使用Biotage微波合成仪进行微波辅助下的高氧化烯烃的聚合反应by耐士科技

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耐士科技400-188-0725www.rysstech.comEuropean Polymer Journal 45 (2009) 899-910 Contents lists available at ScienceDirect European Polymer Journal ELSEVIER journal homepage：www.elsevier.com/locate/europ olj Microwave-assisted polymerization of higher alkylene oxides Muhammad Imran Malik， Bernd Trathnigg a.c*， C. Oliver Kappe Institute of Chemistry， Karl-Franzens-University， Heinrichstrasse 28， A-8010 Graz， AustriaChristian Doppler Laboratory for Microwave Chemistry (CDLMC)， Karl-Franzens-University， Heinrichstrasse 28， A-8010 Graz， AustriaCentral Polymer Labortaries / Molecular Characteristics (CePOL/MC)， Karl-Franzens-University， Heinrichstrasse 28， A-8010 Graz， Austria Article history：Received 1 August 2008Received in revised form 18 November 2008Accepted 20 November 2008Available online 3 December 2008 Keywords： Alkylene oxidesMicrowave-assisted polymerizationAnionic ring opening polymerizationChromatography Poly(alkylene oxide)s are commonly used for variousapplications [1]. Depending on the substituents of the oxi-rane ring， polymers with different properties are obtained：while poly(ethylene oxide)s (PEO) are quite polar and solu-ble in water， only the lower poly(propylene oxide)s arewater-soluble， and the polymers of higher epoxides - suchas butene oxide(BO)， hexeneoxide(HO)etc.-arehydropho-bic and can be dissolved only in non-polar organic solvents. As poly(ethylene oxide)s with hydrophobic end groupsshow amphiphilic properties， they are in widespread useas non-ionic surfactants and emulsifiers. The hydrophobicpart of such molecules can be alkyl， acyl or alkyl-arylgroups， but also blocks consisting of propylene oxide(PO) or higher epoxides. The blocks in suchamphiphilic polymers can bearranged in a different way： as AB diblock and as ABA or ( * C orrespon d ing author . A d dress： Central P o lymer Labortaries/MolecularCharacteris t ics (CePQ L /MC)， Karl-Franzens-University， Heinrichstrasse 2 8 ，A-8010 Graz， Austria. Tel.： +43 316 3805328； fax： + 43 316 3 8 09840. ) ( E-mail address： b ernd . trathnigg@uni-graz. a t (B. Trathnigg). ) ( 0014-3057/$ - see front matter @ 20 0 9 Published by Elsevier Ltd. doi：10.1016/j.eurpolymj.2008.11.035 ) BAB triblock copolymers. The most common class of suchblock copolymers are the poloxamers (EO-PO-EO)， whichare commercially available under different trade names(Pluronic， Synperonic， Imbentin etc.) [2-4]. A limitation of otherwise very useful block copolymersof PEO-PPO or PEO-PPO-PEO is their very high critical mi-celle concentration (CMC) due to weak hydrophobicity ofthe PPO block， which restrict their applications becausethe formed micelles are not stable and are easily de-stroyed by dilution. Peihong Ni et al.[5] introduced poly-octafluoropentyl methacrylateas outer blockkoncommercially available triblocks PEO-PPO-PEO (PluronicF127) (making pentablock copolymers) to reduce CMC ofthese products. Another approach to reduce the CMCcould be to make a diblock or triblock by using higherepoxide (such as butene oxide and hexene oxide) ashydrophobic block or making a brush like tri or tetrablock，which can be prepared by subsequent addition of the nextmonomer having otherwise the same backbone with onlythe pendant group changed in order to achieve morehydrophobicity. While polymers of ethylene oxide (EO)and propylene oxide (PO) are produced in large amounts[6，7]， this is not the case with the higher epoxides， eventhough they may be very useful as hydrophobic buildingblock in amphiphilic molecules. www.rysstech.com An important feature of poly(butylene oxide)s (PBO)and higher poly(alkylene oxide)s and their copolymers istheir solubility in highly non-polar liquids (such as siliconeoils) [8]. They have very low glass transition temperaturesand other interesting physical properties， as they have a di-pole moment component parallel to the chain [9]. Epoxides can be polymerized by different mechanisms：coordinate-initiated， cationic or anionic [1]， which involvethe use of different catalysts. In the literature， butene oxideand hexene oxide have been polymerized using lithium ascounter ion and tert. butyl polyiminophosphazene base ascomplexing agent [10，11]， a double metal cyanide (DMC)catalyst [12]， and potassium alcoholate in combinationwith crown ethers [9] etc. The cationic mechanism is not very favorable in thesynthesis of block copolymers because of ring-chain equi-libria (backbiting)，which lead to the formation of productswith a different architecture (from linear and cyclic homo-polymers to multiblocks). Coordinate-initiated polymerization is suitable for theproduction of homopolymers； in the synthesis of blockcopolymers， however， such catalysts may also yield chainswith a different functionalitv. Anionic polymerization may avoid these problems， butthere are also side reactions in the polymerization of pro-pylene oxide， which are due to the relatively high acidityof the methyl group [1，9，13，14]： chain transfer to themonomer (Scheme 1) and to the penultimate unit of thepolymer chain (Scheme 2). Both reactions lead to polymerswith unsaturated end groups， and the chain transfer withinthe chain produces the anion of the corresponding diol be-sides the chain with a terminal allyl group. These reactions cannot be completely avoided， but onemay minimize them by choosing appropriate reaction con-ditions 14]. The polymerization can also be performed under micro-wave irradiation. Microwave dielectric heating is a verypopular technology in organic synthesis [15] and is rapidlyfinding its ways in polymerization reactions [16-18]. It hasmany advantages as compared to conventional heatingapplying conduction phenomena， such as rapid and uni-form heating. In some instances it has been shown to re-duce polymerization reaction timesfrom hoursttominutes [14] and from minutes to seconds in organic syn-thesis [15]. In the first paper of this series [14] we have shown theeffect of reaction conditions on formation of side productsin the polymerization of propylene oxide. Obviously， sidereactions are a real problem in the synthesis of blockcopolymers， as they may result in considerable amountsof the homopolymers. These reactions are supposed to besuppressed in the anionic polymerization of higher epox-ides due to steric effects of the ethyl or butyl pendantgroups on the epoxide ring [9，11]. The length and nature of the pendant group may influ-ence the physical properties of block copolymers consider-ably. As Darling [19] has shown in a recent review， the sizeand shape of their self assembly are tuneable through thesynthetic chemistry of the constituent molecules. Inthis context butene and hexene oxides can be used to syn-thesize very interesting architectures such as combs HO Scheme 2. Chain transfer to the penultimate unit in alkylene oxidepolymerization. Such structures can， in principle， be synthesized step-wise (by subsequent addition of the monomers after com-plete consumption of the previous monomer). For this purpose a very high product purity (at eachstep) is desirable， as the physical properties of an ABC tri-block copolymer may strongly be influenced by the homo-polymers (A， B， C) and AB or BC diblocks， which may beformed by the side reactions mentioned above. While the formation and identification of side productshas been studied intensively in the polymerization of PO[14]，not much attention has been paid to the polymeriza-tion of higher epoxides [9]. This may be due to the fact thatsuitable methods for their characterization are not avail-able in most laboratories. To ensure the desired high prod-uct purity reliable methods for their characterization hadto be developed (which are described in full detail in an-other publication [20]). In the present study， the microwave-assisted anionicring opening polymerization of higher epoxides (such asbutene oxide and hexene oxide) is reported. The effect ofdifferent parameters on unwanted side products is discussed. The procedure for the synthesis of fairly cleanhomopolymers and multiblock (brush and conetype)copolymers is proposed. 2. Characterization of the products Functional polymers and oligomers can be character-ized using different chromatographic techniques， whichseparate according to different criteria. The type of chro-matographic separation is determined by the so-calledinteraction parameter c， which describes the interactionof a structural unit with the stationary phase [21]. This parameter is negative in Size Exclusion Chromatog-raphy (SEC) and positive in Liquid Adsorption Chromatog-raphy (LAC). At the Critical Adsorption Point (CAP) theinteraction parameter equals zero， and the correspondingblock becomes“chromatographically invisible”22-251This effect is utilized in Liquid Chromatography under Crit-ical Conditions (LCCC)， which allows the separation of apolymer homologous series according to their end groups[23，26-281. In the case of amphiphilic polymers the interactionparameters cA and CB of the individual structural units (Aand B) may have different values. If both ca and cg are neg-ative， the molar mass distribution (MMD) can be deter-mined by Size Exclusion Chromatography (SEC)， with Scheme 3. Comb type polymer： poly(hexene oxide). Scheme 4. Brush type copolymer： poly(ethene oxide-b-hexene oxide). Scheme 5. Cone type triblock copolymer： poly(ethene oxide-b-propyleneoxide-b-butene oxide). Scheme 6. Cone type tetrablock copolymer： poly(ethene oxide-b-pro-pylene oxide-b-butene oxide-b-hexene oxide). retention decreasing with increasing molar mass， and onlya minor influence of chemical composition. SEC yields theoverall MMD； with dual detection [29-31] one may obtaininformation on the chemical composition along the MML[31-33]. SEC is， however， not capable of discriminating amixture of block copolymers and homopolymers. In AB diblock copolymers (with A=EO and B=PO or an-other epoxide) one may use the following strategies： At theCAP for the A unit， a separation according to the B blockmay be achieved by a SEC or a LAC mechanism， while atthe CAP for the B block the length of the A block may be ob-tained in the same way. The application of these methodsto EO-PO block copolymers has been described recently[2，14，34，35]. In principle， similar procedures can be applied to ABAand BAB triblocks. It must be mentioned， that the invisibil-ity concept is only applicable to external blocks. At the CAPfor A one can expect the same elution behavior of thehomopolymer B， the diblock AB， and the triblock ABA. Forthe BAB triblock retention still depends on the length ofthe block A. This effect can be utilized to separate the indi-vidual oligomers of the A block， as has been shown fordiesters of PEG with fatty acids [36]. The situation is even more complicated with ABC tri-blocks，where (for example) A=EO，B=PO， and C=BO. Inthis case， one may have critical conditions for one of theblocks， while the interaction parameters of the othersmay be positive or negative. The only useful situation is acombination of two negative interaction parameters (suchas ca<0， CB<0) at the CAP for the third one (cc=0). In this case， the homopolymer A (i.e. the initiator)， theAB diblock and the homopolymer B (from chain transfer)will elute in the SEC range (as also the desired ABC www.rysstech.com triblock)， while the homopolymer C will elute approxi-mately at the void volume， hence it can be identified anddetermined quantitatively. The determination of residual initiator at the CAP is notpossible in an ABC triblock， as there is no CAP for a copoly-mer (AB). This is， however， not really necessary： under theconditions used in this study， the conversion of monomerand initiator is complete before the addition of the nextmonomer. Of course the purity of the diblock used as initi-ator must be as high as possible. Once the diblock has beencharacterized as described above， the content of homopol-ymer B in the diblock can be determined by LCCC (at theCAP for B). Consequently， LCCC at the CAP for C and SEC for A and Bis an excellent tool to determine the content of the homo-polymer C in an ABC triblock， as has been shown in anotherpublication [20]. 3. Experimental 3.1. Polymerizations Propylene oxide (PO)， butene oxide(BO)， hexene oxide(HO) and n-alcohols (butanol， hexanol) were purchasedfrom Fluka (Buchs， Switzerland). The alcohols were driedover anhydrous sodium sulfate while epoxides were re-fluxed over calcium hydride prior to use. PEG-MME 1100was purchased from Fluka (Buchs， Switzerland) and wasdried by azeotropic distillation with toluene. Sodium hy-dride (60% dispersion in mineral oil) from Aldrich was usedas received. The polymerization of PO， BO and HO was performed byanionic ring opening using the different initiators (the cor-responding alcohols and PEG-MME 1100) and sodium hy-dride as co-initiator. All polymerizations were performedin 2-5 ml (filling volume：)) microwave process vials (Bio-tage AB)， which were oven dried under vacuum prior touse. The initiator (alcohol or PEG-MME 1100) and sodiumhydride were added to the vials which were subsequentlysealed with an aluminum crimp top and Teflon septum.The vials were then evacuated and purged with argon (3cycles). The calculated volume of epoxide was added witha syringe through the septum. The quantity of sodium hy-dride was 0.1% of total product and quantities of initiator(alcohol or PEG-MME 1100) and epoxide in differentexperiments depend upon the intended average numberof monomer units in the chain (calculated by molar ratios).The sealed microwave process vials were subsequentlyintroduced into the cavity of the single-mode microwavereactor (Biotage Initiator Eight EXP 2.0， absorbance level：high). The reaction temperatures ranged from 160 to200℃， the monomer-to-initiator ratio in different experi-ments ranged from 30：70 to 70：30. In the following textwe shalluse the common term “monomer-to-initiatorratio”. In the figures we shall always give the amount ofthe initiator first (i.e. the initiator： monomer). We have already reported the optimization of reactionconditions for the microwave-assisted polymerization ofpropylene oxide [14]. In this study we used propyleneoxide for the synthesis of some special structures (cones) where we synthesized tri and tetrablock copolymers，otherwise our main emphasis was on higher epoxides. As propylene oxide has a low boiling point， the auto-genic pressure generated by this starting material in com-bination with the online pressure monitoring of themicrowave reactor [14，15] provides a convenient methodfor monitoring the conversion in these solvent-free poly-merization processes. As the pressure in the vials is measured by deformationof the septum， the precision and accuracy of the measure-ment is， however， limited. Even at complete conversion ofthe monomer the readout was sometimes larger than zero，which was due to a permanent deformation of theseptum. In the case of butene oxide， the initial pressure in themicrowave process vial iat reaction temperatures of160℃ was typically in a range of 3 and 10 bar and de-creased during the polymerization as monomer was con-sumed. The lower vapor pressure of BO and HO does notallow an accurate determination of conversion， hence wehave determined the residual monomer in the samplesby SEC， as will be described later on. For complete conver-sion of butene oxide， the total reaction time ranged from 1o2h. In the case of hexene oxide， the boiling point of themonomer is even higher (118-120℃) so the initial pres-sure in the reaction mixture at 160°℃ was too low to bemeasured. At higher temperatures (180， 200°C) the initialpressure was somewhat higher (at 200 ℃ about 3 bars)，but still too low for an accurate determination of conver-sion. By chromatographic determination of the residualmonomer we could show， that complete conversion isreached at 160°C after 4 h. After the polymerization the active chain end was neu-tralized by adding an equimolar quantity of acetic acid. The same procedure was followed for the synthesis ofamphiphilic polymeric cones where first monomer to bepolymerized was propylene oxide. In each synthesis， weused several vials containing the same reaction mixture.After complete conversion of the monomer one vial wasopened， in which the polymerization was stopped by neu-tralization of the active chain ends (as described above)，and the product was analyzed by SEC and LCCC. Whenthe reaction vials had cooled down to approximately100°C， the next monomer was added to the other vialsby a syringe through the Teflon septum without unsealingthe vials， which were again introduced into the cavity ofthe microwave reactor. This procedure was then repeatedwith the next monomer. The reaction temperature was 160°℃ during synthesisof all cones (triblock and tetrablock copolymers). Afterpolymerization of last monomer the active end was neu-tralized by adding an equimolar quantity of acetic acid. 4. Chromatography These investigations were performed using a densitydetector (according to the mechanical oscillator principle)DDS70 (CHROMTECH， Graz， Austria) in all chromato-graphic systems. Data acquisition and processing was performed using the software CHROMA， which has beendeveloped for the DDS70. In SEC， we used a modular system consisting of aGynkotek 300 C pump， a VICI injector equipped with a100 pl sample loop， two column selection valves Rheodyne7060 (Rheodyne， Cotati，CA， USA)， a density detection sys-tem DDS 70 (Chromtech， Graz， Austria) coupled with aBischoff 8110 refractive index detector (Bischoff， Leonberg，Germany). All SEC measurements were performed in chlo-roform on a 7.8x600 mm Phenogel 100 A column (Phe-nomenex， Torrance， CA，USA) at a flow rate of 1.00 ml/min. Sample concentrations were 3.0-10.0 g/l. The columns were calibrated with PPGs from Sigma-Al-drich and Fluka (Buchs， Switzerland). Molar mass distribu-tions were calculated using CHROMA. In LCCC， the mobile phase was delivered by an ISCO2350 HPLC pump (ISCO， Lincoln， Ne， USA) at a flow rateof 0.5 ml/min. Samples were injected using an autosamplerSpark SPH 125 Fix (Spark Holland， Emmen， The Nether-lands) equipped with a 20 pl loop. A refractive index detec-tor RI 2414 (Waters) or a SEDEX 45 ELSD (Sedere， France)was connected to the DDS 70. Nitrogen was used as carriergas， and the pressure at the nebulizer was set to 1.0 barEvaporator temperature： 30°C. LCCC was performed onthe following columns： Jordi Gel DVB 500 RP column (100% poly-divinylben-zene； 250×4.6 mm； particle diameter=5 um；poresize=500 A， from Jordi， Bellingham，MA， USA) Nucleosil 100-5 OH 5 um， silica-baseddiolphase；250×4.6 mm； particle diameter： 5 um； pore diameter：100 A (Macherey-Nagel， Dueren， Germany). In all systems， the columns and density cells wereplaced in a thermostatted box， in which a temperature of25.0C was maintained for all measurements in LCCCand30.0°C in SEC. Mobile phases were mixed by mass and vacuum de-gassed， their composition was controlled by density mea-surement using a DMA 60 density meter equipped with ameasuring cell DMA 602 M (A. Paar， Graz， Austria). The solvents (chloroform， acetone， THF and water， allHPLC grade) were purchased from Roth (Karlsruhe， Ger-many) and Merck (Darmstadt， Germany). PEG and PPGstandards were purchased from Sigma-Aldrich and Fluka(Buchs， Switzerland). 5. Results and discussion As has been discussed in several publications， the anio-nic ring opening polymerization of epoxides is accompa-nied by several side reactions [13，14，35]. This may lead to a final reaction mixture containingimpurities like unreacted reagents (initiator and mono-mer)， homopolymer diols (from chain transfer to water)and homopolymers with butenyl or hexenyl end group(from chain transfer to monomer or to the penultimateunit of the polymer chain) along with the block copolymer(targeted product). Althoughthese impurities cannottbecompletelyavoided， their concentration should at least be kept aslow as possible. This is especially important， if the diblock copolymers are used as starting material in the synthesis oftri and tetrablocks. First of all， complete conversion of monomer and ini-tiator should be achieved in order to avoid the presenceof unreacted reagent in the products. In the case of POthe internal pressure gives a good estimate for monomerconversion， this is， however， not very precise for higherepoxides， the vapor pressure of which is considerablylower. As has been described previously [20]， residual mono-mer can be determined by SEC on a narrow pore column.If a low molecular initiator is used， its concentration mayalso be determined by SEC (as long as its peak does notcoincide with the monomer peak). As the first step in this study we have tried to find reac-tion conditions (reaction time and temperature)， underwhich complete conversion of monomer and initiator isachieved. For this purpose we have synthesized variouspolymers with alkyl end groups by using n-alcohols as ini-tiator and sodium hydride as the catalyst. As can be seen in Fig. 1，the peaks of the monomer andthe alcohol can be separated from the polymer peak by SECon a narrow pore column， as both molecules are suffi-ciently smaller than the first oligomer. If the alcohol hasthe same number of carbon atoms as the monomer， itelutes somewhat later. In this case both peaks are reason-ably separated from each other to be detected easily. Oneof the samples shown here contains still residual monomerand initiator， in the second one there is still some mono-mer， but no initiator， and in the third one both have beenconsumed completely. In the synthesis of amphiphilic block copolymers， theinitiator will be a monofunctional hydrophilic polymer(such as PEG-MME). In this case the monomer peak willbe separated from the solvent peak and the polymer peakhence it can be easily integrated. Evidently， this sampledoes not contain residual monomer. The peaks of the(polymeric) initiator and the copolymer will， however，overlap (see Fig. 2). This problem can be solved by LCCC， as has been shownpreviously [20]： on a normal phase column (Nucleosil 100-5 OH) critical conditions for EO are found in acetone-watercontaining 77 wt-% acetone. In such a mobile phase the polymers of the more hydro-phobic alkylene oxides (PO， BO and HO) elute in SEC mode.As can be seen in Fig. 3， the initiator (PEG-MME)， whichelutes close to the void volume， is sufficiently separatedfrom the peak of the copolymer (which co-elutes withthe homopolymer of the hydrophobic monomer). Eventhough the samples shown here were synthesized with ahigh initial concentration of the initiator， they do not con-tain residual initiator. Consequently， the appropriate conditions for furtherinvestigations were selected， under which complete con-version of monomer and initiator was achieved. At a poly-merization temperature of 160°C the required reactiontime was 1 h for PO， 2 h for BO and 4 h for HO. In the synthesis of block copolymers by anionic ringopening polymerization of epoxides the main problem isthe formation of homopolymers by chain transfer to themonomer. As has been shown previously[14]， the amount Fig. 1. Overlay of SEC chromatograms of poly(butene oxide)： Phenogel 100 A； chloroform， 1.0 ml/min； density detection. Fig. 2. SEC of a diblock copolymer of hexene oxide and PEG-MME 1100， the monomer and the initiator (PEG-MME 1100). Detection： RI. of homopolymer in a copolymer depends on the monomerconcentration in the reaction mixture and on the polymer-ization temperature. In the case of propylene oxide， themonomer-to-initiator ratio has a strong effect on the chaintransfer reaction leading to homopolymers with allylic endgroup [14]. This reaction could be suppressed by slowaddition of the monomer to the reaction mixture， whichwould， however， require long reaction times. In microwave-assisted batch polymerization reactionsusing sealed vessels， generally all the reagents have to beadded at the beginning， as it is not easy to introduce re-agents during the microwave experiment [15]. In the case of higher epoxides the probability of chaintransfer to the monomer is somewhat lower than with pro-pylene oxide due to the steric effect of ethyl or butyl pen-dant groups on the epoxide ring[9，11]. Although this chaintransfer reaction is less important with higher alkyleneoxides， it can not be completely avoided. In order to see the effect of reagent ratios on the MMDand the homopolymer content of the products we have syn-thesized block copolymers of BO and HO with PEG-MME1100 as initiator and different monomer-to-initiator ratios. Fig. 4 shows a comparison of the MMD of PEG 1100-b-PBO samples with different monomer-to-initiator ratiosand the initiator (obtained by SEC). As can be seen， the molar mass of the samples increasewith the monomer-to-initiator ratio. The MMDs of thecopolymers appear to be monomodal， but as they overlapwith the MMD of the initiator， it is not clear from SEC(with just one detector)， whether they contain somehomopolymer. Additional information can be provided bydual detector SEC. When a mass m ofa copolymer consisting of the mono-mer units A and B elutes in the interval i of the peak，theweight fractions WA and WB (=1-WA) of the components Aand B (wA) can be calculated from the areas x， obtained elution volume (mL) Fig. 3. LCCC at the CAP for PEG-MME and SEC conditions for the hydrophobic block. Nucleosil 100-5 OH in 77.93 wt-% acetone-water， ELS detection.Reaction temperature=160 °C， monomer： initiator=30：70). Fig. 4. Effect of monomer-to-initiator ratio on MMD of diblock copolymers， as obtained by SEC on Phenogel 100 A (reaction temperature=160C). for this interval from both detectors， provided that the re-sponse factors fA and f B are sufficiently different (whereini denotes the individual detectors) From the composition of a fraction one may calculate themass mi using equation 2 As can be seen in Fig. 5， the lower molecular fraction ofthis sample seems to contain more BO than the main frac-tion， which indicates， that the sample contains indeedsome homopolymer. This can be proven by LCCC the CAP for poly(buteneoxide)， which has been found on the Jordi columns84.55 wt-% THF [20]. As can be seen in Fig. 6， the contentof the homopolymer (which elutes at 2.9 mL) increaseswith the monomer-to-initiator ratio： while the samplewith PEG-MME to BO of 70：30 contains only traces of thehomopolymer， the sample with 30：70 contains a consider-able amount of PBO. The latter case is， however， less important： in the syn-thesis of tri and tetrablocks， the monomer-to-initiator raticin each step will be rather low. Another important parameter is the polymerizationtemperature. As the reactivity of the monomers decreasesin the order PO>BO>HO， the required reaction time at agiven temperature increases in the same order. Thereforewe have polymerized HO with PEG-MME 1100 as initiator mo molar mass， M [g/mol] . at different temperatures (160℃，180℃ and 200℃) andanalyzed the products thus obtained by SEC and LCCC. With an excessofmonomer(initiator： mono-mer=30：70) a pronounced shoulder appears at the lowmolecular side of the MMD， which gets higher withincreasing temperature (Fig.7). As expected， the effect is much less pronounced at amonomer-to-initiator ratio of 50：50. As can be seen inFig. 8， the shoulder is almost negligible at the lowest tem-perature in this series (160 ℃). Dual detector SEC shows，that the low molecular shoulder contains the homopoly-mer of HO (Fig. 9). Again LCCC is an excellent tool to determine the amountof the homopolymer： at a monomer-to-initiator ratio of 50：50， the homopolymer peak increases with reactiontemperature (Fig. 10). Obviously，lower temperatures would result in a lowercontent of the homopolymer. Below 160C there was，however， an enormous increase in reaction times. We havealready reported that total reaction time dramatically in-creases with decreasing temperature in the polymerizationof propylene oxide，which is undoubtedly the most reac-tive monomer of this class [14]. As microwave instruments are not recommended to usefor too long reaction times (our limit in this context is 4 h)the following experiments were performed at 160℃. In the synthesis of tri and tetrablock copolymers thepurity of the intermediates (di and triblock， respectively)， PEG-MME 1100-b-PHO (30：70) molar mass， M [g/mol] Fig. 7. Effect of temperature on the MMD of PEG-MME1100-b-PHO， as obtained by SEC on Phenogel 100 A (monomer： initiator=70：30). Fig. 8. Effect of temperature on the MMD of PEG-MME1100-b-PHO， as obtained from SEC on Phenogel 100A (monomer：initiator=50：50). has to be determined. For the diblock this can be done asdescribed above. The analysis of an ABC triblock (and ABCDtetrablocks) is， however， a more difficult task. In principle， the analysis of an ABC triblock with respectto chemical composition would require three signals fromconcentration detectors with different sensitivity for theindividual monomer units (this may also be IR- or UV-absorption at different wavelengths). In the case of ali-phatic polyethers such a detector combination cannot beapplied. For a highly homogeneous diblock dual detectionusing an average response factor of AB might be applied.In practice， this does not work， as both blocks in the AB di-block are typically polydisperse in molar massandcomposition. SEC with single detection provides only an informationon the overall increase in molar mass after each step， ascan be seen in Fig. 11. If considerable amounts of homopoly-mers were present，one should see them as a shoulder on thelow molecular end ofthe MMD (as the homopolymer formedby chain transfer should have a lower molar mass). In thesamples shown here this seems to be not the case. This information can be obtained by LCCC： if the homo-polymers A and B elute in SEC mode at the CAP for C， thiswill also be the case for the AB diblock [20]. Under such conditions， the homopolymer of C will beseparated from all other polymers SpPresent in the product(A， B， AB， AC， ABC)， hence it can be easily identified andquantified. m molar mass， M [g/mol] o Fig. 9. Molar mass distribution (MMD) and chemical composition along MMD of PEG-MME 1100-b-PHO (30：70，200℃， see Figure 7)， as determined by SECwith dual detection. elution volume (mL) Fig. 10. Effect of reaction temperature on homopolymer content of PEG-MME 1100-HO block copolymers (50：50) at the CAP for poly(hexene oxide) (Jordi，87.04% THF-water； ELS detection). The same will be true for ABCD tetrablocks： at the CAPfor D and SEC conditions for A， B， and C one may determinethe content of the homopolymer D in the product. If the precursor polymers (which are used as initia-tor) are sufficiently pure (which can be verified as de-scribed above)， one can easily determine the amountof the homopolymer formed ineach step of thesynthesis. Fig. 12 shows an overlay of chromatograms of di tri andtertrablock copolymers (corresponding to Scheme 4-6)，which were obtained on the Jordi column at the CAP for the last block， respectively. As can be seen， the shoulderbetween 2.5 and 3.0 mL shows the presence of the corre-sponding homopolymer. The phase behavior and properties in solution of thesetri and tetrablock copolymer (amphiphilic polymeric conessee Scheme 5 and Scheme 6) and diblock copolymers(amphiphilic polymeric brushes with different lengths ofpendant group on main chain， see Scheme 4) having PEGas hydrophilic and propylene oxide， butene oxide and hex-ene oxide as hydrophobic moieties are under investigationand shall be reported soon. elution volume (mL) Fig. 11. Overlay of SEC chromatograms after each step of polymerization for tetrablock copolymer on Phenogel 100 A；1.0 ml/min； density detection. Fig. 12. LCCC of an ABCD tetrablock copolymer and its precursors， as obtained on the Jordi column in THF-water mobile phases of different composition(corresponding to the CAP for the last monomer unit). ELS detection. 6. Conclusion Microwave-assisted polymerizations are quite fast so itis convenient to study the effects of different reactionparameters on the quality of polymer products and to opti-mize conditions in a reasonably short time frame. The an-ionic ring opening polymerization of higher alkyleneoxides under microwave irradiation is a rapid way of syn-thesis of fairly clean multiblock amphiphilic copolymerbrushes and cones. Acknowledgements M.I.M. thanks the Higher Education Commission ofPakistan for a Ph.D. scholarship. We also thank Biotage AB((IUppsala， Sweden) for providing the microwaveinstrumentation. References [1] Bailey FE， Koleske JV. Polymerization of 1，2-Epoxides.NewYork： Marcel Dekker； 1991. p. 35-103. ( [2] Trathnigg B. Polymer 2005；46：9211-23. ) ( [3] Alexandridis P， Alan Hatton T. Colloids Surf A： Physicochem E ng A spects 1995；96：1-46. ) [4] PerreurC， Habas J-P， Lapp A， Peyrelasse J. Polymer2006；47：841-8. [5] He J， Ni P， Liu C. J Polym Sci A： Polym Chem 2008；46：3029-41. 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