Macromolecules 1994, 27, 4908-4913
4908
Anionic Polymerization of Acrylic Monomers. 21. Anionic Sequential Polymerization of 2-Ethylhexyl Acrylate and Methyl Methacrylate J.-S. Wang, R. J6r6me, Ph. Bayard, and Ph. Teyssi6' Centre for Education and Research on Macromolecules (CERM), University of Likge, Sart - Tilman, B6, 4000 Likge, Belgium Received March 30, 1994; Revised Manuscript Received June 15, 1994'
ABSTRACT A mixed complex of a polydentate lithium alkoxide, i.e., lithium 2-(2-methoxyethoxy)ethoxide (LiOEEM),and of a mono- or bisfunctional organolithium initiator, i.e., (diphenylmethy1)lithium(DPMLi) or lithium naphthalene/diphenylethylene,has been used to synthesize well-controlled AB (BA), ABA, and BAB block copolymers of methyl methacrylate (MMA)(A)and 2-ethylhexyl acrylate (2EtHA) (B),irrespective of the monomer addition order. Although the 2EtHA block copolymerizationinitiated with monofunctional living PMMA macroanions at -78 "C in a 75/25 toluene/THF mixture gives rise to a precisely-tailored PMMA-b-P2EtHAdiblock polymer,the reversesequence,i.e., from P2EtHA anionsto the MMA type monomer, always results in contamination by homo-P2EtHA. This has been interpreted in terms of the short shelf lifetime of P2EtHA anions present. Nevertheless, a pure P2EtHA-b-PMMA type diblock copolymer can be produced at -100 "C, while keeping other conditions unchanged. More importantly, a well-controlledPMMAb-P2EtHA-b-PMMAtriblock copolymer has been prepared through a three-stage processwith a monofunctional initiator, acutally providing a potential pathway toward the direct synthesis of a novel type of fully acrylic thermoplastic elastomer, instead of the more complicated hydrolysis/transalcoholysis process previously demonstrated by us.
Introduction It has been recently demonstrated by us that d o dual polydentate lithium alkoxides, such as 242-methoxyethoxy) ethoxide (LiOEEM), can efficiently form a stable and bulky p l a complex with lithium ester enolates in solution' and should consequently be very effective in promoting the living anionic polymerization of (meth)acrylic ester monomers.2 For instance, it has been reported for the first time that a living anionic polymerization process of 2-ethylhexyl acrylate (2EtHA) becomes pos~ i b l ewhen , ~ some of these LiOEEM-complexed organolithium initiators are used. Moreover, a highly syndiotactic living poly(methy1methacrylate) (PMMA) (75-85 % ) can be efficiently produced even in pure toluene at moderate temperature by using the same initiator system^.^ On those bases, attention has been paid to the potential block copolymerization of these two monomers. Indeed, soft P2EtHA (Tg -60 "C) and hard syndiotactic PMMA (- 80% ' ) syndiotacticity; Tg 130"C)represent an acrylic adhesive elastomer and a methacrylic plastic, respectively. Also, a novel type of fully acrylic-based thermoplastic elastomer (TPE) can thus be generated by the association of PMMA and P2EtHA in a triblock structure. The present work aims at producing A-B (B-A) diblock and ABA or BAB triblock copolymers by using (diphenylmethy1)lithium or lithium naphthalenamide as a monoor bisfunctional initiator, and LiOEEM (10 mol equiv) as a ligand, in a 75/25 toluene/THF mixture at low temperature. To our knowledge, this is the first report on well-controlled block copolymers of PMMA and P2EtHA synthesized by means of "ligated" living anionic polymerization, whichever the order of monomer addition.
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Experimental Section Reagents and Solvents. 2EtHA and MMA obtained from Norsolor (France) were first vacuum distilled after reflux over CaH2and stored under a nitrogen atmosphere at -20 "C. Before
* To whom all correspondence should be addressed.
* Abstract published in Advance ACS Abstracts, August 1, 1994. 0024-9297/94/2227-4908$04.50/0
polymerization, the monomer was added with 10 wt % AlEtS solution in hexane until a persistent yellowish green color was observed and then distilled under reduced pressure. THF was purified by refluxing over a fresh sodium benzophenonecomplex (a deep purple color indicating an oxygen- and moisture-free solvent). Toluene and hexane were refluxed over CaH2 for 48 h and redistilled over oligo(styry1lithium)immediatelyprior to use. Alcohols from Aldrich were distilled from CaH2 and stored under nitrogen. Initiator. (Diphenylmethy1)lithium(DPMLi) was prepared at room temperature by treating diphenylmethane with lithium naphthalene in THF for 24 h. The lithium naphthalene/THF solutionwas prepared by reacting lithium metal with naphthalene in THF at room temperature. Preparation of LiOEEM. Two different methods were envisionedto prepare the ligand. (1)Under oxygen-and moisturefree argon, an equimolar amount of lithium metal (wire) and corresponding alcohol were added into THF in a dry flask equipped with an inert gas inlet, a magnetic stir bar, and a watercooled condenser. The solution was heated to reflux and the reaction allowed to continue at reflux overnight. (2) Lithium alkoxide was generated by treating the suitable alcohol in dry hexane with an equimolar amount of n-BuLi at 0 "C. In this case, a few drops of 1M dihenylethylene in hexane were used as a color indicator. Polymerization. Anionic block copolymerizationof 2EtHA and MMA was carried out in a flamed glass reactor under a nitrogen atmosphere. Solvent, ligand, and initiator were transferred into the glass reactor byusing a rubber septum and a stainless steel capillary or a syringe. The initiator solution was added dropwiseto the solvent,containingthe polydentate lithium alkoxide ligand, until the initiator color persisted; ca. 5 drops of a 0.2 M initiator soluton to 100 mL of solution was generally required to get a persistent color. After adding the desired amount of initiator, the solution was cooled to -78 or -100 "C, the required quantity of MMA or 2EtHA was introduced, and the polymerization was performed at that temperature for a few minutes. Upon addition of the monomer, a sudden change of the initiator color from deep red to light yellow indicates that initiation was practically instantaneous. An aliquot of the reaction medium was withdrawn for anlysis by SEC in order to determine the molecularweight and molecularweight distribution of the first block. The second monomer was then added to the solution containing the first living polymer at -78 or -100 " C , 0 1994 American Chemical Society
Anionic Polymerization of Acrylic Monomers. 21 4909
Macromolecules, Vol. 27, No. 18, 1994
Table 1. Characterization Data for PMMA-bP2EtHA Diblock Copolymerizations in a 75/25 Toluene/THF Mixture at -78 OC, Using a 10 Molar Ratio LiOEEM/DPMLi Initiator System monomer (mol) % total for PMMA for diblock copolymer DPMLi no. (108mol) MMA 2EtHA yield Mn (talc) Mn (SEC) Mw/Mn (SEC) f" Mn ( c ~ c )Mn(SEC) Mn ('H NMR) Mw/Mn(SEC) fb 0.364 0.428 0.129 0.129
1 2 3 4
0.02 0.03 0.04 0.08
0.027 0.015 0.0105 0.0071
99 100 100 100
5500 7000 31000 62000
6000 7200 33000 64500
1.03 1.06 1.08 1.10
0.92 0.97 0.94 0.96
18500 12500 45 200 72700
19600 13600 46 400 76600
19 200 13 000 46 000 77 000
1.09 1.09 1.09 1.10
0.96 0.96 0.98 0.94
a f (initiatorefficiency) = M,(calc)/M,(SEC),whereM,(calc) = (molesof monomer/moles of initiator)(M,of monomer). f (initiatorefficiency) = M,(calc)/M,(lH NMR).
Table 2. Characterization Data for P2EtHA-bPMMA Diblock Copolymerization in a 75/25 (v/v) Toluene/THF Mixture, Using a 10 Molar Ratio LiOEEM/DPMLi Initiator System for P2EtHA for diblock copolymer DPMLi monomer temp % total no. (108mol) 2EtHA MMA ("C) yield M.(calc) M,,(SEC) Mw/Mn(SEC) f M,(calc) M,,(SEC) M.(lHNMR) Mw/Mn(SEC) f 1 2 3 4 5 6 a
0.392 0.217 0.375 0.455 0.472 0.426
0.0135 0.0135 0.0201 0.0135 0.0168 0.0268
0.0375 -78 0.0461 -78 0.0596 -78 0.110 -100 0.0552 -100 0.0213 -100
96 100 99 95 100 100
5100 9200 8000 4400 5300 9400
5600 loo00 8100 4900 5300 loo00
1.09 1.09 1.09 1.10 1.06 1.05
0.91 0.92 0.99 0.90 1.00 0.94
15000 31 200 24000 29000 17000 15000
81 1W 122oooO 43 3 W 31400 19100 16000
1.04O 1.07O 1.090 1.10 1.04 1.06
31 500 19 000 16 500
0.92 0.90 0.91
Only one peak on the high molecular weight side (i.e., diblock copolymer) is considered.
n
and the copolymerization was carried out for an additional few minutes. It was then stopped by adding acidic methanol, and the copolymer was recovered by precipitation into methanol. The crude polymer was dried under vacuum at 60 O C for 48 h. Characterization. SEC was carried out by using a HewlettPackard 1090liquid chromatograph equipped with columns (106, 109, 500 and 100 A) and a Hewlett-Packard 1037A refractive index detector. Polystyrene standards weLe used for calibration and the number- (M,) and weight-average(M,) molecular weights as well as polydispersity of the polymer were accordingly calculated. The copolymer's composition was determined by lH NMr using a Bruker AM 400 spectrometer.
Results and Discussion Synthesis of AB and BA Diblock Copolymers of Methyl Methacrylate (MMA) (A) and 2-Ethylhexyl Acrylate (2EtHA) (B). Tables 1 and 2 report the characteristic features of diblock copolymers of PMMA and P2EtHA prepared from polyanions of either the MMA or the 2EtHA type. When these precursors are synthesized in a 75/25 toluene/THF mixture at -78 and/or -100 "C by using a 10-fold excess LiOEEM-complexed DPMLi as an initiator, the data summarized in Tables 1 and 2 show that the expected molecular weight is actually obtained for both PMMA and P2EtHA; the moleuclar weight distribution is very narrow (
