Radical Polymerizations II Special Cases Devon A. Shipp Department of Chemistry, & Center for Advanced Materials Processing Clarkson University Potsdam, NY 13699-5810 Tel. (315) 268-2393, Fax (315) 268-6610 [email protected] 46 © Copyright 2016 Devon A. Shipp Living Polymerizations • Objectives • Requirements – Initiation must be fast – Continuous chain • Ri >> Rp growth – Termination must be – No termination eliminated • Or at least reduced to – Well-defined chains insignificance • Chain structure • Chain length • Problems with free • Molecular weight radical polymerization distribution – Initiation is slow • Block copolymer – Radical-radical synthesis termination is fast 47 Reversible-Deactivation Radical Polymerizations (RDRP) • Often called living radical polymerization activation Dormant polymer Active polymer radical + capping group deactivation Monomer (Propagation) t P h o g l i y e Δ[M] Monomer repeat unit Functional terminal d DP = i W s ( ω ) end ("capping") group [In] r 0 p a e l r u s c X i t e y In l n o M Y Y < 1.5 Initiator ( α ) end 48 % Monomer Conversion Features of Living Polymerizations • Linear increase in • Low polydispersity molecular weight – Mw/Mn < 1.5 (often ~ 1.1) – Vs. monomer conversion • First-order monomer • Pre-defined molecular consumption weight – Same as conventional radical polymerization – DPn = Mn/FWM = Δ[M] / [Initiator] 49 Terminology/Tests for Living Polymerization • Controversy over terminology – Controlled, living, pseudo-living, quasi-living, living/controlled, living/controlled, reversible-deactivation,… • IUPAC definition of living polymerization: – Absence of irreversible transfer and termination • Cannot be applied to any radical polymerization! • Some tests for RDRP: – Continued chain growth after addition monomer added – Molecular weight increases linearly with conversion – Active species (i.e. radicals) conc. remains constant – Narrow molecular weight distributions (low Mw/Mn) – Block copolymers may be prepared (subset of first test) – End groups are retained – yields end-functionalized chains 50 Moad and Solomon The Chemistry of Radical Polymerization, 2nd edition, 2006, page 453. Examples of RDRP Data • Atom transfer radical polymerization (ATRP) – Styrene. Mn at 100% (theoretical) = 10,000 (DP = 100) T.E. Patten, J. Xia, T. Abernathy, K. Matyjaszewski, Science, 1996, 272, 866. 51 K. Matyjaszewski, T.E. Patten, J. Xia, J. Am. Chem. Soc., 1997, 119, 674. Calculated MWDs 52 Moad and Solomon The Chemistry of Radical Polymerization, 2nd edition, 2006, page 454. Materials From RDRP 53 D.A. Shipp, Polym. Rev., 2011, 51, 99-103. RDRP: Types & Requirements • Main types • Requirements – Nitroxide-Mediated – All chains begin at the Polymerization same time • NMP • Fast initiation – Atom Transfer Radical Polymerization – Little or no termination • ATRP • Low radical concentration – Reversible Addition- – All chains grow at the Fragmentation Chain same rate Transfer Polymerization • RAFT • Fast exchange • Other types – Metal-mediated polymn – Iniferter – Iodo and methacrylate-based – Group transfer polymn degenerate transfer 54 Nitroxide-Mediated Polymerization • Nitroxides • TEMPO – Stable free radicals – 2,2,6,6 – Do not react with O-centered N O tetramethylpiperidim-N- radicals oxyl – React fast with C-centered radicals – Only good for styrene 6 9 -1 -1 (co)polymers • kd ~ 10 – 10 M s – Do not initiate polymerization • αH nitroxides N O – Styrene, acrylates, ka Pn-X Pn + X acrylonitrile, 1,3- kd Ph butadiene kp Monomer ka O N O N n k n Rizzardo, Solomon, et al. Ph Ph Ph d Ph Ph Ph US Patent 4,581,429, 1986. TEMPO Aust. J. Chem., 1990, 43, 1215-1230. Ph 55 Chem. Aust., 1987, 54, 32. Propagation Georges/Xerox Approach to NMP N O O O O O Heat O O n N O O O n Ph Ph Ph Ph TEMPO BPO n+1 Ph Heat O O O n N Ph Ph Propagation Molecular weight distributions of polystyrene produced by NMP using BPO and M.K. Georges et al., Macromolecules, 56 n TEMPO. M and Mw/Mn of samples I – IV are as follows: (Mn:Mw/Mn) 1700:1.28, 1993, 26, 2987-2988. 3200:1.27, 6800:1.21, 7800:1.27. Hawker Approach to NMP (Using TEMPO) Alkoxyamine 57 C.J. Hawker, J. Am. Chem. Soc., 1994, 116, 11185-11186. Newer108 Nitroxides: R. B. Grubbs α-Hydrido-Derivatives commercialized by Arkema in 2005 points to the continued interest in NMP from re- searchers who would rather not synthesize their own alkoxyamine initiators. As TIPNO and related alkoxyamines are now also commercially available (Sigma-Aldrich), research • Widerinvolving polymers range prepared by NMPof should monomers continue to grow. & functionalities Common nitroxides/ alkoxyamines 110 R. B. Grubbs Scheme 3. 2.2.1 In Situ Generation of Nitroxide Radicals and Alkoxyamines. In the earliest examples of NMP, alkoxyamines were generated during the course of the polymerization through the use of conventional radical initiators in the presence of nitroxides.51 Shortly thereafter, strategies to isolate unimolecular alkoxyamines from nitroxides and radical precursors were Functional developed.9,54 This general strategy has recently been modified to allow the low temperature R alkoxyamines synthesis of the BlocBuilder⃝ alkoxyamine by photolysis of a diacid-functional azo initiator in high yield.55 Anumberofsyntheticroutestoalkoxyamineshavebeendevelopedinwhichan active alkoxyamine is generated during the course of the polymerization from precursors that are not nitroxides (Scheme 4, color scheme available online).13 The propensity of nitric oxide (Scheme 4a), nitrosoalkanes/arenes (Scheme 4b) and nitrones (Scheme 4c) to react with two equivalents of a carbon-centered radical to form alkoxyamines has been exploited as an alternative route to new nitroxide controlling agents that have allowed the controlled preparation of polymers and block copolymers,56–58 including copolymers of methyl methacrylate (MMA), and, in some cases, have facilitated polymerization at lower temperatures.59 Density functional theory has been used in combination with electron spin 58 Figure 2. Structures of selected new alkoxyamines used for NMP: 17–18,69 19–20,70 21–22,71 P. Tordo, Y. resonanceGnanau, et spectrometry al., J. Am. Chem. to better Soc. understand, 2000, 122 these, 5929-5939 systems. and to predictR.B. Grubbs, the properties Polym. ofRev., 2011, 51, 104-137. 23–24,61 25,20 26,62 27.63 C.J. Hawker,the et resultingal., J. Am. nitroxides. Chem. Soc.60 , 1999, 121, 3904-3920. Downloaded By: [Grubbs, Robert B.] At: 13:14 2 May 2011 alkoxyamine to other polymers (to form block copolymers), to surfaces (to create polymer brushes), and to fluorescent labels (Figure 2).62,69,72–75 Alcohol (17),69 carboxylic acid (18),62,69 thiol/disulfide (20, 21),70 bis(thioether) (21),71 pyridyl (22),71 ketone (25),20 and carboxylate ester (24, 26, 27)20,61–63 functionalized alkoxyamines are among such NMP initiators recently prepared. In a number of these examples, the functional alkoxyamines (24–27) were prepared by the addition of functional group-bearing radical species to nitrosoalkanes.20,61–63 2.3 Improving NMP Rates Scheme 4. In general, efforts to increase the rate and/or lower the temperature for NMP have involved pushing the equilibrium between the dormant alkoxyamine and the active radical toward 2.2.2 One Step Routes to Alkoxyamines. When the above reactions of radicals with amine the propagating radical (increasing k /k in Scheme 1). In all of these manipulations, there oxide derivatives are carried out atd temperaturesa below those where alkoxyamine dissocia- Downloaded By: [Grubbs, Robert B.] At: 13:14 2 May 2011 is a finetion occurs, line between alkoxyamine a slow initiators but well-controlled can be prepared polymerization in one step—a number and a of fast, such uncontrolled examples polymerization; any changes that increase the concentration of propagating radicals are likely to decrease the livingness of the polymerization. As the earliest published examples of NMP were centered on bimolecular benzoyl per- oxide/TEMPO initiating systems, most of the earliest rate acceleration strategies involved the use of various additives (e.g., camphorsulfonic acid,76,77 2-fluoro-1-methylpyridinium 78 79 p-toluenesulfonate, acetic anhydride )toincreasetherateofhomolysis(increasekd in Scheme 1) of the alkoxyamine, to retard the recombination of the nitroxide radical with the polymer radical (decrease ka in Scheme 1), or to decompose excess nitroxide that might build up due to polymer radical termination.9 Nitroxides, such as TIPNO and DEPN, that are believed to decompose at polymerization temperatures at a slow enough rate to prevent the build up of excess nitroxide without negatively affecting control over the polymerization process have similarly proven useful in increasing NMP rates.37,80 Subsequent approaches have used a wider range of initiating radicals and mediating nitroxides to control the dormant alkoxyamine/active radical equilibrium. With bimolecular initiating systems, the use of lower temperature radical initiators, such as tert-butylperoxy 2-ethylhexyl carbonate with TEMPO,81 has shown some success. It has been demonstrated ARTICLE IN PRESS W.A. Braunecker, K. Matyjaszewski / Prog. Polym. Sci. 32 (2007) 93–146 101 successful moderators of vinyl acetate polymeriza- cross-coupling (deactivation) of the transient radical tion, although the mechanism of control is not R with persistent radical Y (kc), and termination of based on the SFRP principle but rather follows a two transient radicals to form product P (kt). degenerative transfer mechanism (cf. below) [73–76]. In this case, the rates of formation of the persistent radical and of loss of the transient radical 4.2.4. Metal mediated polymerization are given by the expressions in Eq. (3) (where the Transition metal compounds that can mediate term 2kt is used because a single termination step radical polymerization include those based on Co consumes two radicals) [47],Mo[77], Os [78], and Fe [79]. Controlled dR 2 d kdI kcRY 2ktR ; polymerization of methyl acrylate with Co porphyr- t ¼ À À (3) dY k I k RY dR 2k R2: ins is one of the most successful SFRP systems, dt ¼ d À c ¼ dt þ t having produced well-defined high molecular weight The two coupled differential equations were polyacrylates.