Scott G. Gaynor

CONTROLLED RADICAL POLYMERIZATION

Various methods which lead to the control of molecular weight and polydispersities, and which allow for the preparation of block copolymers by radical polymerization are discussed. Thermal polymerizationof styrenes in the presence of stable radicals, polymerization of vinyl acetate and methyl methacrylate in the presence of chromium complexed by macrocyclic ligands polymerization of vinyl acetate initiated by organoaluminum compounds complexed by dipyridyl and activated by stable radicals, as well as in the presence of phosphites, are described in detail.

Simple and effective one‐pot synthesis of (meth)acrylic block copolymers through atom transfer radical polymerization

The synthesis of di- and triblock copolymers using atom transfer radical polymerization (ATRP) of n-butyl acrylate (BA) and methyl methacrylate (MMA) is reported. In particular, synthetic procedures that allow for an easy and convenient synthesis of such block copolymers were developed by using CuBr and CuCl salts complexed with linear amines. Polymerizations were successfully conducted where the monomers were added to the reactor in a sequential manner. Poor cross-propagation between poly(n-butyl acrylate) (PBA) macroinitiators and MMA was minimized, and therefore control of molecular weights and distributions was realized, by using halogen exchange—a technique involving the addition of CuCl to the MMA during the chain extension of the PBA macroinitiator. High molecular weight (Mn ∼ 90,000) and low polydispersity (Mw /Mn < 1.35) ABA triblock copolymers were also prepared and their structure and properties in bulk have been preliminary characterized indicating the potential of ATRP for the production of all-acrylic thermoplastic elastomers.

Chain Transfer to Polymer and Branching in Controlled Radical Polymerizations of n-Butyl Acrylate

Chain transfer to polymer (CTP) in conventional free-radical polymerizations (FRPs) and controlled radical polymerizations (ATRP, RAFT and NMP) of n-butyl acrylate (BA) has been investigated using (13) C NMR measurements of branching in the poly(n-butyl acrylate) produced. The mol-% branches are reduced significantly in the controlled radical polymerizations as compared to conventional FRPs. Several possible explanations for this observation are discussed critically and all except one refuted. The observations are explained in terms of differences in the concentration of highly reactive short-chain radicals which can be expected to undergo both intra- and inter-molecular CTP at much higher rates than long-chain radicals. In conventional FRP, the distribution of radical concentrations is broad and there always is present a significant proportion of short-chain radicals, whereas in controlled radical polymerizations, the distribution is narrow with only a small proportion of short-chain radicals which diminishes as the living chains grow. Hence, irrespective of the type of control, controlled radical polymerizations give rise to lower levels of branching, when performed under otherwise similar conditions to conventional FRP. Similar observations are expected for other acrylates and monomers that undergo chain transfer to polymer during radical polymerization.

Synthesis and characterization of graft copolymers of poly(vinyl chloride) with styrene and (meth)acrylates by atom transfer radical polymerization

Graft copolymers of poly(vinyl chloride) with styrene and (meth)acrylates were prepared by atom transfer radical polymerization. Poly(vinyl chloride) containing small amount of pendent chloroacetate units was used as a macroinitiator. The formation of the graft copolymer was confirmed with size exclusion chromatography (SEC), 1H NMR and IR spectroscopy. The graft copolymers with increasing incorporation of butyl acrylate result in an increase of molecular weight. One glass transition temperature (Tg) was observed for all copolymers. Tg of the copolymer with butyl acrylate decreases with increasing content of butyl acrylate.

Hydrogels by atom transfer radical polymerization. I. Poly(N‐vinylpyrrolidinone‐g‐styrene) via the macromonomer method

Atom transfer radical polymerization has been used to prepare well-defined vinyl macromonomers of polystyrene using vinyl chloroacetate as an initiator. Because styrene and vinyl chloroacetate do not copolymerize, no branching or incorporation of the initiator into the backbone was observed. Macromonomers of several molecular weights were prepared and copolymerized free radically with N-vinylpyrrolidinone in varying feed ratios in order to produce poly(NVP-g-Sty) graft copolymers. The macromonomers used were of sufficiently high molecular weight to form physical crosslinks in solvents which favor the hydrophilic NVP, such as water, which prevent the copolymer from dissolving and cause it to swell. These materials, therefore, formed hydrogels of swellabilities in water exceeding 95%, depending on the amount of styrene that was incorporated into the copolymer. Limitations of and alternatives to this method are also discussed.

Functionalization of polymers prepared by ATRP using radical addition reactions

Low molecular weight linear poly(methyl acrylate), star and hyperbranched polymers were synthesized using atom transfer radical polymerization (ATRP) and end-functionalized using radical addition reactions. By adding allyltri-n-butylstannane at the end of the polymerization of poly(methyl acrylate), the polymer was terminated by allyl groups. When at high conversions of the acrylate monomer, allyl alcohol or 1,2-epoxy-5-hexene, monomers which are not polymerizable by ATRP, were added, alcohol and epoxy functionalities respectively were incorporated at the polymer chain end. Functionalization by radical addition reactions was demonstrated to be applicable to multi-functional polymers such as hyperbranched and star polymers.

Preparation of hyperbranched polyacrylates by atom transfer radical polymerization, 4: The use of zero-valent copper

The addition of zero-valent copper to the self-condensing vinyl polymerization (SCVP) of novel AB* (meth)acrylic monomers using atom transfer radical polymerization (ATRP) catalyst systems has allowed for their successful polymerization. Polymerization under homogeneous and heterogeneous catalyst conditions without additional Cu(0) were unsuccessful. The catalyst system that was designed comprised of Cu(I) bromide, 4,4′-bis(5-nonyl)-2,2′-bipyridine, and Cu(0) metal (powder or turning). From 1H NMR spectroscopy, the degree of branching was estimated for the acrylic polymers, DB = 0.48. The degree of branching could not be determined for methacrylates by this method due to overlapping signals in the 1H NMR spectra.

Controlled Radical Polymerization in the Presence of Oxygen

Living polymerizations occur without termination or transfer reactions and have the advantage of being able to form well-defined polymers with predictable molecular weights and narrow polydispersities. The first examples of this were living anionic polymerizations,1 which require the exclusion of moisture and oxygen and are run at low temperatures. Radical polymerization methods have the advantage of being insensitive to the presence of water and have even been carried out in aqueous media. This allows for less rigorous reaction conditions and is convenient for industrial application. Free radical polymerizations typically have slow initiation and form a high molecular weight polymer limited by transfer and termination reactions leading to poorly controlled molecular weights and broad molecular weight distributions.2 Also, in contrast to living ionic polymerization, it is very difficult to prepare well-defined homopolymers and block copolymers. In recent years, radical polymerizations have been developed into controlled/“living” polymerizations yielding well-defined polymers. Currently, nitroxide-mediated,3 metal-mediated,4 and either rutheniumor coppercatalyzed atom transfer radical polymerization (ATRP)5 are at the forefront of controlled radical polymerizations. Improvements to these processes have been aimed toward application to new monomers, new initiators and new architectures, compositions, and functionalities.6 In ATRP, recent advances have also been in the direction of new ligands7 and new metals8 which affect the activity and selectivity of the ATRP catalysts for various monomers. Also, improvements have been made in ATRP by the addition of small amounts of zerovalent metal.9 Up to this point, radical polymerizations need to be carried out in an oxygen-free environment. ATRP, in fact, requires less stringent conditions since O2 can react with the catalyst as opposed to reacting with the free organic radicals which should be present in a much lower concentration. However, oxidation reduces the active catalyst concentration. For example, the Cu(I) catalyst is oxidized to a Cu(II) species which is not an active ATRP catalyst and can even be a deactivating species, if a halogen ligand is present, and further slow the polymerization.7b In this communication, we report that controlled radical polymerizations with polymers having low polydispersities (Mw/Mn < 1.2) can be prepared without any removal of oxygen or inhibitor and does not require purging with inert gas, if a sufficient amount of zerovalent metal is present. If Cu(I)Br/dNbpy complex is added (in excess), the polymerization occurs but at a slow rate. This is due to two factors: first, the amount of Cu(I) is reduced by oxidation to Cu(II), second, the concentration of Cu(II), which is a deactivator, is increased further slowing the polymerization.7b Adding Cu(0) to the system, reduces the Cu(II) to Cu(I) and allows for a smaller concentration of catalyst to be added initially.

FREE RADICAL POLYMERIZATION

1. Polymerization of methyl methacrylate in the presence of a nonpolar hydrocarbon solvent. I. Construction of a ternary phase diagram through in situ polymerization (Y. 2. Inverse free radical suspension polymerization as a route to encapsulate biologically active materials (C. 3. Polymer particles with a pomegranate-like internal structure via micro-dispersive polymerization in a geometrically confined reaction space. I. Experimental study (C. 4. Experimental and theoretical study of the reaction locus during the dispersion polymerization of methyl methacrylate in a nonpolar hydrocarbon solvent at low temperature (5. Modeling of phase inversion and particle stability in the dispersion polymerization of methyl methacrylate in a nonpolar hydrocarbon solvent (C. 1. Modeling of free radical polymerization of styrene by bifunctional initiators (5. Kinetics of free radical styrene polymerization with a symmetrical bifunctional initiator 2,5-dimethyl-2,5-bis(2-ethyl hexanoyl peroxy) hexane (W. 6. Free radical polymerization of styrene with a binary mixture of bifunctional initiators

Controlled/“Living” Radical Polymerization Applied to Water-Borne Systems

Atom transfer radical polymerization (ATRP), a controlled/“living” radical polymerization, has been extended to water-borne polymerization systems. In order to obtain a controlled ATRP reaction in a water-borne system, various criteria must be met, which are not necessary when conducted in organic solvents. The effect of surfactant, monomer, catalyst and initiator employed will be discussed, as each had a profound effect on the success of the ATRP reaction.