydrocarbons. The specific examples are hexane, heptane, pentane, octane, 
decane, cyclohexane, benzene, toluene, xylene, chlorobenzene, ethylene 
dichloride, kerosine, etc. 
The preferable polymerization conditions in the present invention are a 
polymerization temperature of from -78.degree. to 200.degree. C., and a 
polymerization pressure of from 1 to 200 kg/cm.sup.2 G. 
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The present invention is described below in more detail referring to 
examples without limiting it in any way. 
Regarding the structure of the polyethylene produced according to the 
present invention, the short chain branches are identified by .sup.13 
C-NMR with the assignment on the basis of J. C. Randall: J.Polym. Sci., 
Polym. Phys. Ed., 11, 275 (1973). 
The existence of long chain branches in the structure of the polyethylene 
is indicated by the value of g=[.eta.]/[.eta.].sub.1, where [.eta.].sub.1 
denotes a limiting viscosity of a linear polyethylene and [.eta.] denotes 
a limiting viscosity of the branched polyethylene having the same melt 
index. The g-value of less than 1 suggests the existence of long chain 
branches. 
Example 1 
Into a 2-liter magnetic-stirrer-type stainless steel reactor, which had 
been purged sufficiently with nitrogen, there were added 500 ml of 
toluene, 2.0 mmol of bis(1,5-cyclooctadiene)nickel, and 2.0 mmol of 
bis(trimethylsilyl)amino-bis(trimethylsilylimino)phosphorane. The inside 
temperature was adjusted to 20 .degree. C. Thereto 65 ml of 1-hexene was 
added. Then ethylene was fed to maintain the inside pressure at 25 
kg/cm.sup.2 G, and to cause polymerization reaction for 3 hours. 
After completion of the reaction, the unreacted ethylene was removed, and 
the catalyst was decomposed with a hydrochloric acid solution in methanol. 
The reaction mixture was poured into methanol to recover the polymer. The 
recovered polymer was dried under vacuum for 8 hours to give 130 g of 
polymer. The melting point was 94.5.degree. C. as determined by 
differential scanning calorimeter (DSC), and the limiting viscosity was 
1.08 dl/g at 140.degree. C. in dichlorobenzene. 
FIG. 1 shows the .sup.13 C-NMR spectrum of the resulting polymer. The peaks 
resulting from methyl branches are found at 20.4 ppm, 27.5 ppm, 30.4 ppm, 
33.2 ppm, and 37.5 ppm, and those resulting from hexyl and higher branches 
are found at 14.3 ppm, 23.1 ppm, 27.4 ppm, 30.7 ppm, 32.4 ppm, 34.6 ppm, 
and 38.3 ppm. No other peak resulting from a branched structure is 
observed. From the spectrum, the number of the methyl branches and the 
hexyl and higher branches were each found to be 22 per 1000 carbon atoms. 
No branches having 2 to 5 carbon were found in the polymer. The g-value 
was 0.70. 
Example 2 
Into a 2-liter magnetic-stirrer-type stainless steel reactor, which had 
been purged sufficiently with nitrogen, there were added 500 ml of 
toluene, 2.0 mmol of bis(1,5-cyclooctadiene)nickel, and 2.0 mmol of 
bis(trimethylsilyl)amino-bis(trimethylsilylimino)phosphorane. The inside 
temperature was adjusted to 20.degree. C. Thereto 50 ml of 1-butene was 
added. Then ethylene was fed to maintain the inside pressure at 25 
kg/cm.sup.2 G, and to cause polymerization reaction for 3 hours. 
After completion of the reaction, the unreacted ethylene was removed, and 
the catalyst was decomposed with a hydrochloric acid solution in methanol. 
The reaction mixture was poured into methanol to recover the polymer. The 
recovered polymer was dried under vacuum for 8 hours to give 140 g of 
polymer. The melting point was 93.6.degree. C. as determined by DSC, and 
the limiting viscosity was 0.86 dl/g at 140.degree. C. in dichlorobenzene. 
The number of the methyl branches was found to be 23 per 1000 carbons and 
the number of the hexyl and higher branches were found to be 24 per 1000 
carbon atoms. No branches having 2 to 5carbons were found in the polymer. 
The g-value was 0.69. 
Example 3 
Into a 2-liter magnetic-stirrer-type stainless steel reactor, which had 
been purged sufficiently with nitrogen, there were added 500 ml of 
toluene, 1.3 mmol of bis(1,5-cyclooctadiene)nickel, and 1.3 mmol of 
bis(trimethylsilyl)amino-bis(trimethylsilylimino)phosphorane. The inside 
temperature was adjusted to 20.degree. C. Thereto 47 ml of 1-decene was 
added. Then ethylene was fed to maintain the inside pressure at 25 
kg/cm.sup.2 G, and to cause polymerization reaction for 24 hours. 
After completion of the reaction, the unreacted ethylene was removed, and 
the catalyst was decomposed with a hydrochloric acid solution in methanol. 
The reaction mixture was poured into methanol to recover the polymer. The 
recovered polymer was dried under vacuum for 8 hours to give 60 g of 
polymer. The melting point was 92.5.degree. C. as determined by DSC, and 
the limiting viscosity was 1.03 dl/g at 140.degree. C. in dichlorobenzene. 
The number of the methyl branches was found to be 18 per 1000 carbons and 
the number of the hexyl and higher branches were found to be 16 per 1000 
carbon atoms. No branches having 2 to 5 carbons was found in the polymer. 
The g-value was 0.73. 
Example 4 
Into a 2-liter magnetic-stirrer-type stainless steel reactor, which had 
been purged sufficiently with nitrogen, there were added 500 ml of 
toluene, 2.0 mmol of bis(1,5-cyclooctadiene)nickel, and 2.0 mmol of 
bis(trimethylsilyl)amino-bis(trimethylsilylimino)phosphorane. The inside 
temperature was adjusted to 20 .degree. C. Thereto 65 ml of 
4-methyl-1-pentene was added. Then ethylene was fed to maintain the inside 
pressure at 25 kg/cm.sup.2 G, and to cause polymerization reaction for 3 
hours. 
After completion of the reaction, the unreacted ethylene was removed, and 
the catalyst was decomposed with a hydrochloric acid solution in methanol. 
The reaction mixture was poured into methanol to recover the polymer. The 
recovered polymer was dried under vacuum for 8 hours to give 120 g of 
polymer. The melting point was 94.1.degree. C. as determined by DSC, and 
the limiting viscosity was 1.15 dl/g at 140.degree. C. in dichlorobenzene. 
The number of the methyl branches was found to be 21 per 1000 carbons and 
the number of the hexyl and higher branches were found to be 22 per 1000 
carbon atoms. No branches having 2 to 5 carbons were found in the polymer. 
The g-value was 0.68. 
Example 5 
Into a 2-liter magnetic-stirrer-type stainless steel reactor, which had 
been purged sufficiently with nitrogen, there were added 500 ml of 
toluene, 2.0 mmol of bis(1,5-cyclooctadiene)nickel, and 2.0 mmol of 
bis(trimethylsilyl)amino-bis(trimethylsilylimino)phosphorane. The inside 
temperature was adjusted to 20.degree. C. Thereto 5 ml of 1-butene was 
added. Then ethylene was fed to maintain the inside pressure at 25 
kg/cm.sup.2 G, and to cause polymerization reaction for 3 hours. 
After completion of the reaction, the unreacted ethylene was removed, and 
the catalyst was decomposed with a hydrochloric acid solution in methanol. 
The reaction mixture was poured into methanol to recover the polymer. The 
recovered polymer was dried under vacuum for 8 hours to give 105 g of 
polymer. The melting point was 90.3 .degree. C as determined by DSC, and 
the limiting viscosity was 1.12 dl/g at 140.degree. C. in dichlorobenzene. 
The number of the methyl branches and the number of the hexyl and higher 
branches were each found to be 20 per 1000 carbons. No branches having 2 
to 5 carbons were found in the polymer. The g-value was 0.67. 
Comparative Example 1 
Ethylene was polymerized in the same manner as in Example 1 except that 
1-hexene used in Example 1 was not added. 16 g of polymer was produced. 
As described above, the present invention produces a polyethylene having 
short chain branches and long chain branches by low-pressure 
polymerization of ethylene by employing a specific catalyst and by adding 
an .alpha.-olefin with superior polymerization activity.