On the anionic homo‐ and copolymerization of ethyl‐ and butyl‐2‐cyanoacrylates

On the Anionic Homo- and Copolymerization ofEthyl- and Butyl-2-Cyanoacrylates

ZLATAN DENCHEV,1 MILENA TOMANOVA,1 ALBENA LEDERER2

1Department of Polymer Engineering, IPC—Institute for Polymers and Composites, University of Minho,4800-058 Guimaraes, Portugal

2Institute of Polymer Research Dresden, 01069 Dresden, Germany

Received 30 December 2007; accepted 29 April 2008DOI: 10.1002/pola.22842Published online in Wiley InterScience (www.interscience.wiley.com).

ABSTRACT: Ethyl-(ECA) and butyl-2-cyanoacrylate (BCA) monomers of high purityand acidic stabilization were synthesized and anionically polymerized to homo- andcopolymers in two different ways: by piperidine-catalyzed bulk polymerization leadingto transparent, brittle films (method A) and by polymerization in aqueous medium inthe presence of sodium bicarbonate to obtain white powders (Method B). The molecu-lar structure of the synthesized monomers, homopolymers and copolymers werecorroborated by spectral methods. The polymers were studied further by thermalgravimetric analysis (TGA), differential scanning calorimetry (DSC), size exclusionchromatography (SEC) and proton nuclear magnetic resonance (1H NMR). Control-ling the composition of the monomer feed and the way the polymerization wasperformed, it was possible to obtain phase separated or homogeneous cyanoacrylatecopolymers with glass transitions varying between the Tg of polyECA and that ofpolyBCA. VVC 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 5142–5156, 2008

Keywords: alkyl-2-cyanoacrylates; anionic polymerization; copolymerization; DSC;proton-NMR; SEC

INTRODUCTION

Monomer 2-cyanoacrylates (CAs) are being usedas fast setting special adhesives since more than40 years.1 A key property of CAs is that the liq-uid monomer is able to polymerize when appliedon the surface or between various polar sub-strates, also including living tissues.2 Therefore,these monomers have been extensively used astissue adhesives for skin wounds closure.3–5

Alkyl polyCAs were also found to be very promis-ing as drug delivery nanoparticle carriers of anti-tumor medicines.6–8 Other compounds of interestsuch as peptides and nucleic acids have been

combined with polyCA nanoparticles for target-ing purposes.9,10 Nowadays, polyCA nanopar-ticles are considered the most promising polymercolloidal drug delivery system and are already inclinical development for cancer therapy.11

It is known that changing the length of thealcohol residue of the cyanoacrylate monomercan have a threefold impact: on the biodegrad-ability,12,13 on the strength of the polyCA adhe-sive layer14 or on the polymer hydrophilic/hydro-phobic properties, the latter providing variouscompatibilities with different kinds of hydropho-bic drugs.15 Copolymeric poly(CAs) containinglong ester radicals and poly(ethylene glycol) moi-eties have been prepared in the form of nano-particles16 and studied for brain tumor target-ing.17

In most of the cases the polyCA nanoparticlesare obtained by polymerization of the respective

Correspondence to: Z. Denchev (E-mail: [email protected])

Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 46, 5142–5156 (2008)VVC 2008 Wiley Periodicals, Inc.

5142

monomer by nucleophiles. There exist a numberof studies on the mechanism of these reactionssummarized by Robello et al.18 This same work,as well as the preceding paper of Ryan andMcCann,19 deal with the depolymerization andrepolymerization occurring in poly(alkyl cyanoa-crylates) obtained by various polymerizationtechniques.

The use of the CA monomers as medicaladhesives or raw material for nanoparticlesdepends on their polymerizability, which, on itsturn, is related to the stabilization againstanionic or radical polymerization. The stabiliza-tion of the CAs occurs typically during theirsynthesis. The reaction pathway with biggestindustrial importance includes a Knoevenagelcondensation of the respective cynoacetic esterwith formaldehyde to get an oligomeric polyCA.Furthermore, a controlled depolymerization ofthese oligomers is realized producing the rawmonomer, which is afterwards purified by addi-tional distillations.20,21

As a part of a broader study on the prepara-tion of copolymeric polyCA nanoparticles, wesynthesized a number of alkyl- and alkoxyalkyl-2-CA by using a four-component depolymeriza-tion system allowing the preparation of thesemonomers in high yields and purity controllingthe amounts and types of stabilizers.22 In thepresent work ethyl- (ECA) and butyl-2-cyanoa-crylate (BCA) monomers were prepared withthis depolymerization system and anionicallypolymerized in two different ways to homo- andcopolymers. All monomers and polymers werecharacterized by SEC, 1H NMR, and FT-IR. Thethermal transitions in the two homo polyCA andof ECA-BCA copolymers of various compositionswere studied by thermogravimetric analysis(TGA) and differential scanning calorimetry(DSC), associating them with the copolymerstructure.

EXPERIMENTAL

Preparation of the CA Monomers

ECA and BCA monomers with known amountsand types of stabilizers were prepared in thefollowing way. The corresponding ester of thecyanoacetic acid was reacted with paraformalde-hyde (pFA) in almost equimolar ratio, in thepresence of catalytic amounts of piperidine andtoluene as a solvent. The mixture was refluxed

while stirring and removing azeotropically thecondensation water. Upon separation of the the-oretical amount, the toluene solution of the re-spective polyCA oligomers was cooled down toroom temperature. The components of the de-polymerization system were then added understirring, namely P2O5, hydroquinone, para-tol-uenesulfonic acid (pTSA), and ortho-phosphoricacid. After distillation of the solvent, the polyCAoligomers were depolymerized by heating in the180–220 8C range under a vacuum of 0.67–1.33kPa. The raw CA monomer was subjected toadditional vacuum distillations to get the pureCA monomer. The procedure described results inECA and BCA monomers stabilized with 80–120ppm of SO2 and 100–230 ppm of hydroquinoneand containing up to 0.5% of pTSA. More detailsof the procedure are given elsewhere.22

Preparation of the PolyCAs

The CA polymers were prepared in two differentways: by anionic piperidine-catalyzed bulk poly-merization (Method A) or by anionic polymeriza-tion in aqueous medium in the presence ofNaHCO3 (Method B) as described by Kattiet al.23 In the first case, 0.02 mL of freshly pre-pared 0.1% solution of piperidine in chloroform(2.4 3 10�4 mol/L) was added at room tempera-ture to 1 g of monomer (ECA or BCA, or theirmixtures in various proportions). The systemswere rapidly stirred and transferred into alumi-num circular molds with a diameter of �2 cm.The polymerization process started no soonerthan 15–20 min after the addition of the initia-tor. The curing was performed at room tempera-ture typically for 24 h. In such a way, transpar-ent films with thicknesses in the 0.2–0.5 mmrange were obtained and further characterizedby various analytical methods.

In the second case, 1 g of ECA, BCA or of themixture thereof was added dropwise to 20 mL ofdemineralized water containing 2.5 3 10�6 gNaHCO3 (1.8 3 10�6 mol/L), at 30 8C. The poly-mers obtained in the form of white powderswere filtered, washed with a diluted HCl watersolution to pH ¼ 5–6, than with methanol anddried in a vacuum oven. Mixtures of polyECAand polyBCA obtained by either of the methodswere prepared by dissolution of equal amountsof each homopolymers in acetone, followed byadmixing of aliquot parts of the two polymer sol-utions, homogenization and evaporation of thesolvent.

COPOLYMERIZATION OF CYANOACRYLATES 5143

Journal of Polymer Science: Part A: Polymer ChemistryDOI 10.1002/pola

Characterization Techniques

The molecular structure of the synthesized CAsand polyCAs was corroborated by means ofinfrared and 1H NMR spectroscopy. The infraredspectra were obtained in a Perkin-Elmer 1600FTIR spectrometer with an accumulation timeof 16 s. The monomers were studied in con-densed state pressing a drop between two KBrwindows. The immediate polymerization wasavoided by adding minor amounts of H2SO4 tothe monomers. The polymer films (Method A)and powders (Method B) were studied asobtained from their anionic polymerization. Forthe 1H NMR traces a Varian UNITY PLUS ap-paratus operating at 300 MHz was employed.The spectra were obtained at 25 8C in CDCl3(the monomers) or acetone-d6 (all polymers)against tetramethylsilane as the internal stand-ard. The glass transition temperatures (Tg) weredetermined by DSC experiments carried out in aPerkin-Elmer Diamond Pyris instrument at aheating rate of 10 deg/min. The typical sampleweight was 10–15 mg. The weight retentionmeasurements were made in a thermogravimet-ric analyzer Q500 of TA at a heating rate of10 deg/min.

The molecular weights were obtained using amodularly build SEC system with HPLC-Pump(KNAUER, Germany) in combination with RI-detector (KNAUER, Germany) and multi anglelight scattering (MALLS) detection (DAWN-EOS, Wyatt Technologies, USA). The solventTHF was pumped at a flow rate of 1.0 mL/minthrough a 2-PL-Mixed B-LS column system(POLYMER LABORATORIES, UK). Despite thefact that absolute detection method was used,for some of the samples good quality light scat-tering (LS) signals could not be obtained andmolar mass calculation by calibration wasneeded. Thus, the relative molar masses weredetermined using calibration with a standardand the absolute molar masses were found usingLS data. Comparing the absolute masses ofMethod A poly(cyanoacrylates) and those foundby using trivial PS standards shows that two tofour times lower molar masses were obtained inthe latter case with the same samples. This ob-servation can be explained with the differentstate in solution of the PS and polyCA mole-cules. As a standard for the sample specific cali-bration we used the statistically distributedsample ECA, 100% prepared by method A,where the concentration (RI) and the LS detec-

tion were fitting well to each other leading to anabsolute molar mass distribution. The calibra-tion curve was than calculated using the realmolar masses from the LS data and the elutiontime as shown in Figure 1.

For the calculation of the absolute molarmass interpretation of the LS data using ASTRA4.9 Software was carried out. For the detectioncomplete covering of the injected and detectedmass of sample was postulated. This calculationresulted in dn/dc of 0.79 to 0.82 calculated fromconcentration (RI) and LS data after separation.

RESULTS AND DISCUSSION

Monomer and Polymer Characterization by FT-IR

Figure 2 displays the FT-IR spectra of the ECAand BCA monomers (lower curves), the respec-tive polymers obtained by Method A (uppercurves) and by Method B (middle curves). Ascompared to the curves of homopolymers pre-pared by Method B, the spectra of Method ApolyECA and polyBCA display some minor dif-ferences in the form and positions of the mainabsorption bands, more particularly in the3600–3400 cm�1. Supposedly, this has to do withthe various chain initiation—by piperidine(Method A) or OH� (Method B).

The spectra of the monomers display peaks ofstretching vibrations of conjugated CN groupwith mCN ¼ 2238–2244 cm�1, and the mC��H andmC¼¼C at 3126–3128 and 1614–1618 cm�1, respec-tively, characteristic for a CH2��C(R1R2) (R1 andR = H). The strong absorption bands found inthe 1740–1744 cm�1 range in ECA and BCAbelong to an ester C��O group, conjugated witha double bond. Upon polymerization, a shift to-ward bigger wave numbers is registered withthe CN group vibration (mCN ¼ 2244–2248 cm�1)and with the C��O stretch observed in bothpolymer spectra at 1748 cm�1. This effect is at-tributable to the elimination of the conjugationof the nitrile triple and of the ester doublebonds, due to the almost complete disappearanceof the C��C double bond. The FT-IR spectra ofthe ECA-BCA monomer mixture, of the 50:50copolymers and of the homopolymer mixturerepresent superposition of the signals of themonomers and homopolymers, do not add anystructural information and are therefore notpresented in Figure 2.

5144 DENCHEV, TOMANOVA, AND LEDERER


Monomer and Polymer Characterizationby 1H NMR

Figure 3 shows the 1H NMR traces of ECA(curve a) and BCA monomers (b). The exactpeak positions and intensities are listed below(s ¼ singlet; m3, m4, m5, and m6 ¼ multipletscomprising respectively, three, four, five, and sixpeaks; J - coupling constant, Hz):

1H NMR (300 MHz, CDCl3) of ECA: d 7.07 (s,1H); 6.63 (s, 1H); 4.40–4.33 (m4, 2H, J ¼ 2.37Hz); 1.41–1.36 (m3, 3H, J ¼ 2.37 Hz);

1H NMR (300 MHz, CDCl3) of BCA: d 7.05 (s,1H); 6.62 (s, 1H); 4.31–4.27 (m3, 2H, J ¼ 2.21Hz); 1.76–1.67 (m5, 2H, J ¼ 2.35 Hz); 1.50–1.37(m6, 2H, J ¼ 2.48 Hz); 0.99–0.94 (m3, 3H,J ¼ 2.45 Hz).

The designation of the protons of the mono-mers is given in Figure 3. The designation ofthe signals of the f and e protons observed at d¼ 6.63–7.05 ppm is confirmed by calculating thetheoretical chemical shifts24:

dHe¼ 5:28þHgemþCNcisþCOORtrans¼ 6:62 ppm

dHf¼ 5:28þHgemþCNtransþCOORcis¼ 7:01 ppm

The proton NMR studies corroborate theexpected chemical structure of both ECA andBCA monomers. Scheme 1 gives the positions ofthe substitutions in gem, cis, and trans positionsin respect to one of the H-atoms attached toC¼¼C bond in the CA monomer (1) as well as thechemical structures of an alkyl CA homopoly-mers (2).

As seen from the survey spectra in Figure 4,after polymerization according to Scheme 1, thetwo singlet signals of the monomers appearingin weaker fields at 7 ppm and 6.6 ppm disap-pear and the clear multipets of the ester residueprotons typical of the ECA and BCA monomersbecome less resolved maintaining their chemicalshifts. At the same time, a broad resonancepeak centered at d 2.5 ppm appears in thehomo- and copolymers, which is attributable to

Figure 2. Comparison of the FT-IR spectra of: (a)ECA and polyECA and (b) BCA and polyBCA. Thebottom curves are the spectra of the monomers (thinlayer, transmittance mode), the middle—of powdersobtained by polymerization method B (powder, reflec-tion mode), and the upper—thin polymer films(method A), transmittance mode.

Figure 1. RI-chromatogram and calibration curve(molar mass versus elution volume) of the sampleECA, 100%, obtained by Method A.



the backbone methylene protons. Similar spec-tra were observed in the previous work of Steinobtained with homopolymeric polyECA- andpoly(ethoxyethyl)CA.25 As it will be shown fur-ther in this study, based on the studies of Robeloet al.,26 the shape of this peak can be relateddirectly to the tacticity of the polymers formed.

It is important to note that in the spectra ofECA and BCA monomers weak resonance linesat 7.27 and 2.3 ppm were observed. All polymersamples revealed a signal between 5.27 and 5.35ppm. These signals were ascribed to pTSA, afteranalyzing the spectrum of the pure compoundobtained under similar conditions. The inset inFigure 3 shows the signals of the benzene ringprotons (2H) of the pTSA. The amount of pTSAin the CA monomers was estimated to be in therange of 0.15–0.5%.

Thermogravimetric Studies

The thermal stability of the homo- and copoly-mers was studied by TGA. Figure 5 shows theTGA curves of polyECA and polyBCA preparedby the two different polymerization techniques.The curves of the copolymers have similar formsand appear in the same temperature interval.The temperatures of the incipient and the maxi-mum mass loss for all polymers studied aregiven in Table 1.

It can be seen that the polyECA and polyBCAsamples obtained by piperidine-catalyzed anionic

bulk polymerization (Method A) are quite stableuntil 160 and 130 8C, respectively. Compared topolyECA, polyBCA starts degrading at lower tem-perature, but its maximum degradation rate isregistered later. The trend was similar with therespective homopolymers obtained in aqueousmedia (Method B): the temperatures of the 2%mass losses were lower and those of the maxi-mum losses higher than the respective values ofthe Method A samples. The copolymers behave ina similar way (Table 1). We did not find any proofof low-temperature degradation with release ofvolatiles in either of the polymers studied.

Characterization of the Homo- and Copolymersby DSC

Figure 6(a,c) shows the thermal behavior ofpolyECA and polyBCA films obtained in bulk(Method A). During the first DSC scan, weakendothermic processes can be identified in bothhomopolymer samples. Also, a stronger endo-thermic peak superimposed over the Tg transi-tion is registered in both (a) and (c) samples. Asshown by the TGA traces, neither of these tran-sitions could be due to degradation with weightloss. Most probably the endothermic peaks closeto Tg could be related to structural relaxationdue the intensified chain mobility often found inother polymer systems.27 The second DSC scanshows one well-expressed Tg transition in bothhomopolymers samples.

Scheme 1. Polymerization of alkyl-CA to homopoly(alkyl-CA).

Figure 4. 1H NMR of CA polymers obtained in aqueous medium (Method B): (1)poly(ECA); (2) poly(BCA); (3) copoly(ECAþBCA), 1:1 by weight; (4) physical mixtureof poly(ECA) þ poly(BCA), 1:1 by weight. The missing peak at �1.6 ppm belongs tothe CDCl3 solvent. [Color figure can be viewed in the online issue, which is availableat www.interscience.wiley.com.]

Figure 3. 1H NMR spectra of CA monomers with peak designations (solventCDCl3). [Color figure can be viewed in the online issue, which is available at www.interscience.wiley.com.]



Figure 4

Figure 3

The homopolymers obtained by Method B[the solid curves in Fig. 6(b,d), corresponding tothe second scan] showed rather unclear glasstransitions. To reveal better the exact position ofthe Tg, the first derivative of the DSC curveswas calculated [Fig. 6(a–d), dashed lines], show-ing peaks instead of steps. This method wasused to determine the temperature positionsand the number of glass transitions in all poly-mer samples studied. The respective data arerepresented in Table 2.

Figure 6 and Table 2 allow the following con-clusions about the glass transitions in the polyECA and polyBCA homopolymers of Methods Aand B. The temperature position of Tg decreaseswith increasing the length of the ester residue.Thus polyECA, depending on the method ofpolymerization and the number of scans dis-plays a Tg between 138.5 and 140.6 8C, whereasthe Tg polyBCA is registered between 82.5 and92 8C. Furthermore, the increased widths of thepeaks corresponding to the glass transitions af-ter calculating the first derivative (especially inthe case of homopolymers of Method B) maysuggest the presence of several slightly differentamorphous phases.

It was interesting to follow the changes in thetemperature position and number of glass tran-sitions in poly(ECA-co-BCA) systems of variouscomposition obtained byMethod A (Fig. 7, Table 2).For the copolymer with composition [ECA]:[BCA] ¼ 75:25, the strongest transition isslightly below the Tg of the neat poly(ECA),whereas for the 25:75 system the Tg is slightlyabove that of the neat poly(BCA). One can con-

sider the dependence of the glass transitions inFigure 7 as a proof for the formation of misciblecopolymers, most probably with the ECA andBCA repeat units forming relatively short blocksor being randomly distributed. In the copoly-mers obtained by Method A, only one glass-tran-sition was observed when polymerizing a mix-ture richer in one of the comonomers, that is, nowell-expressed phase separation between poly(ECA) and po(BCA) phases occurred. The poly-merization of the 50:50 monomer blend resultedin a system revealing three though close butseparate glass transitions. The DSC measure-ment was repeated with three different samplesof this composition to make sure that the multi-ple transitions were not an artifact due to highnoise level in the derivatized curves. It might besupposed that the three transitions are causedby polyECA- and poly BCA-rich phases (blockcopolymers), and a third, random copolymerphase giving rise the transition in between.

The polymers obtained by Method B dis-played a rather different thermal behavior.There, the first DSC scan always revealed twoglass transitions corresponding to two differentphases (Table 2). The second scan resulted inone single transition, typically closer to thehigher Tg. This thermal behavior is illustratedwith the curves of the 50:50 physical mixture ofthe two homopolymers before and after anneal-ing [Fig. 8(a)] and the polymer produced in sus-pension feeding equal amounts of ECA andBCA. Apparently, in the latter case one initiallyobtains blends of homopolymers or phase sepa-rated copolymers with very long blocks. Duringthe second DSC scan, miscible random copoly-mers are formed characterized by only one glasstransition.

Summarizing the results from the DSCexperiments one may conclude that the flexibil-ity of the copolymeric cyanoacrylates can be suc-cessfully tailored by controlling the amounts ofthe copolymerizing monomers, the polymeriza-tion method and the chemical structure of themonomers employed.

Size-Exclusion Chromatography Studies

The DSC results discussed in the previous sec-tion were obtained during the second DSC scan.This means that all polymers passed through aheating ramp to 160 8C (homo-polyECA andECA-rich copolymers) and to 130 8C (homo-polyBCA and BCA-rich copolymers), performed

Figure 5. TGA curves at a heating rate of 10 deg/min of: (1) polyECA, Method A; (2) polyECA, MethodB; (3) polyBCA, Method A; polyBCA, Method B.



Figure 6. DSC curves of the neat poly(cyanoacrylates): (a) poly(ECA), Method A;(b) poly(ECA), Method B; (c) poly(BCA), Method A; (d) poly(BCA), Method B. Withthe polymers of Method A (a, c) the first and second DSC scans are presented desig-nated with 1 and 2, respectively; for those of Method B the second scan curves areonly given. The dashed lines in a-c represent the 1st derivative of the second scanDSC curve. [Color figure can be viewed in the online issue, which is available atwww.interscience.wiley.com.]

Table 1. Temperatures of Initial (2%) and Maximal Weight Loss inPoly(cyanoacrylate) Homo- and Copolymers as a Function of the Monomer Feed

Monomer feed, %

Weight Loss Temperatures in the Homo- and Copolymers

Method A Method B

2% loss, 8C Max. loss, 8C 2% loss, 8C Max loss, 8C

ECA, 100 161.8 216.7 136.8 222.2BCA, 100 129.1 222.7 124.3 234.6ECA:BCA ¼ 50:50 147.0 209.5 131.0 223.8ECA:BCA ¼ 25:75 136.3 217.6 128.2 228.3ECA:BCA ¼ 75:25 142.1 204.2 131.5 218.4



at 10 deg/min. The TGA studies showed onlyminor mass losses under these conditions, sug-gesting no or very limited depolymerization. Toverify the occurrence of degradation processesnot related to liberation of volatiles, SEC studiesof all homo- and copolymers of methods A and Bwere performed. The samples were divided intotwo groups—(i) as prepared bulk polymerizedfilms and suspension-polymerized powders and(ii) the same samples after a gradual heatingfrom 30 to 160 8C (or 30–130), followed by aslow cooling down to room temperature. Thisheat treatment was intended to model the tem-perature regime during the first scan in theDSC apparatus. Two types of molar mass detec-tion were used: by RI with subsequent samplespecific calibration or by LS obtaining the abso-lute molar masses (see Experimental part). Thelatter method gave reliable results only with theMethod A samples. The samples synthesized insuspension by Method B did not lead to reliableLS signals due to their very broad molar massdistributions. The data for all CA polymersbefore and after annealing are given in Table 3.

Analyzing the SEC results one should bear inmind that previous works,18,19 have renderedenough evidence about the existence of low-tem-perature depolymerization/repolymerizaton proc-esses in alkyl(polycyanoacrylates) made by ani-onic polymerization. Reportedly, these reactionscan occur in solution or even in solid state, ad-ventitious bases being sufficient to promote deg-radation. It was also pointed out that samplespolymerized by weak bases or post-treated withstrong acids (acetic, HCl, methanesulfonic) are

stable either in solution or in solid state. That iswhy all polymer in this work obtained in aqueoussolution (Method B) were acidified (quenched).Such acidification was not attempted with theMethod A homo- and copolymers since the ECAand BCA monomers already contained a bigexcess of pTSA (8.7 3 10�3 mol/L (0.15% pTSA)in respect to the piperidine used (2.4 3 10�4

mol/L).Analyzing the SEC data, it may be concluded

that Method A apparently produces highermolar mass polyECA and poly(ECA-co-BCA)compared to that of the respective polymersobtained by suspension polymerization (MethodB). With polyBCA however, it is the method Bthat gives the higher molar mass product. Thepolydispersity ratio Mw/Mn of all Method A poly-mers before annealing is between 2.3 and 3.8being significantly narrower than that of thesuspension polymerization products whose val-ues range between 14.5 and 19.0. When poly-merized in bulk, ECA produces a homopolymerwith a molar mass being three times higherthan of polyBCA of the same method; the massof the 50:50 copolymer remains higher than thatof the poly BCA. When polymerized in suspen-sion, it is polyECA that gives the lowest massand poly BCA—the highest. Again, the mass ofthe 50:50 ECA-BCA copolymer is intermediatein relation to those of the two homopolymers.

The heating-cooling cycle similar to that ofthe first DSC scan has an important impact onthe molar masses and their distribution. Thus,all Method A products when studied by RI andafter sample specific calibration display a well-

Table 2. Transition Temperatures of Various Polymers and Copolymers Dependingon the Method of Preparation

Startingmaterials, wt %

Polymers

Method A (film) Method B (powder)

Tg, 8Cb Tg, 8C

a Tg, 8Cb

ECA, 100 138.4 140.6 140.3BCA, 100 82.5 90.2 92.0(ECA þ BCA) ¼ 50:50 87.3; 94.2; 101.0 58.2; 98.4 94.5(ECA þ BCA) ¼ 75:25 125.1 60.4; 92.4;128.0 106.0(ECA þ BCA) ¼ 25:75 86.7 65.2; 85.0 80.0Poly(ECA) þ poly(BCA) – 72.4; 100.4 90.0

a Tg values after the first DSC scan.b Tg values after the second DSC scan.



expressed drop in both Mn and Mw values withthe exception of Mw of polyBCA, which growsfrom 24,300 to 37,900 g/mol. The LS detectionused with the bulk polymers, reveals a differenttrend. In polyECA homopolymer, both Mw and Mn

decrease upon annealing while the polydispersitygrows from 1.5 to 3.3. With polyBCA obtainedunder the same conditions, there is an increasein Mn and Mw upon heating. The bulk 50:50 co-polymer displays almost constant masses beforeand after heating.

Heating results in a tremendous augmenta-tion in the polydispersity of polyBCA and the50:50 copolymer obtained in suspension. Thetrend with the Method B polyECA is inversed.This leads to the conclusion that BCA contain-ing polymers are obviously more sensitive toannealing than the ECA-homopolymer. The deg-radation of the BCA-containing polymers canaffect to some extent the thermal behavior. Thehigh polydispersity of the most of the samplescan explain their vague DSC signal of Tg. How-ever the multimodality in some of the molarmasses cannot recover the thermal behavior ofthe samples, which means that reordering of co-polymer units from random to block segments isthe most plausible explanation for the DSCobservations.

Figure 7. First derivatives of the second scan DSCcurves of poly(ECA-co-BCA) samples obtained byMethod A as a function of the starting monomer mix-ture composition (in wt %). The derivatized DSCcurves of the neat poly(ECA) (100:0) and poly(BCA)(0:100) are also presented for comparison. [Color fig-ure can be viewed in the online issue, which is avail-able at www.interscience.wiley.com.]

Figure 8. Fist derivatives of the 1st and 2nd scan DSC curves of poly(cyanoacry-late) samples obtained by Method B: (a) physical blend of polyECA/polyBCA ¼ 50/50wt % obtained by admixing of neat polymer solutions and subsequent film casting;(b) samples obtained by Method B polymerizing a 50:50 monomer mixture. For moredata see Table 1. [Color figure can be viewed in the online issue, which is availableat www.interscience.wiley.com.]



Figure 9 represents selected GPC curvesillustrating the impact of the polymerizationmethod and the heat treatment upon the molmasses and their distributions. Summarizingthe GPC results, it may be concluded that thesuspension polymerization products are definitelymore heterogeneous than the polymers obtainedin bulk. Heating has a much stronger impact onthe polymers obtained in aqueous mediumbroadening even more their mass distributions.Along with some degradation/repolymerization,which cannot be excluded entirely, the calibra-tion seems to be another source of artifacts indetermination of the correct molecular masses ofpoly(cyanoacrylates) by SEC.

Tacticity Studies on Homo- and Copoymersby 1H NMR

Along with the heterogeneity caused by varia-tions in the length of the macromolecules and/orthe sequence of the ECA and BCA units, polyCAcopolymers and also the polyECA and polyBCAhomopolymers are built up of various stereoiso-mers. Figure 4 shows the appearance, uponpolymerization, of a broad resonance in the 2.5–3.5 ppm region related to the backbone methyl-ene protons (marked with asterisk in Scheme 1).Analyzing the shape and the position of thepeaks in this region information about the spa-tial placement of the CN- and COOR estergroups (E) in respect to each other could beextracted. Scheme 2 displays two poly(CA)macromolecules with rrm and mmr tetrads

(r ¼ racemic, meso ¼ m) There are four morepossibilities—the rrr and mmm tetrads (corre-sponding to syndiotactic and isotactic place-ments, respectively), as well as to the mrm andrmr sequences.

Figure 10(a–f) (black lines) reveals theextended methylene protons region of all homo-and copolymers obtained by the two methods(solvent: d6-acetone). Figure 10(a) relates the sixresonance lines with the respective tetrad typesin polyECA (Method A) according to the desig-nations of Robello et al.26 Using a combinationof 13C and 1H NMR techniques, the authorsindicated that the signals of the rrr sequenceswill be observed in the strongest fields (in thepresent case, slightly higher than 2.5 ppm)accompanied in descending order by the signalsof rmr and mmm isomers. In weaker fields (2.8–3.0 ppm) the peaks of the rmr þ rrm and mrm,mmr and rrm isomers are expected to appear.26

In this work we extended the findings ofRobello et al. to the 1H traces of al cyanoacry-late homo- and copolymers obtained in bulk orin solution [Fig. 10(a–f)]. This figure comparesthe spectra of the respective polymers beforeannealing (black lines) and after it (gray lines).Figure 10(a) displays the spectra of the bulkpolymerized polyECA with the respective tetraddesignations according to ref. 26. It can be con-cluded that the as-prepared method A polyECAis richer in mmm, rrr and rmr tetrads and con-tains less rrm, mmr and mrm tetrads. Afterannealing at elevated temperature, the contentof the former three tetrads drops while the

Table 3. Molecular Masses of Various polyCA Homo- and Copolymers Before and After Annealing for 10 min at100 8C with RI and LS Detection

Sample

Mn, g/mol Mw, g/mol Mw/Mn, g/mol

Before Anneal After Anneal Before Anneal After Anneal Before Anneal After Anneal

Sample specific calibration* and RI detectionECA, 100%, A 32,100 26,200 81,000 82,600 2.5 3.2ECA, 100%, B 6,800 11,300 104,300 141,600 15.3 12.5BCA, 100%, A 10,600 7,000 24,200 37,900 2.3 5.4BCA, 100%, B 28,400 19,700 538,000 857,000 18.9 43.5ECA-co-BCA, A 16,000 11,600 60,800 50,900 3.8 4.4ECA-co-BCA, B 12,200 5,800 176,700 284,500 14.5 49.1LS detection (absolute masses)ECA, 100%, A 67,300 23,300 97,500 76,800 1.5 3.3BCA, 100%, A 17,900 25,000 52,100 54,800 2.9 2.2ECA-co-BCA, A 35,700 35,400 67,100 58,100 1.9 1.6

*Sample specific calibration was carried out on the basis of RI- and LS data from the SEC measurements of the sample ECA,100% (see the experimental part).



Figure 9. Selected SEC traces of polycyanoacrylates obtained by suspension (a–c)or bulk polymerization (e–f). Black curves, as prepared powders or films; gray curves,samples after heat treatment modeling the 1st DSC scan. Molecular masses aregiven in Table 2.

Scheme 2. Some of the stereoisomers of a poly(ECA) (E ¼ C(O)CH2CH3) andpoly(BCA) (E ¼ C(O)CH2CH2CH2CH3) homopolymers.



Figure 10. Selected 1H NMR traces (solvent acetone-d6) of polycyanoacrylatesobtained by bulk polymerization (a–c) or in suspension (e–f). Black curves, asprepared films or powders; gray curves, samples after heat treatment modeling the1st DSC scan. Sample (a) shows the tetrad assignment according to ref. 26. BA,before annealing; AA, after annealing. [Color figure can be viewed in the online issue,which is available at www.interscience.wiley.com.]

latter three maintain their concentrations. InpolyBCA obtained in bulk [Fig. 10(b)] the sameannealing results in a well-expressed increase ofthe intensities of the three tetrads between 2.8and 3.00 ppm, whereas in those three appearingin stronger field the amount of rrr and mmr tet-rads slightly diminishes. The respective 50:50copolymer [Fig. 10(c)] before and after heatingbasically reproduces the spectra of the respec-tive polyBCA samples.

The as-prepared and the thermally treatedsuspension polyECA showed almost coinciding1H NMR curves similar to that of the startingMethod A polymer [Fig. 10(d)]. The Method BpolyBCA [Fig. 10(e)] displays a spectrum similarto that of the bulk polymer [Fig. 10(b)]. How-ever, the effect of heating upon the suspensionpolyBCA is different: the relation between theconcentrations of the stronger and weaker fieldtetrads before and after annealing is not as dif-ferent as in the bulk product; there is only aslight increase of the rrr tetrads. As regards theECA-BCA suspension copolymer (10f), heatingdoes not seem the change the amounts of thestereoisomers.

CONCLUSIONS

On the basis of the spectral, thermal and chro-matographic studies the following conclusioncould be drawn:

(1) Bulk polymerized polyECA: Its molarmass decreases slightly upon heating to160 8C suggesting moderate degradationwithout liberation of volatiles and releaseof internal strain with no change in Tg.The more ordered syndiotactic and isotac-tic tetrads are affected stronger by thisprocess.

(2) Bulk polymerized polyBCA: Upon heatingto 130 8C, there occurs a significantincrease of the molar mass with broaden-ing of its distribution accompanied by anabrupt increase of the asymmetric tetrads.At the same time, there is evidence fordegradation associated with a slightdecrease of the rrr and rmr tetrads. TheTg values before and after annealingremain the same releasing some internalstrain as in method A polyECA.

(3) Bulk polymerized ECA þ BCA ¼ 50:50:Heating of this sample results in the crea-

tion of larger amount of asymmetric tet-rads eliminating some rrr and rmr tetrads.The molar mass of the copolymer beforeand after annealing remains almostunchanged. The multiple glass transitionsare, most probably related to the presenceof polyBCA and polyECA segments of vari-ous lengths and tacticities.

(4) Suspension polymerized polyECA: Anneal-ing does not change the Tg of this sample,nor the content of stereo isomers. Themolar mass increases slightly upon heat-ing but this is not enough to change theTg value. The polydispecity index alsoremains nearly constant. In other words,heating affects the suspension polyECAvery slightly.

(5) Suspension polymerized polyBCA: Thereis a well-expressed increase of the polydis-percity upon annealing due to degradationof some medium-length macromoleculesleading to a bimodal mass distribution.The more affected stereostructure is theisotactic one.

(6) Suspension polymerizedECAþBCA¼ 50:50:The coinciding NMR traces before and af-ter annealing of this sample suggests nochanges of the stereoisomer content. TheDSC traces are consistent with a transi-tion from two- to one phase system. Uponheating, the molar mass decreases accom-panied by an abrupt jump in polydisper-sity of the system. Initial formation ofphase-separated blend of homopolymerscan be suggested transforming into a ho-mogeneous system with only one unclearglass transition.

The authors thank for the financial support by thePortuguese Ministry of Education (SIUPI grant‘‘BioAd’’). MT is grateful for the tenure of a PhD grantby the Portuguese Science Foundation (SFRH/BD/13,432/2003). The technical support of P. Treppe(Institute of Polymer Research, Dresden) is gratefullyacknowledged.

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On the anionic homo‐ and copolymerization of ethyl‐ and butyl‐2‐cyanoacrylates

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Transcript of On the anionic homo‐ and copolymerization of ethyl‐ and butyl‐2‐cyanoacrylates