Access to Hydroxy-Functionalized Polypropylene through Coordination Polymerization

Preparation of polyethylenes containing hydroxy groups has been already industrialized through radical copolymerization under harsh conditions followed by alcoholysis. By contrast, hydroxy-functionalized polypropylene has proven a rather challenging goal in polymer science. Propylene can't be polymerized through a radical mechanism, and its coordination copolymerization with polar monomers is frustrated by catalyst poisoning. Herein, we report a new strategy to reach this target. The coordination polymerization of allenes by rare-earth-metal precursors affords pure 1,2-regulated polyallenes, which are facilely transformed into poly(allyl alcohol) analogues by subsequent hydroboration/oxidation. Strikingly, the copolymerization of allenes and propylene gives unprecedented hydroxy-functionalized polypropylene after post-polymerization modification. Mechanistic elucidation by DFT simulation suggests kinetic rather than thermodynamic control.     

Graft polymerization of epoxide and alternating graft copolymerization of epoxide–acid anhydride onto a polystyrene backbone having an aminimide moiety

Graft polymerization of glycidyl phenyl ether (GPE) and alternating graft copolymerization of GPE–succinic anhydride (SA) onto a polymer‐supported aminimide were examined. The polymer‐supported aminimide was synthesized by radical polymerization of 1,1‐dimethyl‐1‐(2‐hydroxy‐3‐(4‐vinylbenzyloxy)propyl)amine 2‐benzoylimide, which was prepared by the reaction of methyl benzoate with equimolar amounts of 1,1‐dimethyl hydrazine and 4‐glycidylmethylstyrene. This aminimide could initiate the polymerization of GPE and alternating copolymerization of GPE with SA to give the corresponding graft copolymers. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 1041–1048, 1999     

Synthesis of polypropylene graft copolymers by the combination of a polypropylene copolymer containing pendant vinylbenzene groups and atom transfer radical polymerization

This article discusses a facile and inexpensive reaction process for preparing polypropylene‐based graft copolymers containing an isotactic polypropylene (i‐PP) main chain and several functional polymer side chains. The chemistry involves an i‐PP polymer precursor containing several pendant vinylbenzene groups, which is prepared through the Ziegler–Natta copolymerization of propylene and 1,4‐divinylbenzene mediated by an isospecific MgCl₂‐supported TiCl₄ catalyst. The selective monoenchainment of 1,4‐divinylbenzene comonomers results in pendant vinylbenzene groups quantitatively transformed into benzyl halides by hydrochlorination. In the presence of CuCl/pentamethyldiethylenetriamine, the in situ formed, multifunctional, polymeric atom transfer radical polymerization initiators carry out graft‐from polymerization through controlled radical polymerization. Some i‐PP‐based graft copolymers, including poly(propylene‐g‐methyl methacrylate) and poly(propylene‐g‐styrene), have been prepared with controlled compositions. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 429–437, 2005     

Behavior of nitroxyl radical in the polymerization process forming copolymers bearing diarylnitrone pendants and photocontrol of refractive indices for these copolymers

To establish the reaction condition under which the radical copolymerization of methyl methacrylate (MMA) with α‐(2‐hydroxy‐4‐methacryloyloxyphenyl)‐N‐(2,6‐dimethylphenyl)nitrone (HMDN) proceeds smoothly to give photoreactive copolymers, the effects of the nitrone chromophore on the extent to which the radical polymerization of MMA is inhibited were investigated. It was found that the reversible addition of initiating radical to the CH?N⁺(? O^?) moiety in the nitrone chromophore readily occurs to give the nitroxyl radical. It was also found that the latter radical undergoes an efficient coupling reaction with propagating radical to inhibit the radical copolymerization of MMA with HMDN. However, on raising the reaction temperature and the radical concentration, the copolymerization was successfully carried out. This polymerization condition allowed us to prepare the HMDN/MMA, HMDN/styrene, and HMDN/cyclohexyl acrylate copolymers in good yields. The photoirradiation of the copolymer film prepared on a silicon wafer lowered its refractive index by 0.003–0.023, depending on the relative composition of the diarylnitrone chromophore in these copolymers. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 88–97, 2006     

Reversible Addition‐Fragmentation Chain‐Transfer Polymerization for the Synthesis of Poly(4‐acetoxystyrene) and Poly(4‐acetoxystyrene)‐block‐polystyrene by Bulk,Solution and Emulsion Techniques

The living radical polymerization of 4‐acetoxystyrene via the RAFT process has been achieved employing bulk, solution and emulsion techniques. The rate of polymerization was studied between 60°C and 90°C. Increasing the temperature increases the rate of polymerization without affecting the polydispersity. Poly(4‐acetoxystyrene) with narrow polydispersity (1.08) was obtained. Various novel dithiocarboxylic esters and dithiocarbamates were screened as chain‐transfer agents for the RAFT polymerization of 4‐acetoxystyrene. The block copolymerization of poly(4‐acetoxystyrene) with styrene leading to poly(4‐acetoxystyrene)‐block‐polystyrene confirmed the presence of active chain ends in the first block. The acetoxy polymers were hydrolyzed to the corresponding hydroxy polymers under mild basic conditions.     

Living/controlled copolymerization of acrylates with nonactivated alkenes

The living/controlled copolymerization of methyl acrylate with 1‐alkenes and norbornene derivatives through several radical polymerization techniques has been achieved. These techniques include atom transfer radical polymerization, reversible addition–fragmentation transfer polymerization, nitroxide‐mediated polymerization, and degenerative transfer polymerization. These systems display many of the characteristics of a living polymerization process: the molecular weight increases linearly with the overall conversion, but the polydispersity remains low. Novel block copolymers have been synthesized through the sequential addition of monomers or chain extension. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 6175–6192, 2004     

Thermosensitive polymer structures based on segmented copolymer networks

Segmented polymer networks with LCST‐behavior have been prepared by free radical initiated copolymerization of α,ω‐bis‐methacrylate terminated poly(methyl vinyl ether) (PMVE) with 2‐hydroxy ethyl methacrylate (HEMA) or butyl acrylate (BA). The PMVE bis‐macromonomers have been obtained via a semi‐continuous process by end‐capping the living cationic polymerization of methyl vinyl ether (MVE) with HEMA. The phase separation temperature can be varied by changing the PMVE/comonomer ratio. Incorporation of PMVE‐grafts in the hydrogels increases the rate of deswelling and improves the mechanical properties. The application of the segmented networks for thermo‐controllable solid phase extraction has been demonstrated by their thermosensitive adsorption behavior of toluene from a water solution.     

Stereo‐ and Temporally Controlled Coordination Polymerization Triggered by Alternating Addition of a Lewis Acid and Base

Significant progress has been made with regard to temporally controlled radical and ring‐opening polymerizations, for example, by means of chemical reagents, light, and voltage, whereas quantitative switch coordination polymerization is still challenging. Herein, we report the temporally and stereocontrolled 3,4‐polymerization of isoprene through allosterically regulating the active metal center by alternating addition of Lewis basic pyridine to “poison” the Lewis acidic active metal species through acid–base interactions and Lewis acidic AlⁱBu₃ to release the original active species through pyridine abstraction. This process is quick, quantitative, and can be repeated multiple times while maintaining high 3,4‐selectivity. Moreover, this strategy is also effective for the switch copolymerization of isoprene and styrene with dual 3,4‐ and syndiotactic selectivity. Tuning the switch cycles and intervals enables the isolation of various copolymers with different distributions of 3,4‐polyisoprene and syndiotactic polystyrene sequences.     

Regio‐ and Enantioselective Photodimerization within the Confined Space of a Homochiral Ruthenium/Palladium Heterometallic Coordination Cage

The photoinduced regio‐ and enantioselective coupling of naphthols and derivatives thereof is achieved in the confined chiral coordination space of a Ru^II metalloligand based cage. The racemic or enantiopure cages encapsulate naphthol guests, which then undergo a regiospecific 1,4‐coupling, rather than the normal 1,1‐coupling, to form 4‐(2‐hydroxy‐1‐naphthyl)‐1,2‐napthoquinones; moderate stereochemical control is achieved with homochiral cages. The photoreactions proceed under both aerobic and anaerobic conditions but through distinct pathways that nevertheless involve the same radical intermediates. This unusual dimerization constitutes a very rare example of asymmetric induction in biaryl coupling by making use of coordination cages with dual functionality—photoredox reactivity and stereoselectivity.     

Novel dinitroxide mediating agent for stable free‐radical polymerization

A novel dinitroxide mediating agent that was suitable for stable free‐radical polymerization was synthesized and used in the block copolymerization of styrene and t‐butyl styrene. Quantitative yields of a novel dinitroxide based on 1,6‐hexamethylene diisocyanate and 4‐hydroxy‐2,2,6,6‐tetramethyl‐1‐piperidinyloxy were obtained. Various experimental parameters, including the nitroxide‐to‐initiator molar ratio, were examined, and it was determined that the polymerization was most controlled under conditions similar to those of conventional 2,2,6,6‐tetramethyl‐1‐piperidinyloxy‐mediated stable free‐radical polymerization. Moreover, the dinitroxide mediator proved to be a viable route for the facile two‐step synthesis of triblock copolymers of styrene and t‐butyl styrene. However, the dinitroxide mediation process resulted in a higher than expected level of nitroxide decomposition, which resulted in polymers possessing a terminal alkoxyamine and an adjacent hydroxylamine rather than a preferred internal bisalkoxyamine. This decomposition resulted in the formation of diblock copolymer species during the triblock copolymer synthesis. Gel permeation chromatography was used to monitor the chain‐end decomposition kinetics, and the determined observed rate constant (5.89 × 10^?5 s^?1) for decomposition agreed well with previous studies for other dinitroxide mediating agents. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 1547–1556, 2004     

A Comparative Study of Methyl Methacrylate/Butyl Acrylate Copolymerization Kinetics by Atom‐Transfer and Conventional Radical Polymerization

Methyl methacrylate and butyl acrylate monomers are copolymerized by atom‐transfer radical polymerization, affording polymers with well‐controlled molecular weight and low polydispersity. A kinetic analysis of this system is compared with the corresponding free‐radical polymerization system. The copolymerization rate follows an opposite trend to that observed in conventional copolymerization. This fact is attributed to a smaller population of radicals generated in the reaction, since the relative fraction of propagating radicals is the same as that in classical copolymerization.     

Olefins and Vinyl Polar Monomers: Bridging the Gap for Next Generation Materials

The inherent differences in reactivity between activated and non‐activated alkenes prevents copolymerization using established polymer synthesis techniques. Research over the past 20 years has greatly advanced the copolymerization of polar vinyl monomers and olefins. This Review highlights the challenges associated with conventional polymerization systems and evaluates the most relevant methods which have been developed to “bridge the gap” between polar vinyl monomers and olefins. We discuss advancements in heteroatom tolerant coordination–insertion polymerizations, methods of controlling radical polymerizations to incorporate olefinic monomers, as well as combined approaches employing sequential polymerizations. Finally, we discuss state‐of‐the‐art stimuli‐responsive systems capable of facile switching between catalytic pathways and provide an outlook towards applications in which tailored copolymers are ideally suited.     

Controlled radical polymerization in the presence of N‐oxyls

Recent development in controlled radical polymerization has provided a tool to combine a relatively robust radical polymerization technique with structural control. This contribution focuses on stable free radical polymerization in the presence of nitroxides. The influence of 2,2,6,6‐tetramethyl‐piperidine‐N‐oxyl (TEMPO) and temperature on the copolymerization of styrene and acrylonitrile will be discussed. In the second part a new class of nitroxide stable free radicals will be presented that shows enhanced performance in styrene polymerizations.     

Synthesis and Characterization of Poly(ϵ‐CL)‐block‐poly(MMA‐co‐St)‐block‐poly(ϵ‐CL) by Combination of Coordination and Controlled Radical Polymerization

The triblock copolymer poly(ϵ‐caprolactone)‐block‐poly[(methyl methacrylate)‐co‐styrene]‐block‐poly(ϵ‐caprolactone) was synthesized by a combination of coordination polymerization and controlled radical mechanism. The poly(ϵ‐caprolactone) prepolymers (PCL_BP) were first obtained by coordination polymerization using benzopinacol as the initiator and aluminium triisopropoxide as the promoter at room temperature. It was determined by means of UV and NMR spectroscopy that the benzopinacolate groups are left intact in the PCL_BP prepolymers; no isomerization was found. The benzopinacolate groups incorporated into the poly(ϵ‐caprolactone) then initiate the copolymerization of styrene (St) and methyl methacrylate (MMA) via a controlled radical mechanism at 95°C. The desired block copolymers were characterized by GPC, IR, UV and NMR spectroscopy in detail.     

Intramolecular Hydrogen Bonding: The Case of β‐Phosphorylated Nitroxide (= Aminoxyl) Radical

Alkoxyamines and persistent nitroxide (= aminoxyl) radicals are important regulators of nitroxide‐mediated radical polymerization. Since polymerization times decrease with the increasing homolysis rate constant of the C? ON bond homolysis between the polymer chain and the aminooxy moiety, the factors influencing the cleavage rate constant are of considerable interest. It has already been shown that the value of the homolysis rate constant k_d is very sensitive to the stabilization of both released radical species. X‐Ray, EPR, and kinetic data showed that the intramolecular H‐bonding radical in the 1‐(diethoxyphosphoryl)‐2,2‐dimethylpropyl 2‐hydroxy‐1,1‐dimethylethyl nitroxide ( 3a ) (homologue of 2‐hydroxy‐1,1‐dimethylethyl 1‐phenyl‐2‐methylpropyl nitroxide ( 2a )) did not occur with the nitroxide moiety as expected but with the phosphoryl group. However, the polymerization rate of styrene (= ethenylbenzene) was significantly enhanced.     

Multisensitive polymers based on 2‐vinylpyridine and N‐isopropylacrylamide *

Poly(2‐vinylpyridine) (P2VP) containing functionalized end groups was synthesized using nitroxyl‐mediated radical polymerization with a hydroxy‐functionalized stable free radical. It was shown that P2VP could be synthesized with variable molar masses and low polydispersities. The transformation of the hydroxy groups to an acrylic ester led to the formation of macromonomers. A free‐radical copolymerization of these macromonomers with N‐isopropylacrylamide gave a graft copolymer with a poly(N‐ispopropylacrylamide) backbone and P2VP side chains. Polymers containing different amounts of the monomers were synthesized. It was possible to vary both the amount of P2VP side chains at a constant chain length of the macromonomer and the chain length at a nearly constant chain number. The behavior of the multifunctional macromolecules at different temperatures and pH values was investigated using dynamic light scattering and DSC. The macromolecules were found to retain the specific properties of the homopolymers. The hydrodynamic radii of the synthesized graft copolymers were both dependent on the temperature and pH value. © 2001 John Wiley & Sons, Inc. J Polym Sci Part A: Polym Chem 39: 3797–3804, 2001     

Interfacial living radical copolymerization of oil‐ and water‐soluble comonomers to form composite polymer capsules

The suspension copolymerization of methyl methacrylate with hydroxy‐functional poly(ethylene glycol) monomethacrylate (PEGMA) by atom transfer radical polymerization (ATRP) yielded soluble, controlled‐molecular‐weight amphiphilic copolymers (weight‐average molecular weight/number‐average molecular weight <1.3). Despite extensive partitioning of PEGMA into the water phase, copolymers containing up to 24 mol % PEGMA were formed in the oil phase, from comonomer feeds containing 30 mol % PEGMA. Conversions by suspension polymerization were comparable to those obtained by solution polymerization, at over 70%. Suspension copolymers with high PEGMA contents contained high‐molecular‐weight polymer formed by uncontrolled polymerization, unless poly(vinyl pyrrolidone) was added to displace the growing polymer from the interface. The addition of diethylene glycol dimethacrylate gave capsules at 17 mol % PEGMA with ATRP, whereas conventional free‐radical polymerization required 24 mol % PEGMA to form capsules. The lower PEGMA level required for capsule formation with ATRP was attributed to the lower rates of propagation and crosslinking and to improved incorporation of PEGMA into the final gels. Suspension ATRP with 24 mol % PEGMA in the feed gave two‐layer capsule walls consisting of an inner layer visible by transmission electron microscopy and an outer layer visible by both transmission electron microscopy and environmental scanning electron microscopy, which indicated a compositional gradient across the capsule wall. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 156–171, 2006     

Nitroxide mediated living radical polymerization of styrene onto poly (vinyl chloride)

Nitroxide‐mediated ‘living’ free radical polymerisation (LREP) was employed for the first time to prepare graft copolymer by having arylated poly (vinyl chloride) (PVC‐Ph) as a backbone and polystyrene (PS) as branches. The graft copolymerization of styrene was initiated by arylated PVC carrying 2,2,6,6‐tetramethyl‐1‐piperidinyloxy (TEMPO) groups as a macroinitiator. Thus, the arylated PVC was prepared in the mild conditions and these reaction conditions could overcome the problem of gelation and crosslinking in polymers. Then, 1‐hydroxy TEMPO was synthesized by the reduction of TEMPO with sodium ascorbate. This functional nitroxyl compound was coupled with brominated arylated PVC (PVC‐Ph‐Br). The resulting macro‐initiator (PVC‐Ph‐TEMPO) for ‘living’ free radical polymerization was then heated in the presence of styrene to form graft copolymer. DSC, GPC, ¹HNMR, and FT‐IR spectroscopy were employed to investigate the structure of the polymers. Copyright © 2007 John Wiley & Sons, Ltd.     

Synthesis of i‐PP‐based functional block copolymer by a facile combination of styryl‐capped i‐PP and ATRP

This article demonstrates a facile and efficient method to combine olefin coordination polymerization with atom transfer radical polymerization (ATRP) for the synthesis of isotactic polypropylene (i‐PP)‐based functional diblock copolymers. The chemistry involves a styryl‐capped i‐PP precursor prepared through the controlled consecutive chain transfer reaction, first to 1,2‐bis(4‐vinylphenyl)ethane and then to hydrogen in propylene polymerization mediated by an isospecific metallocene catalyst. The i‐PP precursor can be quantitatively transformed into i‐PP terminated with a 1‐chloroethylbezene group (i‐PP‐t‐Cl) by a straightforward hydrochlorination process using hydrogen chloride. With the resultant i‐PP‐t‐Cl as a macroinitiator of ATRP, methyl methacrylate (MMA) polymerization was exemplified in the presence of CuBr/pentamethyldiethylenetriamine, preparing i‐PP‐b‐PMMA copolymers of different PMMA contents. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2010     

Design and synthesis of structurally well-defined functional polypropylenes via transition metal-mediated olefin polymerization chemistry

Functionalization of polyolefins is an industrially important yet scientifically challenging research subject. This paper summarizes our recent effort to access structurally well-defined functional polypropylenes via transition metal-mediated olefin polymerization. In one approach, polypropylenes containing side chain functional groups of controlled concentrations were obtained by Ziegler-Natta-catalyzed copolymerization of propylene in combination with either living anionic or controlled radical polymerization of polar monomers. The copolymerization of propylene with 1,4-divinylbenzene using an isospecific MgCl₂-supported TiCl₄ catalyst yielded polypropylenes containing pendant styrene moieties. Both metalation reaction with n-butyllithium and hydrochlorination reaction with dry hydrogen chloride selectively and quantitatively occurred at the pendant reactive sites, generating polymeric benzyllithium and 1-chloroethylbenzene species. These species initiated living anionic polymerization of styrene (S) and atom transfer radical polymerization (in the presence of CuCl and pentamethyldiethylenetriamine) of methyl methacrylate (MMA), respectively, resulting in functional polypropylene graft copolymers (PP-g-PS and PP-g-PMMA) with controllable graft lengths. In another approach, chain end-functionalized polypropylenes containing a terminal OH-group with controlled molecular weights were directly prepared by propylene polymerization with a metallocene catalyst through a selective aluminum chain transfer reaction. Both approaches proved to be desirable polyolefin functionalization routes in terms of efficiency and polymer structure controllability.