CA1134550A

CA1134550A - Cyclopentene copolymerization process

Info

Publication number: CA1134550A
Application number: CA000308365A
Authority: CA
Inventors: Eilert A. Ofstead; Michael Senyek
Original assignee: Goodyear Tire and Rubber Co
Current assignee: Goodyear Tire and Rubber Co
Priority date: 1977-09-07
Filing date: 1978-07-28
Publication date: 1982-10-26
Also published as: BR7805775A; BE870221A; GB2014588B; IT7850960A0; NL7808807A; IT1106124B; JPS5450599A; JPS5946525B2; FR2402671B1; FR2402671A1; GB2014588A; DE2838302A1; MX148506A

Abstract

INVENTION: CYCLOPENTENE COPOLYMERIZATION PROCESS
INVENTORS: Eilert A Ofstead and Michael L Senyek ABSTRACT OF THE DISCLOSURE
There is disclosed a process for the preparation of gel-free rubbery copolymers of cyclopentene and dicyclo-pentadiene by means of a catalyst system comprising (A) a soluble tungsten halide or oxyhalide, (B) an organo-aluminum compound, (C) an alcohol and (D) a polyhalogenated phenol.

Description

~3~5CI

This inven-tion is directed -to a process for the ring-opening copolymeriza-tion of polycyclic unsaturated hydrocarbons wi-th cyclopen-tene. This invention fur-ther relates -to the preparation of rubbery, essentially gel-free copolymers of polycyclic unsa-tura-ted hydrocarbons and cyclopentene. More specifically, this invention relates to the preparation of rubbery, essentially gel-free, copolymers of dicyclopentadiene and cyclopentene. These rubbery copolymers have a desirable combination of proper-ties which make them well sui-ted for use in a variety of manufac-tured rubber ar-ticles, inc].uding -tires.
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The catalyst systems use~ul in -the practice of this invention are known broadly as olefin metathesis catalysts. The olefin metathesis reaction is a general reac-tion of olefins, both cyclic and acyclic, and is known -to proceed by the catalyzed cleavage of carbon-to-carbon double bonds, and the subsequen-t recombination of the resul-ting fragments to form new olefinic species:

2 CH3CH ,- CHC2~5~-~ CH3CH = CHCH3 + C2H5CH = CHC~H5 When cycloolefins react in the presence of an olefin metathesis catalyst, ring cleavage occurs9 and high -~ molecular weight polymers result. Thus, cyclopen-tene yields the linear polymer polypentenamer:
CH - CH
n CH2 CH2 ~ ~ HCH2CH2c~zc ~
CH2 ~ n The s-tructure of this cyclopentene polymer may also be represented by the equivalent formula:

. .

~3~5~3 --~-CH= CHCH2CH2CH2 ~~n As a ~urther example, when dicyclopentadiene reac-ts, only one of the double bonds normally undergoes the meta-thesis reac-tion and so polymerization occurs to give a predominantly linear polymer:

~ =~ CH ~ CH ~=n The s-tructure o the dicyclopen-tadiene pol~mer may also be represen-ted by -the equivalent ~ormula:

~ CH = CH - < ~
t~ ~

A wide variety of catalysts suitable ~or the metathesis of acyclic ole~ins and ~or the homopolymeriza-tion of cycloolefins are known in the art, but consiclerable difficulties have been encountered in at-tempts to prepare rubbery, soluble copolymers from dicyclopentadiene and cyclopentene which contain a substantial proportion of dicyclopentadiene, i.e., greater than about 10% by weight o~ dicyclopentadiene~ Thus, in U. S. Patent No. 3,598,796 there is disclosed a process for making rubbery homopolymers o~ cyclopentene. However, when a mixture of cyclopentene and dicyclopentadiene containing 20% by volume of dicyclo-pentadiene was polymerized in a solven-t, the product was found to be non-elastomeric and only partially soluble.

~45~C~

It has been found that dicyclopentadiene generally has a grea-t tenclency -to homopolymeriæe in -the presence of o-ther cycloole~ins, and -that the resul-ting blocky segments of non-rubbery homopolymers greatly interfere with -the desired elastomeric properties of the in-tended copolymers, causing them -to become s-tiff and inelastic and ~msuitable for use in ar-ticles where rubbery quali-ties are required.
These copolymers are further generally characterized as being opaque or translucent ra-ther -than -transparent, and are only poorly soluble in customary rubber solvents such as benzene, toluene, hexane, cyclohexane and the like.
In order to overcome problems associated wi-th the copolymeriza-tion of dicyclopentadiene and cyclopentene, cer-tain procedures have been proposed. Thus, in U. S.
Pa-tent No. 3,707,520 there is described a two-stage process ^ whereby, in -the first stage, cyclopen-tene is polymerized to at least 40% conversion, followed by -the gradual intro-duction of dicyclopentadiene during the second stage of the polymerization. However, if -the polymerization of cyclopen-tene is no-t carried to a-t least 40% conversion in -the firs-t s-tage, the final produc-t is found to be unsa-tis-~; factory.
Similarly, U. S. Patent No. 3,941,757 describes a two-stage process wherein the homopolymerization of cyclopentene is first initiated9 and thereafter a solu-tion containing a polycyclic olefin such as dicyclopentadiene and also containing a tungsten or molybdenum compound is very gradual]y in-troduced into -the polymerizing cyclo-pentene solution. It is essential tha-t the transition

-3-~3~

metal cornpound be presen-t in -the solll-tion of the polycyclic olefin when i-t is added -to -the polymerizing mass. If the l transi-tion me-tal compound is not present in -this solu-tion prior to its introduction into the polymerization mix-ture, then only a low yield of polymer having excessive stiffness resulted.
In order to avoid the necessity of using a two-s-tage poly~leriza-tion process and a-t the same time overcome the -tendency to form a substantial amount of insoluble polymer, an al-ternative method has been proposed for -the prepara-tion of cyclopen-tene/dicyclopentadiene copolymers which contain very little insoluble polymer. Thus, U. S. Patent No. L~,002,815 describes a polymerization process wherein organoaluminum lodides, or optlonally, a combination of a trialkylaluminum compound and elemental iodine, are employed as essential cocatalysts in conjunc-tion with soluble -tungsten compounds. Organoaluminum chlorides are unsatisfactory. Addi-tionally, this process requires -the presence of from 1 to 30 mole percent of an acycllc olefin, relative to -the total amount of monomers, in order to avoid the formation of insoluble products.
; However, if less than 1 mole percent of acyclic olefin were ; used, yields were poor or the products contained substantial amounts of insoluble polymer.
Also, see ~. S. Patent No. 3,933,778.
Further, each of the above processes have been demons-trated to be effective when aromatic solvents such as benzene, toluene or chlorobenzene are used. Howe~er, yields and rates of polymerization are much inferior when aliphatic ~ ~3~

or cycloaliphatic solven-ts are employed and consequen-tly ~ ~mdesirably high ca-talys-t concentra-tions and long reaction - times become necessltated. Thus, these processes are unattrac-tive ~or indus-trial applications where aliphatic or cycloaliphatic solvents need be employed.
It is, -therefore, an objec-t of the present inven-tion to provide an efficient me^thod f'or the prepara-tion of rubbery, essentially gel-free copolymers of cyclopentene and dicyclopentadiene, wherein aliphatic or cycloaliphatic solvents may be effectively employed as well as aromatic ` solvents.
SUMMAR~ OF THE INVENTION
Substantially gel-free, rubbery copolymers of cyclopen-tene and dicyclopentadiene containing from lO -to 60 percent by weight of dicyclopentadiene are prepared in aliphatic, cycloaliphatic or aromatic solvents by copoly-merizing cyclopentene with dicyclopentadiene in a singIe-stage continuous polymerization reactor in the presence of a catalyst system comprised of (A) a soluble tungsten halide or oxyhalide, (B) a-t least one compound selected from the group consisting of trialkylaluminum compounds, dialkyl-aluminum chlorides, alkylaluminum dichlorides and alkyl-aluminum sesquichlorides, (C) at least one hydroxy compound selected from the group consisting of aliphatic alcohols and alcohols substituted with alkoxy or aryloxy- groups and (D) a polyhalogenated phenol of the general formula:
N
M ~ P
X~_ X
dH

~3~

where X is chlorine or bromine and M, N and P are selec-ted from -the group consi.s-ting of H, Cl and Br; and wherein the molar ratio of ~:B:C:D lies within the range of 1:0.5-5:0.5-2:1-3.
This process provides excellent rates of poly-merization even at low ca-talys-t concen-trations. In addition, gel-free polymers are obtained even ~hen -the amount of acyclic olefin present is less than 0.1% by weight relative -to the total monomers presen-t. This process fur-ther permi-ts the use of aliphatic and cycloalipha-tic solven-ts such as hexane and cyclopen-tane which are more easily removed during -the drying of -the polymer than are the higher boiling aromatic solven-ts such as benzene, -toluene and -the like.
DETAILED DESCRIPTION
- The process of -this invention comprises the rîng-opening copolymerization of cyclopentene with a-t least one polycyclic unsaturated or polycyclic polyunsa-turated, non - conjugated alicyclic compound. The preferred polycyclic monomers are dicyclopentadiene and -the -trimer o~ cyclo~
pentadiene: ~ ~>

Representative of the tungsten compounds (A) are the chlorides and bromides such as tungs-ten hexachloride~
-tungs-ten hexabromide, -tungsten pentabromide, tungsten oxytetrachloride, -tungsten oxy-te-trabromide and the like.
However, it is preferred to use tungsten hexachloride.
Representative of -the organoaluminum compounds (B) are trime-thylaluminum, trie-thylaluminum, tripropylaluminum, 5G~

-triisobutylal~ninum, dimethylaluminum chlorlde, die-thyl-aluminum chloride, diisobu-tylaluminum chloride, me-thyl-aluminum sesquichloride, e-thylaluminum sesquichloride, isobu-tylaluminum sesquichloride, ethylaluminum dichloride, propylaluminum dichloride, bu-tylaluminum dichloride, and the like.
Represen-tative bu-t not res-trictive of the alipha-tic alcohols and substi-tu-ted aliphatic alcohols (C) of -the present invention are methanol, ethanol, propanol~ iso-propanol, butanol, isobutanol, hexanol, cyclohexanol, 2-methoxyethanol, 2-e-thoxyethanol, 2-phenoxyethanol, 2-phenyle-thanol, and -the like.
Represen-tative of -the polyhalophenols (D) are 2,6-dichlorophenol~ 2,3,6-trichlorophenol, 2,4,6--trichloro-phenol, 2,3,4,6--tetrachlorophenol, pentachlorophenol, 2,6-dibromophenol, pentabromophenol 3 2-chloro-6-bromophenol, and the like.
It is preferred to employ -the aliphatic alcohol (C) in an amount equal to about 0.5 to about 2.0 moles per mole of the tungs-ten compound (A), and -to employ the poly-halophenol (D) in an amount equal to about 1.0 to abou-t 3.0 moles per mole of -the tungsten component (A). The pre-ferred amount of the organoaluminum compound ~B) will depend upon the reaction condition a but i-t ls generally preferred to employ the (B) compound in an amount equal to about 0.5 -to about 5.0 moles per mole of -the tungs-ten compound (A).
A catalytically effective amount of the tungsten component (A) must be employed. This amount will vary with ~39L~

the reaction conditions, purl-ty of -the monorners, e-tc., but generally, an amolmt equal to about 0002 -to abou-t 0.5~ parts b~ weigh-t o:E -the (A) componen-t per L00 par-ts of combined monomers is sa-tisfac-tory.
The catalys-t components of -the presen-t inven-tion may be employed in a variety of procedures k~own in -the ar-t.
Thus, in the con-tinuous polymerization process of the present invention, each of -the ca-ta~Lyst components or solutions thereof may be in-troduced in a continuous ~ashion separately in-to -the polymerization reactor. Alter-natively, -the (A) component may be combined wi-th ei-ther the (C) or -the (D) component, or wi-th both the (C) and -the (D) components prior to introducing the (A) componen-t into -~ -the reactor. However, it is preferred not to con-tact -the r~ 15 organoaluminum (B) component with any of the other ca-talyst components prior to introducing -this (B) component into -the reactor. In the handling and transfer of the various ;~ catalyst components, it is often convenient -to utilize solutions of these components in suitable inert solvents, such as benzene, toluene, chlorobenzene ? hexane, cyclo-hexane, cyclopentane and the like.
It has been found that when catalys-t componen-t (A) is pre~reacted with either componen-t (C) or (D), it is advantageous to remove some of the hydrogen chloride which is formed as a byproduct of this step. Known techniques may be used to remove this hydrogen chloride, these include the use o~ a stream of an inert gas, such as nitrogen, which can be bubbled -through the catalyst solu-tion, or the use of a vacuum to withdraw hydrogen chloride vapors.

'`
~8-~3~

In order -to prepare soluble, rubbery copolymers of cyclopen-tene and dicyclopentadiene con-taining from 10 -to 60 percen-t by weight of dicyclopentadiene, it has been found to be essen-tial -to employ a continuous polymerization procedure rather -than a batch process. According to -this procedllre, solutions of -the various monomers and catalys-t components are continuously introduced in a controlled fashion into a reactor con-taining an efficien-t agitator, while concurrently a con-tinuous discharge of -the poly-merizing mass is ~ain-tained. A means of maintaining con-stan-t -tempera-ture in the reac-tor is desired, such as a cooling coil or an ex-ternal jacket; and air and mois-ture must be excluded.
The solvent or mixture of sol~ents chosen for the process of this invention may vary. However, -the solvent should not adversely affect the action of the catalyst or ;~:
the solubility of the products. Representative e~amples of useful solvents are benzene, toluene, xylene, chloro-benzene, cyclopentane, methylcyclopen-tane, cyclohexane 3 methylcyclohexane, cycloheptane~ pentane, hexane, heptane, octane, and the like.
The concentrations of the monomers in -the mixture o~ monomers and solvent which is continuously fed to the reactor may vary. It is preferred -to use a cyclopentene concentration of from about 10 to about 50 percent by weight of the total weight of solvent and combined monomers. The concen-tration of the dicyclopentadiene may also vary, and it is preferred to use an amount which is less than the anount of cyclopentene. Satisfac-tory results are obtained _g_ .

~3~

when the amoun-t o F the dicyclopentadiene ~aries over the range of about 1 percent -to a~ou-t 20 percent o~ the com-bined weigh-t of monomers and solvent.
The manner in whlch -the con-tinuous polymerization is initiated is of some in-terest. It is preferred to avoid a relativel~ high concen-tra-tion of dicyclopen-tadiene in the reactor when -the copolymerization is ~irst ini-tia-ted, in orde-r to minimize the tendency -to ~orm insoluble products at this stage. A variety of procedures may be emp]oyed, bu-t typically a solution containing from abou-t 15 to about 30% by weight of cyclopentene in the desired solvent, and containing less than 3% by weigh-t of dicyclopentadiene, is preferred for the ini-tia-tion of -the polymerization. The polymerization of this initial charge in the reactor is begun by the introduction of a suf:Eicient amount of catalyst to promote substan-tial polymerization of the initial charge.
The amount is not critical, bu-t an amo~m-t suf~icient to produce an initial rate of polymeriza-tion of cyclopentene of at least about 10% within the first 30 minutes of -the reaction is preferred. During this period or shortly thereafter 7 -the simul-taneous introduction of -the various ~onomer and catalyst solutions should be initiated to com-mence the continuous phase of the polymerization process.
The rate o~ ~eed o~ ingredien-ts should be such as to main-tain an average residence time of from about 15 minute~ to about 6 hours. Polymers of -the in-tended composition gener-ally will not be achie~ed until af-ter about 3 -to 6 residence times for the con-tinuous phase of the reac-tion ha~e passed since the monomer composition in the combined feed to the ;

10~

reac-tor is generally substan-tially dlf-Eerent ~`rom -tha-t used in -the ini-tia-ting charge in -the reacto:r.
I-t may be desirable -to introduce an acyclic olefin or mix-ture of olefins into -the reac-tor during the initia-tion phase of the polymeriza-tion, and also during the continuous phase of -the polymeriza-tion as an aid in controlling polymer molecular weight. The use of olefins for molecular weight control in cycloolefin metathesis polymeriza-tions is kno~n in the artj as demonstrated in U. S. Patent No. 3,754,0460 The olefin or olefins may be present adventitiously in the monomers, or may be added intentionally. Suitable olefins include e-thy]ene, linear a~olefins, such as l-pen-tene, and linear internal olefins such as 2-pen-tene. If branching ; of the carbon chain is present in the olefins, it should not occur at the olefinic carbons. The olefin or mixture of olefins may be introduced into the reactor with the monomers as part of the solution of monomers and solvent, or it may also be introduced separateIy so as to provlde additional freedom in adjusting the polymer molecular weight durlng -the polymerization process. The amount of olefins normally desired in the present process ranges ~rom about 0.01 to about 0.5 percen-t by welght of the to-tal - amount of cycloole~in monomers and is preferably in the range of about 0.02 -to 0.2% by weight~
Moderate amounts of the conjugated dienes such as isoprene, butadiene, piperylene and cyclopentadiene 9 often ~ present as impurities in the reaction mix-ture, can be ; tolerated by -the present polymerization cata]yst system, ~ although conjuga-ted dienes are known -to be acti~e poly-li ~3~k5~

merization inhibitors wi-th other catalys-t systems such as those described in U. S. Patent 4,002,81~. For example, a diene con-ten-t of from 0 -to about 3~G by weight in the dicyclopentadiene can readily be tolerated wi-th -the catalys-t sys-tem of the presen-t invention. This is advantageous since typical commercial grades of dicyclopentadiene frequently contain small amounts of dienes as i~purities.
With the present catalys-t system 9 these impurities need not be removed.
The polymerization process of the present inven-tion may be carried out over a range of tempera-tures ~rom -25C. to ~100C. 9 bu-t -the preferred temperatures range from about 0C. to 75C.
The polymer solu-tion which is discharged con-tinuously from the reactor may be processed in a variety of ways in order to recover the product. It is usually preferred to combine a catalyst deactivating agent, such as an alcohol, water, or other reactive material, with the polymer solution shor-tly after i-t is discharged from the reactor. A stabilizing agent of the type known as an anti-oxidant may also 'be added to -the polymer solution at this time. The resulting mix-ture may be processed to remove volatile solven-ts and unreacted monomers in a varlety of familiar ways, including air drying, vacuum oven drying and treatment with a combination of steam and hot water, followed by further drying to eliminate waterO
The prac-tice of this invention is further illus-trated by the following examples, which are intended to be represen-tative ra-ther than restrictive of the scope of this ~12-inven-tion. All polymerizat.ions and hand:Ling of ca-talys-t solution were conducted in an atmosphere of dry ni-trcgen.
EXAMPLES
~y~e_______n_Apparatus - The one gaIlon glass-lined polymerization reac-tor which was used in the fol-`~ lowing examples was fi-tted with an internal cooling coil ;~ and an efficient agitator. The head of the reactor was fitted with inlet tubes for -the individual introduction of (1) the mixture o~ monomers and solvent, (2) -the tungs-ten ca-talyst solution, and ~3) the organoalumi~um ca-talyst solution. These inlet tubes introduced the reagents below the surface of the polymerizing solution. A dis-charge tube fitted with an adjustable valve was used to control -the ra-te of discharge of polymer solution f'rom the reactor. The head of the reactor was also fitted with connections for providing an atmosphere o~ dry nitrogen, and for relief of pressure as needed. A thermometer in a thermowell was employed ~or temperature measurement.
Three holding tanks were employed ~or storing (1) the mixture of monomers and solvent, (~) the tungsten catalys-t solution, and (3) the organoaluminum catalyst solution. These ta~ks were connec-ted to individual, adjustable metering pumps for the precise control o~ ~low of these solutions to the polymeriza-tion reactor. An atmosphere of dry nitrogen was maintained in each holding tank.
~ me desired mixture of dicyclopentadiene, cyclopentene and solvent, designated as the "feed premixl', was puri~ied by being ~L~3~5~i~

passed through a drying bed consisting of a mixture of anhydrous al~ina and anhydrous silica gel. This solution was then charged direc-tly in-to -the cleang dry, ni-trogen-~illed holding -tank from which i-t was to be metered into the reac-tor.
The tungsten ca-talys-t solution was prepared by dissolving sufficient tungsten hexachloride in toluene and dry cyclohexane to give a 0.018 molar -tungstan hexachloride solution. To this was added ethanol ~molar ratio of ethanol/W~16 = 1) and pentachlorophenol (molar ratio of`
pentachlorophenol/WC16 = 2). After the e-thanol and penta-chlorophenol had reac-ted, a s-tream of ni-trogen was bubbled ;~ through the solu-tion briefly to expel some of the hydrogen chloride which had formed. This solution was then diluted with ~ volumes o~ a dried aliphatic solvent (either hexane ~ or cyclohexane) per volume of toluene, giving a 0.0045 ;~ molar solution of the -tungs-ten component This diluted solution was -then transferred to the designated holding tank from which it was to be metered into -the reac-tor~
A 0.014 molar solu-tion of -the desired organo-aluminum compound in dry hexane was prepared and -transferred into its designated holding tank.
A solution o~ cyclopentene in the desired sol-vent, designated as the "reactor premixl' was prepared and passed through a drying bed consis-ting of a mixture of anhydrous alumina and anhydrous silica gel. This solution was then charged directly into the clean, nitrogen-filled reactor, brought -to -the desired reaction tempera-ture, and a sufficient amount of catalyst was in-troduced into the ' .

~3~

; reac-tor -to inltia-te rapid polymeriza-tion of the cyclopen-tene.
The amount of ca-talys-t required was generally equivalent to a molar ra-tio o~ -tungsten compound/cyclopentene of abou-t 1/4000 .
The continuous phase o~ the polymeriza-tion reac-tion was commenced (i.e., the con-tinuous feeding of catalysts and monomers was inltiated~ wi-thin 30 minutes following the first charging o:E catalys-ts to -the "reac-tor premix" to initia-te polymeriza-tion. A rela-tively high rate of feed of catalys-t componen-ts of about -twice the ultimate desired rate was main-tained during -the first 1-2 hours of the con-tinuous reaction. Thereaf-ter the cataIyst feed rate was reduced to -the desired rate and a s-teady state condition of the continuous reaction phase was achieved after a total of about six hours of reactor operation.
The emerging polymer solutions were terminated continuously wi-th a mixture of alcohol and suf~icient 2,6-ditertiary-butyl-para-cresol (an antioæidant) to provide about 1 part of an-tioxidant per 100 parts of polymer. Af-ter drying~ all polymers were shown to be virtually completely soluble in toluene and were obviously ` elastomeric.
;- EXAMPLES 1-5 Experimental conditions and results are given in ; 25 Table I. The glass transition -temperatures (Tg) reported in Table I were determined by differential scanning calorimetry. Inherent viscosities of the polymers were measured in toluene at 30C. The dicyclopentadiene contents reported in Table-I were determined by inspec-tion of high resolu-tion lH or 13C nuclear magnetic resonance spectra.

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S~ , While certain represen-tative embodimen-ts and details have been shown for the purpose of illustrating the inven-tion, i-t will be apparent -to those skilled in this art that various changes and modifications ma~ be made -therein wi-thout departing from the spirit or scope of the inven-tion.

Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A process for the preparation of substan-tially gel-free rubbery copolymers of cyclopentene and dicyclopentadiene comprising polymerizing mixtures of cyclopentene and dicyclopentadiene containing from 10 to 60 percent by weight of dicyclopentadiene in an aliphatic, cycloaliphatic or aromatic solvent, and wherein the poly-merization is conducted in a single stage continuous poly-merization reactor in the presence of a catalyst system comprising (A) a soluble tungsten halide or oxyhalide, (B) at least one compound selected from the group consisting of trialkylaluminum compounds, dialkylaluminum halides, alkylaluminum sesquihalides and alkylaluminum dichlorides, (C) at least one hydroxy compound selected from the group consisting of aliphatic alcohols and aliphatic alcohols substituted with alkoxy or aryloxy groups, and (D) a poly-halogenated phenol of the general formula:

(D) where X is chlorine or bromine and M, N and P are selected from the group consisting of H, Cl and Br, and wherein the molar ratio of A:B:C:D lies within the range of 1:0.5-5:
0.5-2:1-3.

2. A process according to claim 1 wherein the temperature lies within the range of 0-75°C. and wherein the (A) catalyst component is WCl6, and wherein the solvent is selected from the group consisting of pentane, hexane, heptane, cyclopentane and cyclohexane.

3. A process according to claim 2 wherein the (C) catalyst component is selected from the group consisting of methanol, ethanol, propanol, 2-methoxyethanol, 2-ethoxy-ethanol and 2-phenoxyethanol, and wherein the (D) catalyst component is selected from the group consisting of penta-chlorophenol, 2,4,6-trichlorophenol and 2,6-dichlorophenol.