Poly(macromonomers): Homo- and Copolymerization Koichi Ito 1 , Seigou Kawaguchi Department of Materials Science, Toyohashi University of Technology, Tempaku-cho, Toyohashi 441-8580, Japan 1 E-mail: itoh@tutms.tut.ac.jp Syntheses and characterization of branched polymers prepared by homo- and copolymer- ization of macromonomers are reviewed. A number of macromonomers have so far been available as potential building blocks to design a variety of well-defined, branched homo- and copolymers including comb, star, brush, and graft types. Recent progress in macrom- onomer syntheses, macromonomers' homo- and copolymerization, characterization of the branched polymers obtained, as well as application to design of polymeric microspheres are described. Macromonomers and their homo- and copolymerization appear to provide continuing interest in designing and characterizing a variety of branched polymers and in their unique applications. Keywords. Poly(macromonomers), Graft copolymers, Comb, Star, Brush, Polymeric micro- spheres List of Symbols and Abbreviations . . . . . . . . . . . . . . . . . . . . . . . 130 1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 133 2 Survey of Macromonomer Techniques . . . . . . . . . . . . . . . . 134 3 Syntheses of Macromonomers . . . . . . . . . . . . . . . . . . . . . 136 3.1 Polyolefins . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 136 3.2 Polystyrenes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 137 3.3 Polyacrylates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 139 3.4 Poly(ethylene oxide) . . . . . . . . . . . . . . . . . . . . . . . . . . . 139 3.5 Some Other New Macromonomers . . . . . . . . . . . . . . . . . . . 141 4 Homopolymerization and Copolymerization of Macromonomers . . . . . . . . . . . . . . . . . . . . . . . . . . . 141 4.1 Homopolymerization . . . . . . . . . . . . . . . . . . . . . . . . . . 142 4.2 Copolymerization . . . . . . . . . . . . . . . . . . . . . . . . . . . . 145 5 Characterization of Star and Comb Polymers . . . . . . . . . . . . 148 5.1 Characterization and Solution Properties of Poly(macromonomers) . . . . . . . . . . . . . . . . . . . . . . . . 149 Advances in Polymer Science, Vol.142 © Springer-Verlag Berlin Heidelberg 1999 130 K. Ito, S. Kawaguchi 5.2 Bulk Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 154 5.3 Some Properties of Graft Copolymers . . . . . . . . . . . . . . . . . . 156 6 Design of Polymeric Microspheres Using Macromonomers . . . . . 157 6.1 Dispersion Polymerization . . . . . . . . . . . . . . . . . . . . . . . . 157 6.2 Mechanistic Model of Dispersion Copolymerization with Macromonomers . . . . . . . . . . . . . . . . . . . . . . . . . . . 163 6.3 Emulsion Polymerization . . . . . . . . . . . . . . . . . . . . . . . . . 167 6.4 Chain Conformation of Grafted Polymer Chains at Interfaces . . . . 171 7 Conclusions and Future . . . . . . . . . . . . . . . . . . . . . . . . . . 173 8 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 174 List of Symbols and Abbreviations a exponent in Mark-Houwink-Sakurada equation AIBN 2,2'-azobisisobutyronitrile a' bead spacing a s area occupied by a surfactant molecule a s expansion factor ATR attenuated total reflection B excluded-volume strength b bond length b binary cluster integral BMA n-butyl methacrylate C s surfactant concentration [cmc] critical micelle concentration d m density of monomer d p density of polymer DP degree of polymerization DP n number-average DP DP n o DP n without chain transfer DP w weight-average DP DV differential viscosity ESCA electron spectroscopy for chemical analysis ESR electron spin resonance [ h] limiting viscosity f initiator efficiency f' number of branches FTIR Fourier transform infrared spectroscopy g shrinking factor g ratio of molecular weights of branch and backbone Poly(macromonomers), Homo- and Copolymerization 131 GTP group transfer polymerization HEMA 2-hydroxyethyl methacrylate [I] initiator concentration k d decomposition rate constant KP Kratky-Porod k p propagation rate constant k t termination rate constant k 2 diffusion-controlled rate constant for coalescence between similar- sized particles L contour length LALLS low-angle laser light scattering l –1 Kuhn segment length M molecular weight M o molecular weight of monomeric unit [M] monomer concentration MALLS multiangle laser light scattering M D molecular weight of macromonomer µ rate of particle volume growth M L shift factor MMA methyl methacrylate mp melting point [M] p equilibrium concentration of monomer swelling particle MW molecular weight M w weight average molecular weight n number of bonds N number of particles n – average number of radicals per particle n K Kuhn segment number n' number of grafted chains onto surface n kinetic chain length N A Avogadro's number NAD nonaqueous dispersion NMR nuclear magnetic resonance PAA poly(acrylic acid) PBMA poly(n-butyl methacrylate) PCL poly( e-caprolactone) PDMS poly(dimethylsiloxane) PE polyethylene PEO poly(ethylene oxide) PHBd hydrogenated poly(1,3-butadiene) PHEMA poly(2-hydroxyethyl methacrylate) f m volume fraction of monomers swelling particles PHSA poly(12-hydroxystearic acid) PIB polyisobutylene PIp polyisoprene 132 K. Ito, S. Kawaguchi PLMA poly(lauryl methacrylate) PMA poly(methacrylic acid) PMMA poly(methyl methacrylate) PNIPAM poly(N-isopropylacrylamide) POXZ polyoxazolines PP polypropylene PSt polystyrene PTBA poly(t-butyl acrylate) PTBMA poly(t-butyl methacrylate) P( q) particle scattering factor PVA poly(vinyl alcohol) PVAcA poly(N-vinylacetamide) PVC poly(vinyl chloride) PVP poly(vinylpyrrolidone) P4VP poly(4-vinylpyridine) q persistence length r i reactivity ratio of i species R radius of particle R crit radius of particle at critical point r density r' rate of radical generation ROMP ring-opening methathesis polymerization R p rate of polymerization S surface area occupied by a macromonomer chain <S 2 > mean square radius of gyration SAXS small-angle X-ray scattering SANS small-angle neutron scattering S crit surface area occupied by a macromonomer chain at critical point SEC size exclusion chromatography STM scanning tunneling electron microscopy TBA t-butyl acrylate TEMPO 2,2,6,6-tetramethylpiperidinyloxy q fractional conversion of monomer q crit fractional conversion of monomer at critical point q D fractional conversion of macromonomer q Dcrit fractional conversion of macromonomer at critical point T g glass transition temperature glass transition temperature of polymer with infinite molecular weight v excess free volume at a chain end v m free volume per monomeric unit W D weight of macromonomer polymerized W do initial weight of macromonomer W M weight of monomer polymerized T g ¥ Poly(macromonomers), Homo- and Copolymerization 133 W Mo initial weight of monomer x fraction of disproportionation in termination z excluded-volume parameter scaled excluded-volume parameter 1 Introduction A macromonomer is any polymer or an oligomer with a polymerizable func- tionality as an end group. Formally, the macromonomer homopolymerizes to afford a star- or comb-shaped polymer and copolymerizes with a conventional monomer to give a graft copolymer. Thus the macromonomer serves as a con- venient building block to constitute arms or branches of known structure in the resulting polymer. A large number of macromonomers, differing in the type of the repeating monomer and the end-group, have so far been prepared, thereby offering the possibility of construction of an enormous number of branched polymers in a variety of architectures, combinations, and compositions. Polym- erization and copolymerization of macromonomers have also been studied in great detail in order to understand their unique behavior in comparison with that of conventional monomers. Their useful application in design of polymeric microspheres has also been appreciated recently. Some interesting properties of poly(macromonomers) have also been explored very recently as a simple model of brush polymers which are of increasing interest. Comparatively, however, the characterization and properties of graft copolymers with randomly distributed branches have not been investigated to the same extent in spite of their theoret- ical and practical importance. The present article is intended to discuss the state-of-the-art of the design and characterization of the branched polymers obtained by the macromono- mer technique, with particular stress on the characterization and the proper- ties of the brush polymers obtained by the homopolymerization of macromon- omer. The synthetic aspects of the macromonomer technique, including prep- aration of various kinds of macromonomers, have been recently reviewed by one of the authors [1]. Therefore, we intend here to outline briefly the macrom- onomer technique and describe only the very recent important developments in syntheses. Preparation and characterization of the polymeric microspheres by use of macromonomers as reactive (copolymerizable) emulsifiers or disper- sants will be described in some detail to represent one of their unique applica- tions. Some comprehensive reviews covering earlier references include those by Kawakami [2], Meijs and Rizzard [3], Velichkova and Christova [4], and those in books edited by Yamashita [5] and by Mishra [6] among others. ˜ z 134 K. Ito, S. Kawaguchi 2 Survey of Macromonomer Techniques A macromonomer is usually defined as a polymeric or an oligomeric monomer with a polymerizable or copolymerizable functional group at one end. They af- ford a comb-shaped polymer with regularly and densely attached branches by homopolymerization, and a graft copolymer with randomly and loosely distrib- uted branches by copolymerization with a conventional, low molecular weight (MW) comonomer, as illustrated in Fig. 1a,b, respectively. A formally comb- shaped poly(macromonomer) may actually be forced to take a conformation that looks like a star as in Fig. 1c or a brush as in Fig. 1e, depending on the rela- tive lengths of the macromonomer branch vs the poly(macromonomer) back- bone. A graft copolymer with a relatively short backbone as compared to the branches may also look like a star as in Fig. 1d in a solvent which is selective for the branches, while that with a long backbone with few but long branches may take a flower-like conformation as in Fig. 1f with some of their backbone seg- ments looped outside in a selective solvent for the backbone. These isolated con- formations favored in dilute solutions are expected to coalesce to some organ- Fig. 1a–f. Various branched architectures obtained by the macromonomer technique: a,b comb-like; c,d star-like; e brush; f flower-like. a c, and e are poly(macromonomers) obtaine d by homopolymerization, while b, d, and f are graft copolymers obtained by copolymeriza- tion Poly(macromonomers), Homo- and Copolymerization 135 ized structure or morphology in concentrated solutions or in solids. In fact, a number of possible conformations or morphologies that can be expected from self-organization of the branched polymers has been a matter of increasing study for the macromonomer technique. The variety of branched architectures that can be constructed by the mac- romonomer technique is even larger. Copolymerization involving different kinds of macromonomers may afford a branched copolymer with multiple kinds of branches. Macromonomer main chain itself can be a block or a random co- polymer. Furthermore, a macromonomer with an already branched or dendritic structure may polymerize or copolymerize to a hyper-branched structure. A block copolymer with a polymerizable function just on the block junction may homopolymerize to a double comb or double-haired star polymer. If we extend the definition of the macromonomer to include all polymers or oligomers with a multiple number of (co)polymerizable functional groups at any positions, then we can design an even larger number of branched poly- mers by their polymerization and copolymerization. For example, a “teleche- lic macromonomer” with two (co)polymerizable functional groups, each on one end, may be useful to design a network structure in copolymerization with control over the inter-crosslink length and/or crosslink density. A “mul- tifunctional macromonomer” with a multiple number of (co)polymerizable functional groups along their chain may include already well-known resins such as unsaturated polyesters used in thermosetting. Although these “mac- romonomers” are no doubt practically important in applications, the scope becomes too broad and complicated and the authors prefer to adhere to the original, simpler definition of the macromonomer as that with a single (co)polymerizable end group that affords star- and comb-shaped polymers and/or graft copolymers with their branches (side chains) of known structure as in Fig. 1. So far, a great number of well-defined macromonomers as branch candidates have been prepared as will be described in Sect. 3. Then a problem is how to con- trol their polymerization and copolymerization, that is how to design the back- bone length, the backbone/branch composition, and their distribution. This will be discussed in Sect. 4. In brief, radical homopolymerization and copolymeriza- tion of macromonomers to poly(macromonomers) and statistical graft copoly- mers, respectively, have been fairly well understood in comparison with those of conventional monomers. However, a more precise control over the backbone length and distribution by, e.g., a living (co)polymerization is still an unsolved challenge. Needless to say, the best established architecture which can be designed by the macromonomer technique has been that of graft copolymers. With this tech- nique we now have easy access to a variety of multiphased or microphase-sepa- rated copolymer systems. This expanded their applications into a wide area in- cluding polymer alloys, surface modification, membranes, coatings, etc. [5]. One of the most unique and promising applications of the technique may be found in the design of polymeric microspheres. In this technique macromono- 136 K. Ito, S. Kawaguchi mers are reactive emulsifiers or dispersants in emulsion or dispersion systems, respectively. Since the macromonomers are already polymers, they serve as ef- fective steric stabilizers of the resulting microspheres. They are surface grafts after copolymerization with the substrate comonomer. A number of hy- drophilic or polar macromonomers have been designed for aqueous emulsion or alcoholic dispersion systems. They are the counterpart of the nonpolar mac- romonomers which were indeed the first “macromonomers” developed for the well-known nonaqueous (petroleum) dispersion polymerization (NAD) by ICI [7]. 3 Syntheses of Macromonomers Macromonomers are synthesized by introducing an appropriate (co)polymer- izable end-group, generally by one of the following methods: (a) end-capping of a living polymer (termination method), (b) initiation of living polymeriza- tion (initiation method), (c) transformation of any functional end-group, and (d) polyaddition. Methods (a) and (b) are simple and usually afford most well- defined macromonomers of a controlled degree of polymerization with a nar- row MW distribution, but depend on proper combination of any living polym- erization with an effective terminator or initiator carrying a polymerizing group or its protected one. Method (c) utilizes any end-functionalized poly- mers such as those obtained from chain-transfer-controlled radical polymeri- zation and polycondensation. Method (d) involves the polyaddition reactions between vinyl and silane groups (hydrosilylation), for an example. Since more than one hundred macromonomers have been reviewed previously [1], includ- ing polyolefins, polystyrenes, polydienes, polyvinylpyridines, poly(meth)acr- ylates and their derivatives, poly(vinyl ethers), poly(vinyl acetate) and deriva- tives, halogenated vinyl polymers, poly(alkylene oxides), poly(dimethylsi- loxanes), poly(tetrahydrofuran) and polyacetals, polyoxazolines and poly(eth- yleneimines), polylactones and polylactide, polylactams and poly(amino ac- ids), and macromonomers prepared by polycondensation and polyaddition, only very recent developments will be described here in a way to supplement them. 3.1 Polyolefins End-functionalized polyethylene (PE) [8, 9], polypropylene (PP) [10], and polyisobutylene (PIB) [11] have been transformed to their corresponding mac- romonomers carrying (meth)acrylate, oxazoline, and methacrylate end groups, 1, 2, and 3, respectively. Polybutadienyl lithium was terminated with chlo- rodimethylsilane, followed by hydrogenation to saturated polyolefin (PHBd) [12]. Hydrosilylation of the end silane with allyl glycidyl ether afforded an epox- Poly(macromonomers), Homo- and Copolymerization 137 idized macromonomer, 4, and subsequent hydrolysis gave a dihydroxy-ended macromonomer, 5, to be used for polycondensation to polyester-g-PHBd. (1a) (1b) (2) (3) (4) (5) 3. 2 Polystyrenes Polystyrene (PSt) macromonomers, 6, almost quantitatively functionalized with p-styrylalkyl end groups have been prepared by termination of living polysty- ryllithium with corresponding p-styrylalkyl bromide or iodide [13]. Termina- tion of PSt-Li with epichlorohydrin, in benzene plus tetrahydrofuran, was suc- cessful after end-capping with 1,1-diphenylethylene to afford epoxide-ended PSt macromonomer, 7 [14]. Living polystyryllithium was end-capped with ethylene oxide, followed by reaction with 5-norbornene-2-carbonyl chloride to afford w- norbornenyl PSt macromonomer, 8, which was also successfully subjected to liv- ing, ring-opening methathesis polymerization (ROMP) to afford regular comb PSt, 9, with both the branch and the backbone well-controlled with regard to 138 K. Ito, S. Kawaguchi MW and MW distribution [15]. w-Norbornenyl macromonomers of poly(styrene- b-ethylene oxide) have similarly been prepared, as will be described later in Sect. 3.4. (6) (7) (8)(9) Very recently, a multifunctional, “orthogonal” initiator, 10, has been devel- oped by Puts and Sogah [16]. Living free radical polymerization of styrene, ini- tiated with the styryl-TEMPO moiety as an active site, afforded w-oxazolinyl PSt macromonomer, which was in turn polymerized through cationic ring-opening of the oxazoline end groups by methyl trifluoromethanesulfonate, to give a reg- ular comb PSt with poly(oxazoline) as a backbone, 11. (10)(11) [...]... termination by recombination or 2.0 either for the termination by disproportionation or for the chain transfer to small molecules In this respect, any living polymerization with rapid initiation will afford polymers with a narrow DP distribution of the Poisson type Ring-opening methathesis polymerization of norbornenyl-terminated macromonomers, 8, 15, and 16, appears promising in this regard [22, 23] ... systems in water or in alcoholic media have been recently developed using hydrophilic macromonomers to meet increasing concerns for environmentally friendly systems 6.1 Dispersion Polymerization Dispersion polymerization is defined as a type of precipitation polymerization by which polymeric microspheres are formed in the presence of a suitable steric stabilizer from an initially homogeneous reaction mixture... growth of sterically stabilized particles in dispersion polymerization using macromonomer 164 K Ito, S Kawaguchi solvent (2) Accompanied by the decomposition of the initiator, linear oligomers, polymers, and graft copolymers are all produced by polymerization in the continuous phase The solubility of these polymers is a function of their MW and the composition of the graft copolymer Polymers with a MW larger... (2) [34 , 36 ] The very low initiator efficiency, f, around 0.2 or even smaller, as shown in Table 1, found in the solution polymerization of macromonomers also appears to come from the initiator decomposition in the high viscosity medium, resulting in an enhanced probability of recombination or disproportionation of the primary radicals generated Polymerization of p-styrylalkyl-ended poly(ethylene oxide)... examples of dispersion polymerization using macromonomers are summarized in Table 3 Historically, non-aqueous dispersion (NAD) polymerization of polar monomers was first carried out in aliphatic hydrocarbon media with hydrophobic macromonomers, 32 and 33 [7] These are copolymerized with MMA or other polar monomers to produce comb-graft copolymers which have limited solubility in pure aliphatic hydrocarbons... exaggerated in size Poly(macromonomers), Homo- and Copolymerization 1 63 6.2 Mechanistic Model of Dispersion Copolymerization with Macromonomers According to the aggregative and coagulative nucleation mechanisms which have been derived originally from the homogeneous nucleation theory of Fitch and Tsai [128], the most important point in the reaction is the instant at which colloidally stabilized particles... copolymerization method, anionic or radical, results in the graft copolymers with very different branch distribution The characterization and solution properties of graft copolymers in which the backbone polymers are chemically different from the branches require many difficulties to be overcome, from the viewpoints of the determination of MW, the branching rate, and their distributions In the next section,... copolymerization mechanism which favors an amphiphilic monomer with a more hydrophobic polymerizing moiety to participate more readily in the reaction sites (micelles) To summarize, macromonomers in polymerization and copolymerization are only fairly well understood compared to the conventional monomers Effects, such as conformational, morphological, or due to incompatibility caused by the macromonomer chains,... certain critical value precipitate and begin to coagulate to form unstable particles (3) These particles coagulate on contact, and the coagulation among them continues until sterically stabilized particles form (4) This point is referred to as the critical point, and it occurs when all of the particles of interest contain sufficient stabilizer polymer chains on the surface to provide colloidal stability... chains of the graft copolymers, which must be insoluble in the medium, serve as the anchors into the core The following section presents a general criterion for the size control of polymeric microspheres by the dispersion copolymerization using macromonomers Fig 5 Schematic picture of a microsphere obtained in emulsion and dispersion copolymerization using macromonomer technique The grafted chains . mol- ecules. In this respect, any living polymerization with rapid initiation will afford polymers with a narrow DP distribution of the Poisson type. Ring-opening met- hathesis polymerization of norbornenyl-terminated. from the initiator decomposi- tion in the high viscosity medium, resulting in an enhanced probability of re- combination or disproportionation of the primary radicals generated. Polymerization of. poly(vinyl chloride) PVP poly(vinylpyrrolidone) P4VP poly(4-vinylpyridine) q persistence length r i reactivity ratio of i species R radius of particle R crit radius of particle at critical point