Anionic polymerization of dienes using oligobutadienyl-lithium as initiator

Polymer International 47 (1998) 497È502

Anionic Polymerization of Dienes UsingOligobutadienyl-Lithium as Initiator

C. L. Almeida1 & L. C. Akcelrud2,*

1 Instituto de Qu•�mica, Universidade Federal do Rio de Janeiro, Rio de Janeiro, Brazil2 Instituto de Macromoleculas Professora Eloisa Mano, Rio de Janeiro, CEP 21.945-970, Brazil

(Received 23 January 1998 ; revised version received 25 May 1998 ; accepted 29 July 1998)

Abstract : The aim of this work is to combine the microstructure of a radical-polymerized diene (butadiene and isoprene) with the functionality of an anioni-cally prepared one, which could be used for polyurethane preparation afterfunctionalization. A two-step anionic procedure was used in order to obtain ahydrocarbon-soluble lithium-based initiator. When compared with the relatedanionic polymers obtained under classical conditions, the vinyl content of thepolydienes was three times greater. These Ðndings are discussed on a mechanisticbasis. 1998 Society of Chemical Industry(

Polym. Int. 47, 497È502 (1998)

Key words : anionic polymerization ; anionic initiation ; polyurethane

INTRODUCTION

One of the most important characteristics of poly-urethane elastomers is their processability from theliquid state, rendering the manufacturing of articlesmuch simpler than by conventional vulcanization. Oneof the starting materials for the synthesis of suchelastomers is a macrodiol, the structure of whichcan be a polyether, a polyester or a polybutadiene(hydroxyl-terminated polybutadiene, HTPB). The latterprovides better performance for elastomers in speciÐcapplications1 and can be obtained through two di†erentprocesses : the Ðrst is a free radical process. Typicalcommercial examples are R45M and R45HT (Atochem),and LiquiÑex H and LiquiÑex P (PetroÑex, Brazil). Thesecond is an anionic process.2,3 Owing to transfer reac-tions in the free radical process, the functionality ofHTPB is always greater than 2É04h7 and thereforecrosslinked polyurethanes are formed.8,9 Consequentlylinear HTPB-derived polyurethanes5 can only beobtained from an anionic HTPB which is essentiallydifunctional.10

* To whom all correspondence should be addressed.

When a comparison between linear and crosslinkedHTPB polyurethanes is to be made, in addition to func-tionality, another structural factor must be taken intoconsideration : whereas the microstructure of com-mercial free-radical HTPB consists predominantly of1,4-enchainment,5h7 commercial anionic HTPBscontain mainly the 1,2 (or vinylic) conÐguration.2,3 Inthis context it seemed to be of interest to synthesize anessentially difunctional HTPB with a similar micro-structure to that of free radical HTPB. Thereby linearHTPB polyurethanes with a structure similar to that ofcrosslinked ones could be prepared.

Besides permitting the functionalization of both chainends, anionic polymerization allows for a certain degreeof microstructure control. The most relevant data withrespect to correlations between polymer microstructureand structural or reaction factors for these systems havebeen reviewed.2,11h14

From a generic point of view, the most importantfactor for microstructure control is the ionic characterof the carbonÈmetal bonding in the growing chain,which in turn depends upon the solvent and thecounter-ion. When polar solvents are used, a highvinylic unsaturation content (1,2 or 3,4) is obtained for

4971998 Society of Chemical Industry. Polymer International 0959È8103/98/$17.50 Printed in Great Britain(

498 C. Almeida, L . C. Akcelrud

all alkaline metals and derivatives15h21 due to theincrease in ionic character of the terminal bond by sol-vation of the counter-cation.15 With regard to themetal, a higher content of 1,4 enchainment is alwaysfound when lithium is the counter-ion. For all dienes,including 1,3- and 1,2-butadiene, isoprene, 2,3-dimethyl-butadiene and 1,3-pentadiene, a lithiumÈhydrocarbonsolvent system yields a high content of 1,4 enchainmentin a non-polar solvent.

In this work we aimed to synthesize a difunctionaloligomeric polybutadiene with a microstructure similarto that of free radical HTPB (e.g. 22% 1,4-cis ; 57% 1,4-trans ; 21% vinyl)5h7 by means of a difunctional initiatorwhich would provide reactive extremities for furtherfunctionalization. As shown above, to achieve thisrather high 1,4 content, an apolar (hydrocarbon)solventÈdifunctional lithium-based initiator had to beused.

Hydrocarbon-soluble difunctional organolithium ini-tiators for diene polymerization may be prepared bytwo general methods : the Ðrst involves the coupling ofradical anions such as those formed by the reaction ofthe monomer with lithium dihydronaphthylide in tetra-hydrofuran, using a cosolvent such as an aromatic etheror a tertiary amine. It was found that the cosolvent,necessary for initiator solubility, had a negligible e†ecton the desired 1,4 enchainment.22h24

The second method employs a precursor having tworeactive double bonds which are reacted with a mono-functional organolithium compound (generally sec-butyl lithium) producing a hydrocarbon-insolubledilithium adduct. Upon addition of monomer the car-banionic species is solubilized. A variety of precursorshave been reported for these preparations.25h32 Forexample, dilithium adducts were prepared by Tung etal.27 from the reaction of compounds such as bis[4-(1-phenylethenyl)phenyl]ether, 1,4-bis(1-phenylethenyl)benzene, 4,4@-bis(1-phenylethenyl)-1,1@biphenyl or 2,2-bis[(4, 1-phenylethenyl)phenyl]propane with sec-butyllithium, which were successful for the synthesis ofpoly(styrene-block-butadiene-block-styrene) with goodmechanical properties. Guyot et al.26 used bis(phenylvinylidenyl)alkanes or diisopropenyl alkanes as precur-sors, whereas Quirk and Ma32 used 1,3-bis(1-phenyl-ethenyl)benzene. In the latter case monomodal distribu-tions were achieved for high molecular weights, and theaddition of tetrahydrofuran or lithium butoxide wasnecessary when lower molecular weights with narrowdistribution were desired. Sanderson et al.30 obtainedhigh molecular weight poly(1,3-butadiene) with narrowpolydispersity from 1,3-bis(1-dilithio-3-methylpentyl)benzene which was prepared from divinylbenzene asprecursor. More recently Teyssie and co-workers31 used1,3-diisopropenylbenzene and obtained an oligomerrather than the desired di-adduct. When used in a polarsolvent, mono-, di-, tri- and tetra-functional specieswere obtained, but in an apolar medium, polystyrene

with a narrow polydispersity and a functionality of twowas found.

A variation of the latter strategy may also beachieved in a two-step procedure. First, a lithiumÈaromatic complex (such as lithium dihydronaphthylide)is formed in a polar medium (e.g. tetrahydrofuran), anda small amount of diene is added forming an oligo-dienyldilithium carbanion which is soluble in apolarmedia. In the second step, the polar solvent is replacedby a hydrocarbon solvent and the polymerization iscarried out with further monomer addition.33 In thepresent work, this latter technique was used because itdid not require the preparation of a precursor whichinvolves a series of reaction steps.

EXPERIMENTAL TECHNIQUES

The high vacuum equipment used for polymerizationwas described elsewhere.34

Purification of solvents and monomers

T etrahydrofuran. Tetrahydrofuran (Grupo Qu•�mica,Brazil) was reÑuxed over sodium wire for 2 days anddistilled into a Ñask containing small lumps of metallicsodium. The Ñask was connected to a high vacuum line,degassed at 10~6mbar, and exposed three times to afreshly prepared sodium mirror. The solvent was thentransferred to a 500 ml Ñask containing a sodiumÈnaphthalene complex in tetrahydrofuran, and storedthere before use.

Hydrocarbon solvents (n-hexane, cyclohexane andtoluene). The solvents (Grupo Qu•�mica, Brazil) werereÑuxed with a 3 : 1 mixture of concentrated sulphuricacid and nitric acid for 72 h, and distilled into a Ñaskcontaining After Ðltration under the solventsCaH2 . N2were transferred to a 500 ml Ñask containing smalllumps of metallic sodium. The Ñask was connectedto a high vacuum line, degassed at 10~6mbar andexposed three times to freshly prepared sodium mirrorsbefore use.

Isoprene. Isoprene (98% purity, containing 50 ppmp-tert-butyl catechol as inhibitor) was obtained fromMatheson Colleman & Bell. After distillation oversodium wire to remove the inhibitor, the monomer wasdegassed at 10~6È10~8mbar. It was then cryoscopicallydistilled and collected in ampoules covered with freshlyprepared sodium mirrors. These last two steps wererepeated up to three times in order to remove all tracesof water, dimers and occluded air.

POLYMER INTERNATIONAL VOL. 47, NO. 4, 1998

Anionic polymerization of dienes 499

TABLE 1. Polymerization conditions for polydiene

syntheses

Initiator First stepa Second stepb

preparation

Temperature (¡C) 25 É32 25

Time (h) 24 2 24

Polymerization reactionc

Temperature Time

Monomer (¡C) (h)

Butadiene É32 25

2 24

Isoprene 0 25

2 24

a Formation of a lithium-dihydronaphthylide complex in

THF.

b Formation of an oligobutadienyl-lithium complex in THF.

c Initiator concentration was 7·8 Ã10É2 M in hexane.

Butadiene. Butadiene (99É43% purity ; containing 6 ppmp-tert-butylcathecol) was obtained from PetroÑex Ind.,Brazil. The gaseous monomer was transferred from thestorage cylinder to a Ñask at [72¡C containing HgSO4to remove the inhibitor, washed with 40% KOH toremove traces of peroxide, and after passing through atube containing molecular sieve was collected in a4 ÓÑask containing It was degassed at 10~6ÈCaH2 .10~8mbar, cryoscopically distilled and collected inampoules covered with freshly prepared sodiummirrors, where it was kept before use at [72¡C. Thislast operation was repeated three times to remove alltraces of water, dimer and occluded air.

Preparation of oligobutadienyl-dilithium initiator

Oligobutadienyl-dilithium was prepared in two steps :Ðrst, a 500 ml reaction Ñask containing 0É0148 mol ofnaphthalene or 1,1-diphenylethylene and 2É0 g lithiumwas degassed at 10~6È10~8mbar. 100 ml of THF wasthen added by cryoscopic distillation, forming thelithium dihydronaphthylide or 1,1-diphenylethylenecomplex. In the second step, 20% of the total monomerto be used was added to the complex, thereby formingthe oligobutadienyl-dilithium initiator in THF. Thelatter was removed and replaced by cyclohexane bymeans of sucessive cryoscopic distillations until all THF

traces were absent. Finally cyclohexane was replaced byn-hexane. After 24 h the oligomeric initiator solutionwas Ðltered into the reactor Ñask on the vacuum line, inwhich the polymerization was carried out.

Polymerization procedure

The remaining monomer (80%) was slowly transferredby cryoscopic distillation into the initiator solution.After polymerization, the red carbanions present wereterminated by the addition of 30 ml methanol containingthe antioxidant 2,6-di-tert-butyl-4-methylphenol. Thesolvents were removed by distillation, the polymerwas washed three times with 500 ml of a 1 : 1É5water : ethanol mixture, dissolved in 60 ml ethyl ether,precipitated by a six-fold excess of methanol and driedunder vacuum. The reaction conditions for these prep-arations are shown in Table 1.

RESULTS AND DISCUSSION

Tables 1 and 2 show the results relative to the synthesisand characterization of the polydienes. The molecularweights were within the expected range, given the initi-ator concentrations employed, and the polydispersitieswere in the same range as the reported values for anion-ically prepared polydienes.15

The gel permeation chromatograms (Fig. 1) for poly-butadiene and polyisoprene showed a monomodal dis-tribution, indicating that only one propagating specieswas present in the polymerization reaction.

Several methods for microstructure characterizationof polydienes have been reported in the literature, usinginfrared35h37 or nuclear magnetic resonance38h42 tech-niques. Whereas these methods provide a quantitativedetermination of all conÐgurations for poly-butadiene37,43h45 the reported values for the cis/transratios in polyisoprene are contradictory.46 The infraredabsorptions of the cis and trans units for this polymerare located at approximately the same wavelength ;therefore the IR measurements yield the total 1,4content and do not distinguish one isomer from theother.47

For the microstructure characterization of poly-butadiene we have used the method reported by Haslanand Willis,37 which focuses on absorptions at 10É3 l(967 cm~1), 11É0 l (887 cm~1) and 13É6 l (760 cm~1) cor-responding to the out-of-plane angular deformation of

TABLE 2. Characterization data of the polydienes

Polydiene Units (%) Molecular weight Ã10É3

1,4 cis 1,4 trans 1,2 3,4 Mn

Mw

Mw/M

nPolybutadiene 60 13 27 0 1·2 1·7 1·4

Polyisoprene 74 0 26 5·3 7·2 1·3



Fig. 1. Gel permeation chromatograms of the polydienes : (A)polybutadiene ; (B) polyisoprene.

1,4 trans, 1,2 vinyl and 1,4 cis units, respectively. TheÐrst two absorptions are strong, constant in locationand independent of sample origin. However, the 12lÈ16 l (800È670 cm~1) region varies in shape and peakposition according to the amount of cis conÐguration,particularly for high cis content samples.48 The concen-tration of cis and trans units was determined using eqns(1)È(3). The molar absorptivities (E) were 109, 143 and23 l cm~1 g mol~1 for absorptions (A) at 10É3, 11É0 and13É6 cm~1, respectively.37

1,4-trans (%)\ (A/E)10.3(A/E)10.3] (A/E)11.0

] (A/E)13.6

] 100% (1)

1,2-vinyl (%)\ (A/E)11É0(A/E)10É3 ] (A/E)11É0

] (A/E)13É6

] 100% (2)

1,4-cis (%)\ (A/E)13É6(A/E)10É3 ] (A/E)11É0

] (A/E)13É6

] 100% (3)

For polyisoprene microstructure characterization,three di†erent IR methods have been reported in theliterature. The Ðrst due to Corish49 uses the near-IRabsorption of cis 1,4 units at 2É46 l (4050 cm~1) ;the second due to Binder48 uses the weak bandsat 8É85 l (1130 cm~1) and 8É68 l (1152 cm~1) attributedto the absorption of cis and trans, respectively,while the third method (Richardson and Sacher47)uses the absorptions at 12É3 l (811 cm~1) and 11É83 l(835 cm~1) for determining the total amount of 1,4addition and the 1,4-cis/1,4-trans ratio, respectively.In the present contribution, the last method wasemployed using eqns (4)È(7). In Fig. 2 the IR spectra ofthe polydienes are shown. The molar absorptivities (E)used were 149, 145, 21É3 and 11É89 l cm~1 g mol~1 for

Fig. 2. Infrared spectra of the polydienes : (A) polybutadiene ;(B) polyisoprene.

absorbances (A) at 10É98 (1,2-vinyl), 11É25 (3,4-vinyl),11É83 (1,4-cis) and 11É94 (1,4-trans) l cm~1, respectively.

1,2-vinyl (%)\ (A/E)10.98(A/E)10.98] (A/E)11É25

] (A/E)11É83 ] (A/E)11É94

] 100%

(4)

3,4-vinyl (%)\ (A/E)11É25(A/E)10É98 ] (A/E)11É25

] (A/E)11É83 ] (A/E)11É94

] 100%

(5)

1,4-cis (%)\ (A/E)11É83(A/E)10É98 ] (A/E)11É25

] (A/E)11É83 ] (A/E)11É94

] 100%

(6)

1,4-trans (%)\ (A/E)11É94(A/E)10É98 ] (A/E)11É25

] (A/E)11É83 ] (A/E)11É94

] 100%

(7)

In Table 3 the microstructure of selected polydienes isshown, for comparison of the present results with thoserelated to other synthetic routes. In addition to thestructures considered in Table 3, small amounts of oxy-genated functions are present in commercial HTPBsamples as hydroxyl end-groups or in the middle of thechain.6 It is interesting to note that the vinyl content


Anionic polymerization of dienes 501

TABLE 3. Microstructure of some polydienes obtained by different methods

(configurations represented in mol%)

Polydiene 1,4 cis 1,4 trans 1,2 vinyl 3,4 vinyl cis /trans Reference

Anionic

polybutadienea 35 57 8 – 0·61 19·48

Anionic difunctional

polyisopreneb 81 15 0 4 5·4 19

Anionic

polybutadiene 60 13 27 – 4·6 This work

Anionic difunctional

polyisoprene 74 0 26 – This work

Free radical commercial

difunctional

polybutadienec 22 57 21 – 0·38 6

Anionic commercial

difunctional

polybutadiened 12 88 – – 51

Anionic commercial

difunctional

polybutadienee 35 45 55 – 3·5 10

a Hexane as solvent ; alkyl lithium as initiator 2·0 Ã10É4 M; (moles initiator/moles monomer).

b Hexane as solvent ; alkyl lithium as initiator 3·0 Ã10É4 M; (moles initiator/moles monomer).

c Hydroxyl terminated polybutadiene, ethanol as solvent, hydrogen peroxide as initiator, (Liquiflex ‘H’)

Petroflex, Brazil.

d Anionic hydroxyl terminated polybutadiene (Nisson G 2000), Nippon Soda, Japan, prepared with a

difunctional initiator in polar medium.

(1,2 and 3,4 enchainments for polybutadiene and polyi-soprene, respectively) obtained under the conditionsemployed here was much greater than those reportedfor usual anionic conditions.19 This Ðnding can beexplained in terms of the mechanism of Gerbert et al.50depicted schematically in Fig. 3, where three isomericforms coexist in equilibrium with a n-complex. Becauseof the apolar nature of the medium, the anions are pre-dominantly present in pairs or aggregated structures11rather than as free ions.

Fig. 3. Equilibrium forms during propagation for dieneanionic polymerization in non-polar medium.

The vinyl enchainment is formed through the disso-ciation of the n-complex, whereas the formation of the1,4 units requires its isomerization before its disso-ciation. In a non-polar medium the covalent characterof the carbonÈlithium double bond is increased andisomerization is less favoured. We used a rather highinitiator concentration (about 100 times higher thanthat used for polymers (see footnotes a and b of Table3), thus favouring the formation of a great number ofdimeric forms, which in n-hexane give rise to 1,2 or 3,4units.

From Table 3 we can see that there is a great di†er-ence between the microstructure of the two types ofcommercial anionic polydienes. When prepared with amonofunctional initiator the 1,4 enchainment can be ashigh as 92% (Table 3, footnote a) but when it is pre-pared with a difunctional initiator this value drops to12% (Table 3, footnote d). We were able to achieve a1,4-enchainment of 73% which represents a substantialincrease relative to the latter case. However, the cis/trans ratio obtained with the oligodienyl-lithium systemwas 4É6 times higher than that of the radical poly-merized polybutadiene and this is the highest value fordifunctional oligomeric polybutadiene reported so far.

CONCLUSIONS

The use of an oligobutadienyl lithium initiator and aspeciÐc set of reaction conditions made it possible to



achieve a polydiene conÐguration (1,4 and 1,2enchainments) very similar to that obtained with free-radical initiation of butadiene. This method can there-fore be used to make linear polyurethanes withmicrostructures similar to those of the crosslinked onesobtained from radically generated HTPB. The cis/transratio of the new polymers was, however, ten timesgreater than a corresponding radically polymerizedpolybutadiene.

With polyisoprene, this system also provided a 1,4content very close to that obtained with a conventionalanionic difunctional initiator.

ACKNOWLEDGEMENTS

The authors gratefully acknowledge support from theBrazilian Research Council (CNPq) and Professor F. E.Karasz for revising the manuscript.

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Anionic polymerization of dienes using oligobutadienyl-lithium as initiator