Styrene-butadiene block copolymer

Details Styrene-butadiene block copolymer Styrene-butadiene block copolymer

1999-08-04

2001-08-07

Boykin, Terressa M. (Department: 1711)

Synthetic resins or natural rubbers -- part of the class 520 ser

Synthetic resins

Mixing of two or more solid polymers; mixing of solid...

Reexamination Certificate

active

06271313

ABSTRACT:

BACKGROUND OF THE INVENTION
The term “metallocene” was first used in the mid-1950s as a replacement for the colloquialism “iron sandwich,” a name given to Cp
2
Fe, where Cp is cyclopentadienyl, after the &eegr;
5
-bonding mode of the Cps was first described independently by Wilkinson and Fischer in 1954. Now the term is used to describe any transition metal complex which has one or more Cp or substituted Cp ligands bound to it (see K B Sinclair and R B Wilson, Chem Ind 1994, 7, 857). Much of the initial interest and research in the area of early metal bent metallocenes (a metallocene with two Cps where the Cp(centroid)-metal-Cp(centroid) angle is less than 180° C., example Cp
2
TiCl
2
) was due to an effort to model the highly active and stereoselective heterogeneous Ziegler-Natta polymerization catalysts (see H H Brintzinger et al, Angew Chem Int Ed Engl 1995, 34, 1143), which are based on early metals such as TiCl
n
/AlR
m−p
Cl
p
, where R is an alkyl group, for example methyl (Me) or ethyl (Et), n is 3 or 4, m is 3, and p is 1, 2 (see P Locatelli et al, Trends Poly Science 1994, 2, 87).
Bent metallocene models, based particularly on Group IV metals, offered promise of elucidating key features of the homogeneous polymerizations which they catalyzed. It was believed this information could then be related to the field of conventional Ziegler-Natta catalysis. As models, Group IV metal bent metallocenes offered several advantages. These advantages included simple coordination geometries, only two reactive ligand sites with cis orientation, and from a practical standpoint, compatibility with spectroscopic techniques, such as NMR, allowing more direct observation of the active catalyst species. It would now appear that these “models” are replacing the existing Ziegler-Natta polymerization catalyst systems in many applications (see H H Brintzinger et al, Angew Chem Int Ed Engl 1995, 34, 1143).
It has been known since the mid-1950s that Cp
2
TiCl
2
and Et
2
AlCl catalyze the formation of polyethylene under conditions similar to those used in conventional heterogeneous Ziegler-Natta catalysis (see D S Breslow and N R Newburg, J Am Chem Soc 1957, 79, 5072). By 1960, several of the key features in systems like this had been deduced by various spectroscopic techniques. The key features included the formation of Cp
2
TiRCl, where R is Me, Et or a related species, by exchange with the alkylaluminum co-catalyst, polarization of the Ti—Cl bond in this species by Lewis acidic centers forming an adduct of the type Cp
2
TiRCl.A1RCl
2
, and insertion of the olefin into the Ti—R bond of this electron deficient species. However, these types of model systems are only capable of polymerizing ethylene, which is in contrast to the heterogeneous Ziegler-Natta catalysts which can also polymerize propylene. This limitation proved to be a serious obstacle to progress in this field.
A breakthrough occurred in the late 1970s when Sinn and Kaminsky serendipitously observed that the addition of small amounts of H
2
O to the otherwise inactive catalyst system of Cp
2
MMe
2
/AlMe
3
, where M is Ti or Zr, imparted a surprisingly high activity for ethylene polymerization (see H Sinn and W Kaminsky, Adv Organomet Chem 1980, 18, 99). It was suspected that partial hydrolysis of AlMe
3
formed methylaluminoxane (MAO) which is of the general formula Me
2
AlO—[Al(Me)O]
n
—AlMe
2
where n is generally thought to represent an integer from 5 to 20, which then acted as an efficient co-catalyst. This idea was supported by directly synthesizing MAO and successfully utilizing it as a co-catalyst with not only Cp
2
ZrMe
2
, but also Cp
2
ZrCl
2
(see H Sinn et al, Angew Chem Int Ed Engl 1980, 19, 396). Activity in certain examples are even higher than in conventional Ziegler-Natta catalyst systems. Activity as high as 40,000 Kg PE/g metal/h have been reported employing zirconocene catalysts activated with MAO with an Al:Zr ratio of 12,000 (Mw=78,000) (see W Kaminsky et al, Makromol Chem Rapid Commun 1983, 4, 417). Furthermore, Sinn and Kaminsky demonstrated that these types of MAO-activated homogeneous metallocene catalysts are capable of polymerizing propylene and other &agr;-olefins, however, without any stereoregularity (see H Sinn and W Kaminsky, Adv Organomet Chem 1980, 18, 99).
The role of the MAO in early metal metallocene catalysis is now believed to be threefold. First, MAO acts as an alkylating agent for the generation of metal-alkyl adducts. Second, MAO acts as a strong Lewis acid, abstracting an anionic ligand thereby forming the crucial alkyl cationic species. Finally, MAO and especially AlMe
3
impurities in the MAO act as a scavenger for removing catalyst poisons (for example, H
2
O which would react with AlMe
3
, forming more MAO) in the olefin and solvent (see A D Horton, Trends Polym Sci 1994, 2, 158).
The role of MAO as a co-catalyst is now fairly well understood; however, at the time of Kaminsky and Sinn's discovery, this was not the case. The nature of the active catalyst species derived from these MAO-activated early metal metallocene model complexes remained unclear. Compounding this problem is the remarkably complex nature of MAO, as well as the large excess required for high activity. In fact, the exact structure(s) of MAO remains unknown to this day (see J C W Chien et al, J Poly Sci, Part A, Poly Chem 1991, 29(4), 459). Much of the debate at the time revolved around whether or not the active species was bimetallic or cationic. Natta, Sinn, and others supported a theory which suggested the active catalyst was a bimetallic species in which an alkyl group or halide bridged the Group IV metal and the aluminum center promoting olefin insertion (see G Natta and G Mazzanti, Tetrahedron 1960, 8, 86). Shilov and others supported a theory which suggested insertion of the olefin actually occurs at a truly cationic species, such as [Cp
2
TiR]
+
(see A K Zefirova and A E Shilov, Dokl Akad Nauk SSSR 1961, 136, 599).
In 1986, Jordan helped resolve this issue by isolating tetraphenylborate salts of base stabilized zirconocene alkyl cations such as [Cp
2
ZrR(THF)]
+
, where R represents an Me or benzyl (Bz) group, and THF is tetrahydrofuran (see R F Jordan et al, J Am Chem Soc 1986, 108, 1718). Jordan, also demonstrated their ability to polymerize olefins without the presence of any co-catalyst (see R F Jordan, Adv Organomet Chem 1991, 32, 325). Subsequent research by Jordan and other groups gave credence to the idea that an alkyl cation is a crucial intermediate in Group IV metal bent metallocene based olefin polymerizations.
Several requirements are now widely considered critical in the formation of Group IV metal bent metallocene catalysts active for olefin polymerization. An active catalyst must have a d
o
, 14e
−
, Lewis acidic metal center, a coordinately unsaturated metal center, and a vacant coordination site cis to a reactive M—R bond.
Group IV metal bent metallocenes activated with large excesses of MAO can polymerize &agr;-olefins, but the large excess of MAO required is often impractical from an industrial standpoint due to the high cost of MAO, as well as the requisite high catalyst residue left in the resulting polymer. One solution to this problem evolved from the observation that insertion of an olefin into an M—R bond can only occur if the counterion is bound very weakly, noting that even large, bulky counterions, for example, BPh
4
−
and C
2
B
9
H
12
−
, coordinate to the cationic metal centers quite strongly, producing catalysts with only moderate activity (see G G Hanky et al, J Am Chem Soc 1989, 111, 2728). In order to produce weakly or non-coordinating counterions, the general idea of placing electron withdrawing substituents at the periphery of the boron counterion center has worked well. The most successful electron withdrawing substituents have been fluorinated phenyls which produce stable, yet weakly coordinating counterions (for example, B(C
6
F
5
)
3
, [HNMe
2
Ph] [B(C
6
F
5
)
4
], and &lsq

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