846 Macromol. Rapid Commun. 21, 846–852 (2000) Communication: Using 2-hydroxyethyl a-bromoisobuty- rate as initiator, atom transfer radical polymerization (ATRP) of tert-butyl acrylate leads to poly(tert-butyl acry- late) (PtBA) with a hydroxyl group at one and a bromine atom at the other end. Esterification of the hydroxyl group of these heterotelechelic polymers with acryloyl chloride — yields PtBA (Mn = 3060) with a polymerizable double bond at one end and a bromine atom at the other end which can act as an initiator in ATRP (“macroinimer”). Self-condensing ATRP of such a macroinimer leads to hyperbranched or highly branched PtBA. The polymer was characterized by GPC viscosity measurements. Even — — at M = 78800, a rather low polydispersity index of M / — w w Mn = 2.6 was obtained. A significantly lower value for the Mark-Houwink exponent (a = 0.47 compared to a = 0.80 for linear PtBA) indicates the compact nature of the Mark-Houwink plots for linear PtBA (9) and branched PtBA F branched macromolecules. ( ) and the contraction factor of intrinsic viscosity for branched PtBA (H). Column set II Synthesis of hyperbranched poly(tert-butyl acrylate) by self-condensing atom transfer radical polymerization of a macroinimer Guanglou Cheng1, Peter F. W. Simon2, Markus Hartenstein1, Axel H. E. Mu¨ller* 1 1 Makromolekulare Chemie II and Bayreuther Institut fu¨r Makromoleku¨lforschung, Universita¨t Bayreuth, D-95440 Bayreuth, Germany [email protected] 2 Institut fu¨r Physikalische Chemie, Universita¨t Mainz, D-55099 Mainz, Germany (Received: November 25, 1999; revised: March 23, 2000) Introduction and even ring-opening polymerization11). The molecular In the last decade, dendrimers have been extensively parameters of the polymers obtained by SCVP have been studied as materials with novel physical properties1, 2). studied theoretically12, 13), showing that these hyper- These polymers have a very compact structure and can be branched polymers have a high degree of branching, DB L highly functionalized. However, commercialization of 0.5, and exhibit very broad molecular weight distributions dendrimers is prevented by the requirements of multi-step (MWD). The polydispersity index (PDI) is expected to be reactions and intermediate purification. Less regular equal to the number-average degree of polymerization, Pn, hyperbranched polymers obtained by polycondensation partially due to the inevitable presence of oligomers. Slow are more easily available, but their preparation was addition of the inimers to a multifunctional initiator will 3–6) 14, 15) restricted to the polycondensation of ABn monomers . lead to higher DB and rather narrow MWD . It has been Recently, Fre´chet et. al.7) reported a new synthetic also shown theoretically16) and experimentally8, 16) that the method, self-condensation vinyl polymerization (SCVP), self-condensing vinyl copolymerization (SCVCP) of AB* to prepare hyperbranched vinyl polymers. Initiator-mono- monomers with conventional monomers, M, leads to highly mers (“inimers”) have the general structure AB*, where A branched polymers, allowing for control of MWD and stands for a double bond and B* for an initiating group. This degree of branching. This reaction gives way to the incor- general approach has been applied to various types of living poration of functional monomers into highly branched polymerization, i.e. cationic7), radical8, 9), group transfer10), polymers. Macromol. Rapid Commun. 21, No. 12 i WILEY-VCH Verlag GmbH, D-69451 Weinheim 2000 1022-1336/2000/1208–0846$17.50+.50/0 Synthesis of hyperbranched poly(tert-butyl acrylate) ... 847 Structures similar to those obtained in SCVCP should line. 2-Hydroxyethyl-a-bromoisobutyrate (HEBIB) was pre- 25) 1 d be obtained by using macromonomeric initiators pared according to the literature . H NMR (CDCl3): = 1 1 1 1 1 (“macroinimers”). The structure of a macroinimer, 4.31 ( CH2 OCO), 3.85 ( CH2 OH), 1.97–1.92 ( CH3, 1OH) ppm. A(m)nB*, differs from that of an AB* inimer only by the presence of linear monomer units, (m), acting as a spacer between initiator and monomer function. Preparation of HO-PtBA-Br by bulk ATRP of tBA The molecular parameters of polymers made from All operations were carried out under a nitrogen atmosphere. SCVP of a macroinimer have been calculated by Simon CuBr, PMDETA and tBA (molar ratio = 0.5/0.5/27) were 17) and Mu¨ller . Assuming the macroinimer to be monodis- added to a flask and stirred until the system turned homoge- perse and neglecting excluded-volume effects, the PDI of — neously green. Then the initiator HEBIB ([HEBIB]0/[tBA]0 a hyperbranched polymer with a given Mn was calculated = 1/27) was added dropwise. As soon as the initiator was to be (c + 1) times lower than that from an AB* inimer: added, the system turned homogeneously blue, indicating the 8 PDI = Pn/(c + 1), where c is the number-average degree start of the polymerization. The mixture was kept at 25 C of polymerization of the macroinimer. Since the linear for 140 min. After the polymerization, the catalyst was (m)-units reduce the number of branch points by the fac- removed by an adsorption filtration through a neutral alu- tor (c + 1), the expected DB should be correspondingly mina column, and the resulting polymer (HO-PtBA-Br) was — precipitated in a mixture of CH3OH/H2O (3/1 v/v). Yield: lower than that from an AB* inimer for a given M . How- — — — — n 62.5%; GPC: M = 2420, M /M = 1.34; 1H NMR: M = ever, macroinimer units which are incorporated into the n w n n 3060. 1H NMR (CDCl ): d = 4.17 (1CH 1OCO), 4.05 chain only by their vinyl groups are counted as branched 3 2 (1CH(CO2Bu)-Br), 3.81 (1CH21OH), 2.20 (1CH(CO2 whereas they are counted as linear in SCVP of inimers. Bu)1CH21), 2.0–1.0 (1CH3, 1CH21CH1) ppm. MALDI- As a consequence, DB at full conversion of double bonds TOF MS: m/z = 211.05 (initiator) + n N 128.17 (PtBA back- is increased by about 59% to DB L 0.74/(c + 1). This bone) + 79/81 (-Br) + 23 (Na). value is still smaller than the one obtained in SCVCP (DB X 2/c for c S 1). Preparation of the macroinimer Poly(ethylene glycol)18) and poly(tetrahydrofuran)19) macroinimers have been applied to prepare block and graft Acryloyl chloride (2.05 g, 22.6 mmol) was added dropwise branched copolymers, as well as crosslinked networks in to the mixture of HO-PtBA-Br (11.97 g, 4.53 mmol) and 8 conventional free-radical polymerization. These macroini- triethylamine (2.29 g, 22.6 mmol) in 500 ml THF at 0 C dur- ing 60 min. The mixture was stirred for 3 h at 08C followed mers behaved as macromonomers, macroinitiators and ma- by stirring at room temperature for 24 h. THF was then crocrosslinkers. Conventional free-radical homopolymeri- removed by a rotary evaporator. The produced polymer was zation of these macroinimers yielded insoluble material. purified by passing through a SiO2/Al2O3 (1/4 w/w) column In this paper, we wish to report the preparation of hyper- using THF/hexane (4/1) as mobile phase, followed by preci- — branched polyacrylates via the macroinimer technique pitation in CH3OH/H2O (3/1 v/v). Yield: 70%. GPC: Mn = — — 1 — 1 using atom transfer radical polymerization (ATRP). 2760, Mw/Mn = 1.29; H NMR: Mn = 3050. H NMR 20) ATRP allows the controlled polymerization of a wide (CDCl3): d = 6.49–5.68 (H2C2CH1CO1), 4.50–4.20 variety of vinyl monomers to obtain polymers with a vari- (1CH21OCO), 4.08 (-CH(CO2Bu)-Br), 2.20 (1CH(CO2 1 1 1 1 1 1 ety of well-defined architectures. By using AB* inimers, Bu) CH2 ), 2.0–1.0 ( CH3, CH2 CH ) ppm. MALDI- hyperbranched polystyrene8, 21) and polyacrylates22–24) have TOF MS: m/z = 265.05 (inimer) + n N 128.17 (PtBA back- been successfully synthesized via ATRP. By applying this bone) + 79/81 (-Br) + 23 (Na). approach to macroinimers, we have obtained highly branched or hyperbranched poly(tert-butyl acrylate) ATRP of the macroinimer using CuBr/PMDETA as catalyst (PtBA) without oligomers. PtBA macroinimers were also Under inert conditions, 0.0118 g (8.2 N 10–5 mol) CuBr, and obtained by ATRP using an OH-functionalized initiator. 0.0160 g (9.2 N 10–5 mol) PMDETA were added to a solution of 2.50 g (8.2 N 10–4 mol) macroinimer in 3.5 ml ethyl acet- ate. The reaction mixture was then immersed into an oil bath Experimental part at 408C and stirred with a magnetic bar. Samples were taken periodically and were passed through a short neutral alumina Materials column to remove the catalyst. The solvent was evaporated, tert-Butyl acrylate (tBA, BASF AG) was fractionated from the produced polymer was dissolved in benzene and freeze- CaH2 over a 1 m column filled with Sulzer packing at dried. 45 mbar, stirred over CaH2, degassed and distilled in high vacuum. CuBr (95%, Aldrich) was purified by stirring over- night in acetic acid. After filtration it was washed with etha- Analysis nol, ether, and then dried. N,N,N9,N99,N99-pentamethyldiethyl- GPC was performed using THF as eluent at a flow rate of enetriamine (PMDETA, Aldrich) and ethyl acetate (EAc, 1.0 ml/min at room temperature. Column set I: 5 l PSS SDV 99%, Aldrich) was degassed and distilled over a vacuum gel, 100 A˚ and linear: 102 –105 A˚ , 60 cm each; detectors: 848 G. Cheng, P. F. W. Simon, M. Hartenstein, A. H. E. Mu¨ller 26Jasco Uvidec 100 III with variable wavelength, Bischoff peak (a) from the overlapping peaks of (b) and (e9), the RI detector 8110.