Incorporation of free radical inhibitors in polyolefins

Details Incorporation of free radical inhibitors in polyolefins Incorporation of free radical inhibitors in polyolefins

1997-08-27

2002-06-25

Hoke, Veronica P. (Department: 1714)

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

Synthetic resins

At least one aryl ring which is part of a fused or bridged...

C524S353000

Reexamination Certificate

active

06410629

ABSTRACT:

This invention relates to olefin polymerization, more specifically polymerization of olefins using transition metal catalysts; the invention also relates to stabilization of polyolefins especially using free radical inhibitors.
BACKGROUND OF THE INVENTION
Transition metal catalysts include Ziegler type catalysts which are well known to those skilled in the art and metallocene catalysts which are also known to those skilled in the art of polymerizing olefins. Inhibitors, particularly free radical inhibitors, are often added to polyolefins after polymerization using such catalysts to prevent or lessen deterioration caused by free radicals. Addition of free radical inhibitors has followed polymerization to avoid inhibition of the transition metal catalysts. Phenolic compounds are recognized free radical inhibitors but are also believed by those skilled in the art to inhibit transition metal polymerization catalysts.
It would be useful to also avoid free radical induced deterioration during polymerization, preferably without substantially inhibiting transition metal catalysts. Inhibitors added before or during polymerization would advantageously reduce deterioration of the polymer by free radicals during processes that often precede inhibitor addition. Additionally, inhibitor addition during or before polymerization would advantageously result in good mixing and avoid processing steps such as melting formed polymer, a step now needed for good inhibitor addition. In some processes the inhibitor is added in solution and mixing as well as solvent removal, is needed; both would advantageously be avoided by addition of inhibitor before or during polymerization.
In previous attempts to add inhibitors to polymers before or during polymerization, whether or not the inhibitors were monomers to be incorporated into the polymer, the active groups of the inhibitors were inactivated, for instance by reaction with aluminum reagent, for instance diethylaluminum chloride. See EP 466,263 (Olivier and Young) 1992. Inactivation of the inhibitor is disadvantageous in that the inhibitor is not active to avoid deterioration from free radicals, and in that extra process steps and reagents are required for inactivation and removal of the inactivating species. In addition to the costs associated with extra reagents and extra steps, removal often requires acid (as in the case of aluminum compound inactivators) which itself can be detrimental to a polymer, especially in its subsequent uses.
It would be desirable to have a method of inhibiting deterioration in a polymer by adding an inhibitor during or before polymerization with a transition metal catalyst without inactivating the inhibitor or reacting it with one or more substances that reduce its activity with the catalyst.
SUMMARY OF THE INVENTION
It has now been found that certain phenolic compounds can be used as free radical inhibitors during polymerization without deactivation of transition metal catalysts. Use of such compounds during polymerization is particularly advantageous when the inhibitors and/or the resulting mixture with monomer and catalyst are substantially free of catalyst inactivating compounds such as less substituted phenols and quinones and when the transition metal catalysts are activated with compounds which do not react with the inhibitors, especially non-aluminum compounds.
The invention includes a process for inhibiting deterioration in a polymer by admixing an inhibitor with at least one olefin monomer during or before polymerization thereof using a transition metal catalyst. It is not necessary or desirable to inactivate the inhibitor or react it with one or more substances that reduce its activity with the catalyst. The process is preferably preceded by a step of purifying the inhibitor to remove compounds which inhibit catalyst activity such as quinones and phenols which are not sufficiently sterically hindered to avoid inhibiting the catalyst.
Additionally, the invention includes a composition comprising at least one olefin monomer, a polymer of the monomer, a transition metal catalyst and at least one inhibitor having sufficient substituents to hinder the active inhibiting group such that it does not undesirably inhibit the transition metal catalyst. The inhibitor is preferably purified such that compounds which inhibit the catalyst are substantially removed. Preferably neither the monomer(s) nor catalyst contain such compounds. The invention is particularly useful when the monomer comprises propylene, styrene or a derivative of either.
DETAILED DESCRIPTION OF THE INVENTION
Inhibitors useful in the practice of the invention are those having at least one active group capable of inhibiting free radicals and having sufficient substitution to hinder each active group such that it does not undesirably inhibit a transition metal catalyst. Those skilled in the art recognize that some catalyst inhibition may be acceptable however, advantageously, in the practice of the invention only limited catalysts inhibition takes place preferably less than about 75 percent, more preferably less than about 50 percent, most preferably less than about 25 percent of the catalyst is inhibited by the free radical inhibitors used in the practice of the invention. Those skilled in the art recognize that the amount of inhibition is a function of the molar ratio of free radical inhibitor to catalyst. For instance at a ratio of 10000:1, 25 to 75 percent inhibition is sometimes observed. However, in the case of a molar ratio of free radical inhibitor to catalyst of about 1:1, there is preferably less than about 10 percent reduction in catalyst activity measured by monomer converted to polymer per unit time.
Each active group on the inhibitor is suitably any group active in inhibiting free radicals, preferably an active hydrogen group, XH where X is a heteroatom such as oxygen, nitrogen (R′″N where R′″ is any hydrocarbyl group or silyl group (such as trimethylsilyl, triethylsilyl, or H
3
Si)) or sulfur, preferably oxygen. R′″ preferably has from 1 to about 50 carbon atoms and optionally from 1 to about 5 silicon atoms. More preferably the active group is a hydroxyl (including phenol) group, amine, or sulfhydryl, most preferably a hydroxyl group. Those skilled in the art recognize the molecular structures which demonstrate high activity in inhibiting free radicals. For instance, the active group is preferably attached to an aromatic ring, most preferably is a phenol group.
To achieve sufficient steric hindrance to avoid undesirably inhibiting a transition metal catalyst, the inhibitor preferably has at least one hindering group on each carbon adjacent to an active group. Hindering groups are groups sufficiently large to hinder access of a transition metal catalyst to the active inhibiting group and advantageously comprise at least about 3 carbon atoms, preferably at least about 4 carbon atoms. The hindering groups are preferably branched such as isopropyl, tertiary butyl, isobutyl, isopentyl, styryl (—CH
2
—CH
2
—C
6
H
5
) groups and the like. The groups are preferably hydrocarbyl groups or substituted with additional active inhibiting groups which are also sufficiently sterically hindered to avoid inhibiting a transition metal catalyst, but optionally may have other inert substitution such as ethers or trisubstituted amine groups. Inert substitution means substitution which does not undesirably interfere with the action of the active group(s) or with transition metal catalysis of polymerization. The hindering groups are preferably selected from tertiary butyl groups or styryl groups, optionally and preferably further substituted with at least one additional active inhibiting group which is in turn sterically hindered. The preferred styryl group is 1-hydroxy, 2-methylene, 4-methyl, 6-tertiary butyl phenyl, hereinafter referred to as active styryl.
The inhibitors are preferably of the formula:
wherein XH is an active group as previously described; and R is independently a hindering group as previously described; and eac

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