A one-pot synthesis of comb polymers and hydrogels by the ring-opening metathesis polymerization reaction

Macrornol. Rapid Commun. 15,543 -550 (1994) 543

A one-pot syatthesis .of comb polymers and hydrogels by €he ring-opening metathesis polymerization reaction

Brian Bell, James G. Hamilton*, Edwina E. Law, John .i Rooney

School of Chemistry, The Queen's University of Belfast, Belfast, BT9 5AG. (U. K.)

(Received: March 7, 1994)

Introduction

Polymeric materials in which the properties of a basic main chain can be modified beneficially by the introduction of substituents as side chains, either to the monomer prior to polymerization, or, by a graftingkrosslinking reaction after polymerization are the subject of much current interest. Using these methods a wide variety of molecu- lar combs hydro gel^'-'^) and hydrophobically modified water-soluble polymers I I -I6) have been prepared and their properties studied.

The majority of these polymers are derived from polyesters or polyamides and there are few examples of such systems containing olefinic unsaturation as part of the main chain's6). In such systems variation of the double bond geometry could potentially bring about substantial changes in the conformation of the polymer chain and consequently the rheological properties of their solutions, thereby allowing a better understanding of structure-property relationships.

Ring-opening metathesis polymerization (ROMP) 17) affords a route to such materials and the methodology is developed to the point where a wide range of function- alized monomers based on the bicyclo[2.2. llheptene ring system will polymerize '*). Our recent studies in this area have produced a number of such materials using the ROMP reaction and in the light of a recent report6) which deals with the attempted preparation of similar materials we are prompted to describe our preliminary results in this area.

Experimental part

The ex0 anhydride"), 1, and the 7-oxa analog2'), 2 (Scheme I ) were prepared according to literature methods. Refluxing equimolar quantities of 1 with the respective alcohols in benzene for 4 h followed by removal of benzene under vacuum produced the half esters 4 and 6 (numbers refer, in the following, to entries in Tabs. 1 and 2). A similar procedure, using a ratio of one mole of 1 to two moles of alcohol with removal of water as the benzene azeotrope in a Deadstark apparatus produced good yields of the diester 3 but much lower yields of 5; these could be purified with respect to residual half-ester by flash chromatography on silica, eluting with low-boiling petrol ether. The analogous half-esters2') of 2 may be prepared conveniently by forming a paste from liquid alcohols and 2 and allowing this to stand for up to two weeks at 35 OC; progress is monitored using ' H NMR and the product isolated by flash chromatography on silica. Dihexadecyl ester, 7, was prepared by a Zn12-catalysed DieWAlder reaction 22) between furan and dihexadecyl maleate; the latter is obtained quantitatively by esterification of maleic anhydride with hexadecanol as described above for the preparations of 3 and 5 .

Attempted polymerization of monomers 3 to 8 with the RuC13.3 H20, OsC1, and MoC15/(CH3),Sn/(C2H5)20 catalysts (all salts used as received from Aldrich) in chlorobenzene

0 1994, Hiithig & Wepf Verlag, Basel CCC 1022-1336/94/$02.00

544 B. Bell, J. G. Hamilton, E. E. Law, J. J. Rooney

solution were carried out as described previously23). In order to minimize transesterification only very small amounts of alcohol were used with the OsC1, and RuCI, catalysts compared to that reported p rev i~us ly~~) . Thus the catalyst was allowed to stand for 5 min in the presence of = 0,05 mL CH,OH before chlorobenzene solutions of the monomer were added.

In the one-pot synthesis either RuCI, or OsC1, was dissolved, with heating, in the respective hydroxy compound (used as received from Aldrich) in glass tubes, a chlorobenzene suspension of anhydride 1 was added and the tube sealed. The sparingly soluble anhydride tends to precipitate but eventually redissolves on occasional shaking at the reaction temperature of 70 ' C . Reaction times vary, but reasonable yields of soluble polymer can be recovered by precipitation of the reaction mixture in diethyl ether after dilution with acetone following 24 h reaction; longer reaction times improve the yield and the degree of esterification but produce insoluble gels. However, samples of these materials in sheet or rod form, suitable for examination of their hydrogel properties, can be prepared by carrying out the polymerization in narrow tubes or between glass plates.

Results and discussion

We approached the preparation of the polymers by two synthetic strategies both of which make use of the readily available bicyclo[2.2.l]hept-5-ene-2,3-dicarboxylic anhydride (1) or its 7-oxa analog (2) as starting material (Scheme I). In the case of the methylene-bridged derivatives the required monomer may be prepared first by esterification of the anhydride prior to polymerization, or, alternatively, polymerization of the anhydride may be carried out using the specific alcohol required to provide the side chains, as solvent, in a one-pot reaction. The first of these two strategies has led to the preparation of a series of diester or half-ester monomers, 3 to 6 which, with the exception of 4, have been polymerized to yield a series of comb-like polymers. The 'H- and I3C NMR spectra of the prototypal polymer formed from 3 using the OsCI, catalyst are shown in Figs. l(a) and (b), respectively. In the 'H NMR spectrum resonances from each of the groups of protons are well resolved with the exception of one of the cyclopentane ring methylene protons which is obscured by the strong signal from the alkyl chain. Integration of the well resolved signals from either the olefinic

Scheme I:

2 , x = o I

ROH I ROMP

A one-pot synthesis of comb polymers and hydrogels by the ring-opening . . . 545

or the protons at or adjacent to cis and trans double bonds allows us to estimate a &/trans ratio of 0,07 for this polymer. The relative positions of these cis and trans signals are known from the spectra of a number of analogous ROMP polymer^^^-^^). The low intensity singlet marked X (= 3,6 ppm) arises from a small proportion of methyl ester side chain formed by transesterification with the small amount of methanol present in the catalyst (see Experimental part). Fig. l(b) depicts the

polymer in CDCI, 6 5 L 3 2 1 formed from 3 (Tab. l ) , using the OsC1, catalyst; (b) I3C NMR spectrum of the same sample

6 in pprn

(b)

175170 135130 60 50 10 30 20

0

H3

L 1

10 t 6 in pprn


I3C NMR spectrum of the same sample and this, too is consistent with a high-trans diester polymer. The greater segmental motion of the dodecyl side chain relative to the rigid main chain is reflected in the line widths of the various signals, Fig. l(b)27).

The syntheses of the oxygen-bridged derivatives, 7 and 8, require a different approach because of the tendency of the anhydride, 2, to undergo a retro-DieWAlder reaction2') at the elevated temperatures required for diester formation. The dihexadecyl ester, 7, was thus formed by a Zn12-catalysed DieMAlder reaction22) between furan and the easily prepared dihexadecyl ester of maleic acid (Scheme 1). The half-ester monomers, e. g. 8, may be formed directly from the oxygen-bridged anhydride, 2. The resulting half-esters appear to be much less labile with respect to the retro-DieldAlder reaction than the anhydride precursor.

Our second strategy was to combine anhydride 1 with catalyst and a given alcohol which has the dual function of solvent and esterification agent. One can thus form polymers of a type which we have found to be difficult or impossible to form by the conventional route of polymerizing a given monomer which contained the desired functionality, even when these can be prepared. A good example of this is seen in the lactate derivative, 9, and tartrate derivative, 10, Table 2, where we obtained good yields of half-ester polymer but were unable to prepare the equivalent monomer from attempted esterification of 1. In these cases there is substantial overlap of the 'H NMR signals, and 13C NMR provides the best method of microstructure determi- nation. Fig. 2 depicts the spectrum of the lactate derivative which had been isolated after a 48 h reaction period when most of the anhydride groups had been converted to half-ester. The carbons of the lactate moiety Cs, C9, CIO and C", are centered at 69,45, 61,06, 14,59 and 17,34 ppm, respectively (Fig. 2). The most obvious reason for the splitting of the C9 signal is that it can occur in both head-to-head and head-to-tail orientations along the chain; the fine structure due to these effects is well established

9 10 O:C-O-CH,CH,

8 I 11 H-q-CH,

Fig. 2. I3C NMR spectrum of the lactate half-ester polymer 9 in acetone-d, (Tab. 2), produced using the OsC1, catalyst

175170 135130 70 60 50 Lo 30 20 10 .( 6 in ppm


in other ROMP polymers formed from unsymmetrical monomers "). CSs6 and Cts4, centered at 52,96 and 46,24 ppm, respectively, and the carbonyl signals, are quite complex and we attribute this to head-to-tail/head-to-head effects and, in the case of C's4 and C5s6, diastereomeric effects, i. e. C5(R), C8(S) or C5(S), C8(S), etc., C', being achiral and symmetrically positioned in the repeating unit, is uneffected. As expected, these splittings are absent in the spectrum of 3 (Fig. l(b)) where the repeating unit is symmetrically substituted by an achiral moiety. The tartrate derivative 10 and the menthol derivative 11 give analogous spectra which may be similarly interpreted. We hope to utilize the chiral space thus formed, both here and with the analogous lactate derivative, to effect enantiomer separations.

Generally, NMR analyses of our polymers show that those obtained by the one-pot route have a variable ester side chain content which depends on factors such as reaction time, temperature, concentration of 1 and the nature of the alcohol used, e. g. methoxy- ethanol (12) and I-propanol (13) readily form diester polymers whereas the other alcohols and diols (Tab. 2), give essentially half-esters. Although the one-pot synthesis is clearly advantageous, preparation of discrete monomers prior to polymerization, although less attractive synthetically, leads to the formation of well defined polymers which aid characterization and provide standards for comparison purposes.

Choice of catalyst is an important consideration for the polymerization of these compounds because a number of the most common examples, i.e. the halides of tungsten or rhenium are very sensitive to the presence of hetero-atoms in the monomer and are therefore unsuitable. Recent work, however, using molybdenum-based carbene complexes28, and our own experience with the classical MoCI,/(CH,),Sn/(C2Hs),0 system29* ,O), suggests that molybdenum-based catalysts are much more tolerant of hetero atoms in the monomer than tungsten or rhenium catalysts; we were therefore able to form polymers from the didodecyl ester 3, in good yield with this catalyst. Generally, though, noble-metal halides, i.e. RuCI, and OsCI,, are the catalysts of choice when hetero-atoms are present in the monomer and where a simple off-the-shelf catalyst is required. Laschewsky and Schulz-Hanke6 have recently reported the synthesis and attempted polymerization of an analogous series of monomers for use in the preparation of Langmuir-Blodgett films but their work was restricted to the RuCI, catalyst and this proved to be ineffective for a number of their monomers. Our work with this catalyst supports their findings but we have found that 3 readily forms copolymers with norbornenea) in the presence of the RuCI, catalyst, in which norbornene and 3 are incorporated to an equal extent at 20% conversion, indicating that the steric effects of the long alkyl chains are not important as has been suggested6). These observations demonstrate that the RuCI, catalyst is not inherently unreactive but is reluctant to form the initial metallacarbene required for propagation when in the presence of 3 alone. However, when a copolymerization is attempted, an initial metallacarbene is formed from norbornene and the polymerization of 3 and norbornene then propagates at roughly equal rates at the catalyst site.

The OsC1, catalyst does not suffer from the same limitations and we have used it recently to polymerize a number of other hetero-atom containing compounds29. 30). In

a) Systematic IUPAC name: 8,9,10-trinor-2-bornene.


the present study it has been used sucessfully in the polymerization of most of our monomers with the exception of half-esters 4 and 8 (Tab. l) , although copolymers which incorporate these monomers may be formed as described above.

Tab. 1 . Ring-opening metathesis polymerization (ROMP) of di-ester and half-ester monomers iHc$I:i R'OOC. CooR'

ROMP COOR' -

" I X R ' R2 RuCI, a) OsC1, a)

& 3 b) 4 5 6 7 8

a) ,, indicates polymer formed; X, indicates no polymer formed. b, Also polymerized using the MoC15/(CH3),Sn/(C2H5)z0 catalyst system. ') Copolymer formed with norbornene. d, Intrinsic viscosity [q] = 100 cm3 . g - ' (measured in acetone at 25 "C).

Equilibrium water content EWC% = 97 (see text), (measured at pH 7 and 25 "C).

The reactivity of the RuCl, catalyst in the homopolymerization of monomers 3 to 8, discussed above, may also be compromised by the reaction conditions. Our experience suggests that the catalysts work best in the presence of wet alcoholz3) or other source of hydride3') and in the conditions used, both in our work and that of Laschewsky and Schulz-Hanke6), only very small amounts of alcohol were present, a necessary condition if transesterification is to be avoided. This contrasts the conditions used in the one-pot synthesis, 9 to 16 where RuCl, was active and where alcohol, and in some cases diol was used as solvent. In fact we have found diethyl tartrate to be a very efficient catalyst promoter and we have observed that the polymerization of norbornene with the RuCl, catalyst, and diethyl tartrate as solvent, undergoes a substantial rate enhancement as well as a change in the main chain double bond stereochemistry from 95% to 75% trans in comparison to ethanol as solvent 1 7 ) .

Furthermore, a I3C NMR study of copolymers formed from norbornene and the much less reactive cyclopentene, a t low conversion, show a substantially increased incorporation of cyclopentene when diethyl tartrate was used as solvent compared to ethanol3'). This proves that the presence of the tartrate enhances the activity of the catalyst rather than merely increasing the number of active sites. We believe that a diethyl tartrate ligand chelated at the ruthenium ion is sufficiently electron withdrawing to increase the electrophilicity of the carbene carbon atom and hence the reactivity of the metallacarbenes which are known to propagate the reaction "). Similar increases in reactivity have been observed on comparing acetate and the harder trifluoroacetate


ruthenium complexes 3 3 3 3 4 ) . Inter alia the ruthenium complex also catalyses the esterification reaction.

Each of the polymers, with the exception of those formed using glycols 15 and 16 (although these are extensively crosslinked they swell sufficiently in acetone to permit NMR analysis), were soluble in either chloroform or acetone provided that the reactions were stopped before gelation. The polymers could then be purified by precipitation in diethyl ether, and after drying and dissolving in a suitable solvent, good quality NMR spectra were obtained, vide supra.

We have carried out a preliminary survey of the physical properties of these polymers and a number of them show interesting rheological and hydrogel properties.

Polymers disubstituted with hydrocarbon or fluorocarbon ester groups, as expected, are hydrophobic and do not form hydrogels, but we have copolymerized the didodecyl ester 3 with the ex0 anhydride 1 (3 mol-Vo of 3 in copolymer by 'H NMR) and after water solubilization have observed a decreased solution viscosity in the copolymer compared to the dicarboxylate homopolymer, an effect which has been noted with other water-soluble polymers modified with hydrophobic groups 35).

Each of the polymers in Tab. 2 absorb water to varying degrees and their equilibrium

water contents EWC 070 = wt'wet - Wt.dry x 100 are found to be temperature

dependent and strongly pH dependent, with, for example, the lactate half-ester 9 having an EWC of 93% at pH 8 compared to 15% at pH 5 . We are currently investigating the volume phase change behaviour *) of each of these polymers as well as their potential as useful materials for enantiomer separation.

Wt.wet

Tab. 2. Polymers prepared by the one-pot synthesis route

- 9

10 11 12 13 14 15 16

R2 RuC1, b, OsC1, b, EWC%')

(-)-ethyl lactate (-)-diethy1 tartrate (-)-menthol HOCHZCH2OCH3 HOCHzCH2CH3 CH,(OCH,CH,),OH H(OCH2CH2),0H H(OCH2CH2),0H

H H H R1 R' H H H

25 d, 44 10 40 8

65 83 23

a) Corresponding hydroxy compound from which each alkoxy group was derived. b, See footnote a) Tab. 1. ') Equilibrium water content measured at pH 7 and 25 "C. dl Intrinsic viscosity [q] = 155 cm3 . g- ' (measured in acetone at 25 "C).


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A one-pot synthesis of comb polymers and hydrogels by the ring-opening metathesis polymerization reaction