CA1123413A - Bonded organo-pellicular packings for chromatographic columns - Google Patents

Abstract

ABSTRACT

Highly effective chromatographic column packings are prepared by reacting hydroxyl groups on a silica surface with SiC14 and then reacting the chloros?lylaced surface with a polyglycol or polymeric glycol ester in 3 slurry reaction.
Residual chlorosilane groups on the reacted surface are neutralized by reaction with methanol or other lower alkanol.
The resulting modified silica has a bonded, essentially monomolecular organic surface film which provides thermal stability, uniform efficiency, and rapid analysis when the material is used as a column packing in gas-liquid chromatography.

Description

BOND~D O~G~NO-PhLI!ICULPR PACKINGS
FOR C~ROMATOGF~PHIC COL~S

'llhis invention relates to impro~ed silica packin~s for use in chroma-to~raphic analysis and to a process f or makina them. It relates particularly to $inel~ divi.ded silica suppor ~s havillg a very thln pol-ylneric o~-~anic ~ chPmicaily 'oon~e~ to theiL
surf ac:e .

The ~roblem of optimizing chromatGgraphic perlormance is one that has ~ersisted throu~hout the history of chro natographic separations. A1-though si~lificant advances have beell ~.ade in the apP11-cation of g~s-liguid chromatog--aphy to analytical lr~ problems, most of the~.e have r~sulte~1 from improve-ments in apparatus rather tharl in th.~ column packlng itself.

In order t.o avoi~ or minimize problems caused b~ active sites Oll tke surface 0$ a silica chroniato~raphic colum~ packing, ccat~d packings have b2en prepar~d ~ eatir.g an acti~at~d sllica ' , ? 5,5'36~

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~-2--with an ~pproprlc-te alcohol under conditions which allow continuous removal of water, thereby causing ethel-ification of the alcohol with silanol groups on the si ica surface, see ~alasz et al., J. Chrom.
Sci., 12, 161 (197~ owe~rer, the coating obtained is often no~ miform and the method gives poor reproduci~ ty. Stehl, U.S. 3,654,967 describes a method wh2reby a silica or alumina gel is reacted with an organohalosilane, the halosilane groups thus at-tached io the surface are reac-ted with. an alcohol arid the produc-t is halogenated to provide a haloorganic coa~ing ~onded to the support sur-face. Ho~ever, the im~roved chromatographic packings thus obtained are still not entirely satisfactory.

1~ lt has now been found that novel chron~ato-grap~ic packings of uniformly high quality are ~ro~
duced ~y a process which cornprises ~1) contacting an activated silica sup~ort havins a sign1ficant propor-tion of hydroxyl ~roups bond~d to silicon atoms at

2~ the ~upport surface with silicon tetrachloride to react essentially all of said h~rdroxyl g-oups, ihereby - pLO~ucing a chlorosilylated surface, (2~ reactin~
by coiltacting the chlorosilylated surface ~ith an inert solvent soluiion of a pol~ro' hav~Ilg an avera~
molecular weight o fror~l 3,000 -to 100,000 at a tempera ture of from 100 to 250C, (~ ccntaztillg the polyol--chlorosilylated surface reaction product ~ith a lower alkanol in sufIicient quantity to neutralize residual chlorosilyi groups, and (4) separating

3~ t~.e neu-traLized product from the reaction mixture as an esselltially ~ule and dry solid. The reaciion prod~c-~ has 2 ~mifol~, chemicall~ ~onded crganic surface c~atiily ~.r~lich is essenti~-~11y monomolecular ~5,596-F

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in -thickness. This bonded coating provides sub-s~antially complete sulface coverage and reduces ~ surface activity ~o a minimum.

; The chromatog~aphic packings of this inven-tion offer the advantages o:E high thermal stability, increased selectivity, controll~le functionality, reduced analysis time, low reactivity to sample components~ and sharp separation of solute species at lo~er ten~peratures than those required for elutior on conventional coated packings.

These advantages are obtained by followin~
the above-described process steps and -they are maxim ~ed by following those steps in their pre-ferred modGs of operation. For example, the sur~
face of a silica support is preferably specially ac'ci~ated to provide a laryer number of hydroxyl groups bonde~ to the su;-face silicon atoms by treat-ing a cle2n~d silica -~ith vaporized conce~tr~-ted hydrochloric acid at 100 to 300C for 0.5 to S
hours. The vaporized aqueous ~Cl is most conveniently - applied as a mixture with all inert gas, or example, nitro~en, argon or helium.

The hydroxy (or silanol) groups on the silica surfa~e are then reacted with silicon tetra-~5 cnloride in either a slurry r~action with the liquidxeagen~ or, preferably, by a gas-solid reaction in which SiCl4 vapors are contacted with a bed of the silica particles. In either case, the Si~14 reaction is Garried Olit at a temperatur~ Irom 50C to 300C, preferably from 53~ to ~50~ for -he gas phase reaction, at a somewhat lower tem~erature for the 25,59r~F
, ~3~23~3 reaction with li ~lid silicon tetrachloride in order to avo:id excessi~e reactor pressure. This chlo1-o silylation reaction is preferably ~arried to the extent of 0.002 to 0.01 ~ram atoms of silicon-bound chlorine per s~uare meter of silica surface.

The reaction of the chlorosilylate~ produ~t wi-th the polyol or polyester pol~ol is carrie~ out at a temper~ture from 100C to 25~C by contacting the li~uid polyol reactant with the solid chloro-silylated si'ica in a slurry reaction, pref'erablyin the presence of an inert solvent for the polyol.
Suitable solvents are those boiling at or above lOO~C and inert to bot,h reactants under the reaction conditiolls. ~romatic hydroc~rbons su~h as xylene, diethyl~enæene, and durene are examples.

Lo~7er alkylene polyglycols of at least 3,0C0 average molecular weight arc preferred poliol reactants. These include polyethylene glycol, poly-propylene glycol, polybutyle~e glycol, ~lock co-polymers of tWG Ol' more Or these oxyalkylene units,and physical mixtl~res of any of these. The minimum molecular weight is a measure of the mini.mum length o molecule required to give effQctive surace ~overage and conse~uent surface deactivatio~.
For poly~-thylen~ glyccl, the minimum molecular weight indicates a chain of 65-70 oxyethylene units in the average molecule. For pol,vpropylene and poly~
butylene ~lycols, molecular ~eights of about 4,000 and 5,000, respectively, correspond to mole~ules of 3i~ similar lengt'l. Polyglycols having an a~erage molec~
ular weight of about 100, noo represent a pr~ctical maximu~ mol~cular size limit.
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,.. ~ -, i : ~ -~3~..3 Polyester polyols are another class of polyol reactant. Polymers n~ade by esterifying an alkylene diol of 2-16 carbon atoms with rl dicar-boxylic acid of 3-10 carbon atoms are preferred 5 examples. All~ylene diols include ethylene glycol, propylene ~lycol, butylene glycol, diethylene glycol, triethy].ene glycol, dibutylene glycol, trimethylene glycol, 1,4~butanediol, and 1,12--dodecanediol, and mixtures of these. Alipha-tlc dicarboxylic ac.ids such as malonic acid, suc-eini- acid, and sebacic acid are prefexred although aromatic diacids such as terephth~alic acid an~ iso-ph$halic carl also be used, alone or in mixtur~
witll the acids deIined above. The polyesterifi eation reac-tion is normally earried out for con-venience witn tlle diacid chloride. Other reactive dihalides can be mixed in minor proportion ~ith the diaci~ chloride reactant in the polyesterifi eation reaction -~o vary the properties of the ~0 resulting poly.~er, for example, organic silico~
dichlorides, or disulfonyl dichlorides. A minimum molecular wei~ht of about 3,000 is also appro-priate for po:Lyester polyol reaetants. Preferabl~, the polyester polyol is prepared 1 _ itu, in the presence of the chlorosilyla~ed silica so ~hat the polyesterification reaction and the reaetion o~ the polyol molecules with tke chlorosilyl grou~s take place more or less simultaneously.

When the reaction o the polyol or poly-ester polyol reactant -~litll the chlorosilylated si~iea has essf.-nti~lly ceased, a small proportion of unreacted chlorGsil~l srowps renlaiils o~ the sili.ca surface. In crcler to eliminate these highly unde-sirable active ~roups they are ne~tLalized by ~, ~5, 596~F
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~, adding a lo~er al~ nol such as methanol, ethanol, or isopropvl alcoi~ol to the reaction mixture and heating a5 before. Preferably, an intermediate neutrali~ativn with a lower molecular weight and con~e~len-tly more reactive polyol is carried out, most pref~rably with a series of such polyols of progressiv~l-y decreasing molecuLar weight. In this way, the s.ilica surface is blan}ceted to the gr~.atest extent po.ssible with bonded molecules of ma~imum leng~h. For example~ chlorosilylated silica Cail be reacted with polyethylene glycol of about 20,000 molecular weight and remaining si.licon~bound chlorine atoms then neutraliæ~d by successive re-actions with polyethylene glycols of a~out 5,000 and abGut l,Q00 molecular weight, then with tri-ethylene gl-rcol, and inally with methanol to ensure the neutralization of all possible residual chlorosil~l groups.

~3~L~ 9~5~ Surface 2C About 100 g portions of 80-100 mesh Chromo-sorb ~Y, a flux-calcined celite diatomaceous silica specially processed for chromatographic use by Jo~s-M~nville Co~p., were e.~tracted for ~4~72 hours in a Soxhle~ extraction apparatus with constant 25 boiling hydrochloric acid. The extracted silica l;
was pui in a washing column and washed for i~-24 hours with deionized water at room temperature using a fluid bed back~flushing procedure to remove acid and fines. The washed silica was rinsed thoroughly with metr~an31 and then dried by passing filt2red air ~rougn th~ colunln or about two hours. The dried sili.ca was stcred in closed glass bottles uniil sub~ec';ed to su^f;lce reac~ion.

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_ --7--r.~he si].i.ca surface was acti.vated by packing about 30 g of the aci.d-extracted sili~z in a g~ass reactor tllbe hected ~y a clamshell electric furnace and passing about ~5 ml/min nitrogen t:llrou~h the bed while its temperature was raised .n 40C steps to 200GC' over a pexiod of about 40 minutes, t~len the .incoming nitrogen was switched throug~l a conc. HCl bubb].er so that the nitrogen ~.assin~ through the bed was essential`ly saturated with HCl an~ water ~apor. The H~l-satura~ed nitrogen stream was _ontinued at ~OO~C ar. tne s,~ne xate .~r three ho-urs, -then the bubbler WdS bypassed and the bed was flushed witn pur~ nitroger. for one hour, also at 200C.

Reaction with SiCl A~ this point, a bubbler charged with SiCl4 ~as connec-ted into the nitrogen supply line aDcl the silica bed was contacted with SiCl4 vapor in nitrvgen lor 90 mlnutes, the tempe.rature and nitrogen flow rate remainin~ constant at t~e prior lev~e:Ls. 'rhe bed Q~
~0 chlorosilylated silica ~12S th?n flushed with nitrogen 2S before for 15 minutes and allowed to cool to room temperature ~fter removal of the furnace with continuPd 'low of nitrogen.
~:
The followirlg procedure was employe~ wi~h ~i 2~ modifications as noted fGr th~ reaction of the chloro--silylated silica with a polygiycol. A somewhat lnodified procedure was used for the corresponding reaction o~ a polyester polyol ~s described in those exan-.p].e~.

Re~ction Proced.ure ~ glass reac-~or flask ~ ipped with reflux sondenser ald nitr~en ~ .et wa., c~iargefl .~it:h about 25,596 E

;, i 10 g of polyglycol reactant and 300 ml of o-xylene and the contents refluxed for about an hour with dry nitrogen flush to remove small amounts of water, then the contents were cooled to 110C and about 30 g of the chlorosilylated silica were added under nitrogen. Nitrogen flow through the flask was reestablished and the reaction mixture was heated at reflux temperature for 2-24 hours. The reaction was then quenched by successive addition of poly-ethylene glycols as described in the examples while maintaining reflux temperature. The reactor was then cooled to 100C, the heat source was removed, and 50 ml of anhydrous methanol were added slowly to neutralize any residual chlorosilane groups.
After the addition was completed and the reaction mixture had cooled to 55C to 60C, the liquid in the reactor flask, consisting essentially of xylene and unreacted polyglycols, was decanted and the reacted silica was washed by decantation with three portions of methanol followed by three portions of chloroform. The washed silica was then carefully transferred to a washing column where it was throughly washed by gravity flow with successive 300 ml portions of methanol, chloroform, and methy- ~
lene chloride. The washed silica was then dried ~`
by drawing filtered air through the column for about an hour. The finished bonded silica packing was stored in sealed glass bottles until used. The product was a free-flowing fine white powder.

Example 1 Chlorosilylated 100-200 mesh Chromosorb~ W
(a series of screened calcined and flux calcined diatomite aggregates) was reacted with polyethylene Trademark 25,596-F

., . ' g glycol of 2~,0~ average molecular weight ~E 20,000) by the procedure descri~ed above. T~e reaction was ~uenched bv adding about 2 g of melt,ed polyethylene ylycol OI 6,0~0 average molecular weight ~E~,000), refluxing the reaction rllixture for about 2C ~linutes and repe~ Li ng this proceclure with successi.ve 2 g portions of polyethylene glycols E ~,OOo, E-1,000, and E-400 and, finally, diethylene glycol.
The reacted silica was then trea-ted with methanol and washed and dried by the previously described procedure.

Fvr purposes of comp~rison, "bonded" or coated siliGa packings were prepared by coating 100-120 n~esh Chromosorb~ W-HP Witil polyetnylene glycol of 20,000 molecular wei~ht in quanti.ties sufficient to produce loadings of 5 percent and 3 percent by weight using the con~entiona' slurr~
method (described by Halasz et al., J. Chrom. Sci., 12, 161 (1~74). These packings were preconditioned for 12 hours at 220C and 60 ml/min helillm flow.

These packings were compared in ~ G . 1 ~m x 160 cm co]umn maintained at 75C and-using 300 ~g/ml n-dodecane i.n me~hy~.ene chlori.~ as t~e test solute and helium as the carrier gas. Column efficiency for each packing at optimum carrier flow was calculated from the plot o~ test results .~, and is li.sted in Table 1.
.

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- : ' .' ~ ~ :, PY~13 Optimum Column Plate Height, Carrier Flow Packin~ mm/Theoretical Plateml/min _ 5 bonded 0.34 5s coated, 5% 0.67 27 coated, 3% 0.62 32 It is apparent that the bonded packing provided substantially greater ~fficiency in terms of plat~
height and also offers faster analysis ti~.es sin~e the o~timwn carrier flow was about twice that fo~
the coate~ packings prepared by a ~reviously known method.

-Example 2 -A bonded packing was prepaxed as .in Example 1 using 80 lG0 r.~esh Chromosorb~ W and about 8 ~ of polyethylene glycol of 4,000 average molecular weight (E-4,000). ~olyglycols used in the quench cycle of the process were polyethylene glycols of 1,000 and 600 molecular weight, tetraethylene glycol and dieth-ylene glycol, respecti~ely. This pc.cking was conl-pared with a conventionally prepaxed polyester-coated silica packing by the method of Example 1 in the analysis of impurities in 1~2-dibromo-3-chloroplopane, a commercial soil fumigant. Both packings showe~
the presence of allyl chloride and 1,2,3-tribromo-propane in the product but the bonded packing Or this inv~ntion also showed the presence o 1,2~5,6--tetrabromoheYane w~lich was not previously observed 30 USillg the col~vention^~l packing. Ad~itionally, us~
of thc bonded packing cut the analysis time in h~lf.

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~2 Exa~ple 3 Bonded packings were also prepared ky the method of Examples 1 and 2 using polyethylene glycols with average molecular weights o a~out 6,000 and abou-t 1,50G. Chromatographic testing showed excellerlt results for the filst of these, eomparable with results o~tained with the prodllcts of Examples 1 and 2.
However, t~e bonded pac~ing made with E-1,500 showed severe peak tailing, charaeter:istie of high surfaee activity. Evidently, for packings having a bonded polyethvlene glycol layer, the minimum average molecular size that provides adequa-tP surface cover-age is in the molecular weight range of about ~,000, eorresponding to polymers having about 65 to 70 oxyethylene units in the polyglycol moleeule.

This eonelusion was supported by ~he properties o~ a bonded polyglyeol packing descri~ed in Example 4 where the polyglycol reaetant was a pol~ropylene glycol o 4,000 average molecular weight, corresponding to about 65 to 70 oxypropylene ~mits per molecule. The bonded packing was evidently at about the lower moleeular size limit for -the bonded molecules covering the silica surface, for it showed some peak tailing although suecessful chromatographie separations were obtained.

Example 4 A bor.ded ehromatographie eolumn paeking was prepared with polypropylene glyeol of 4000 average molecular weight (P-4000) and subjeeted to the same eva'uation as descri~ed in Example 1.
Wherl examinecl using n-tetradec~lle at 100C as in Example 1, theoretical plate he:ights of 0.88 mm 25,596-F

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were observed -For the P-4000 product as compared to 0.45 ~-n for the bonded E-4000 packing. These hydrocarbons are eluted faster at the same tempe.ra-tures from the bonded polypropylene glycol columns.
~s a result, the number of components that ca~ be separa-ted in a given leng-th o time is virtually identical on both bonded columns and both are signi-ficantly better than a conventionally coated column packing as noted in Table I.

1~ ln addition to the improved efficiency, the lonyer molec~les represented by the polypropylene glycols afford this high efficiency at lower flow rates than the corresponding bonded pol,yethylene glycols, allowi~lg observations and detection of more volatile components.

Example 5 The proc~dure described above for the prepa-ration of the bol-lded polyglycol packings wa modified to make a corresponding silica having surface-bonded diethylene glycol succinate polymex. Equimolar quan~
- tities OI diethylene glycoi and succinyl chloride (0.0472 g mole each) in o-xylene solutio~ were added from separate dropping funnels to a flask reactor containing about 30 g of chlorosilylated Chromosorh~
W-AW in refluxing o-,~ylene under a nitrogen atmo-sphere. The resulting reaction mixture was quenched by adding about 3 g of diethylene glycol and refluxing an additisnal half hour. The mixture was the~ cooied to about 100C and 50 ml of methanol were add?d drop~
wise with ~radual cooling to abvut 55C. Liqui.d was decanted of~ and the c~oated silica was washed and dried as before.

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2~
-~3-The p~lye~ter-coated silica w~s packed into a colun~n similar to that of E~ample 1 and the column was used to separate a mixture of close:Ly related phenols and cresols (~00 ~g in ether). The com~onent~ of the ~ixture ~phenol, o-cresol, o--chlorophenol, p-chlorophenol, 6-chloro-o-cresol,

4-chloro-o-cresol, 2,4-dichlorophenol, and 4,6~
-dichloro-o-cresol) were all separated efficiently and sharply.

- lG Example 6 The proce~ure of Example 5 was followed in the reaction of chlorosilylated Chromosorb~ W-AW
with 1,12-dodecanediol sebacate polymer, the polymer being formed ln situ by reaction of the diol and the acid chloride as before. The reaction mixture ~as c~enched by the addition of 2 g of molten dodecanediol with a one-}lour reflux followed l~
addition OI 2 g of diethylene glycol and another 20 minutes of reflux.

The bonded packing thereby produced was found -to be particularly useful for analyzing mix ;' tures of nonpolar compounds. For example, it was highly effective for the isothermal chromatographic analysis of thr~e alkanes (C14, C15, and C16) in hexane. It also provided efficient separation of polychlorinat~d ~ibenzo-p-dioxin isomers.

Example 7 A bcnded silica packing was made by the pxocedure d~scribe~ in Exam~le 5 except th~t the succinyl chloride ~as replaced by a mixture of 6.6 g (0.042~ mol~) succillyl chloride and 1.2 g (0.0048 mole) y-c~anopl^~p~l phenol dichlorosilane. The 25,595-~

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¢~3 bonded coating thereby obtained was polymeric di-ethylene glycol succinate wherein a ten-th of the succ:inyl groups were replaced by y-cyanopropyl pheneyl silyl moieties. The r~action product was quenched by reacting with diethylene glycol and then with methanol as in Example 5.

The highly polar nature of the bonded silica packing thereby obtained permitted efficient gas chroma-tographic separation of brominated penta-erythritols, the dibromo and t:ribromo compolmdsboth eluting in sharp peaks wi-th minimal tai.ling.
This pac~ing also proYidec improved separating po~er and considerably reduced analysis time as eompared to a conventional coated silicone packing in the separation of components present in crude pentabromochlorocyclohexane.

In all of the bonded polyester packing products described in Examples 5-7 and in other sueh bonded polyester packings prepared similarly from other diol and dibasic acid reactants, t~e polyester moieties had a eomparatively broad dis-tribution of molecular weights .in the approximate rangP of 1,000 to 20,000 based on examination of the nonbonded polyester byproduet of the reaction.

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Claims (14)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A process for making a chromatographic column packing which comprises:
(1) contacting a silica surface having Si-OH groups with SiCl4 to react essentially all of said hydroxyl groups, thereby producing a chlorosilylated surface, (2) reacting by contacting the chloro-silylated surface with an inert solvent solution of a polyol having an average molecular weight of from 3,000 to 100,000 at a temperature of 100°C to 250°C, (3) contacting the chlorosilylated surface--polyol reaction product at 50°C to 150°C with a lower alkanol sufficient to neutralize residual.
chlorosilyl groups, and (4) separating the neutralized product from the reaction mixture as an essentially pure and dry solid.

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2. The process of Claim 1 wherein gaseous SiCl4 is contacted with the silica surface at a temperature of from 50° to 300°C.

3. The process of Claim 2 wherein the chlorosilylated surface polyol reaction product is progressively reacted with at least one polyol of lower molecular weight and finally with methanol.

4. The process of Claim 2 wherein the polyol is selected from polyethylene glycol or polypropylene glycol.

5. The process of Claim 2 wherein the polyol is the polyester of an alkylene diol of 2-16 carbon atoms and a dicarboxylic acid of 3-10 carbon atoms.

6. The process of Claim 5 wherein the polyester is that obtained by reacting the alkylene diol with a mixture of the dichloride of the dicar-boxylic acid and an organic silicon dichloride.

7. A chromatographic column packing com-prising the product of the process of Claim 1.

8. The packing of Claim 7 prepared by reacting the silica surface with gaseous SiCl4 and reacting the chlorosilylated surface-polyol reaction product successively with at least one polyol of lower molecular weight and thereafter with methanol.

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9. A process of making a chromatographic separation of components contained in a sample which comprises:
(1) mixing the vaporized sample with a carrier gas, and (2) contacting the vaporized sample-carrier gas mixture with a packing permeable by said mixture and consisting essentially of a silica support having a chemically bonded organic surface coating, said packing being prepared by (a) contacting a silica surface having Si-OH groups with SiCl4 whereby essentially all of said hydroxyl groups are reacted, thereby producing a chlorosilylated surface, (b) reacting by contacting the chloro-silylated surface with an inert solvent solution of a polyol having an average molecular weight of from 3,000 to 100,000 at a temperature of from 100° to 250°C, (c) contacting the chlorosilylated sur-face-polyol reaction product at a tempera-ture of from 50° to 150°C with a lower alkanol sufficient to neutralize residual chlorosilyl groups, and separating the neutralized product from the reaction mixture as an essentially pure and dry solid.

10. The process of Claim 9 wherein the silica surface of the packing is chlorosilylated by con-tacting with gaseous SiCl4 at a temperature of from 50° to 300°C, and wherein the chlorosilylated sur-face-polyol reaction product is progressively reacted with at least one polyol of lower molecular weight and finally with methanol.

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11. The process of Claim 10, wherein the polyol is selected from polyethylene glycol or poly-propylene glycol.

12. The process of Claim 9, wherein the polyol is the polyester of an alkylene diol of 2-16 carbon atoms and a dicarboxylic acid of 3-10 carbon atoms.

13. The process of Claim 12 wherein the poly-ester is that obtained by reacting the alkylene diol with a mixture of the dichloride of the dicarboxylic acid and an organic silicon dichloride.

14. A chromatographic column containing a packing wherein said packing is prepared by a process according to Claim 1.

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