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Bottlebrush polymers consist of a linear backbone with densely grafted side chains which impact the rigidity of the molecule. The persistence length of the bottlebrush backbone in solution is influenced by both the intrinsic structure of the polymer and the local environment, such as the solvent quality and concentration. Increasing the concentration reduces the overall size of the molecule because of the reduction in backbone stiffness. In this study, we map out the size of a bottlebrush polymer as a function of concentration for a single backbone length. Small-angle neutron scattering measurements are conducted on a polynorbornene-based bottlebrush with polystyrene side chains in a good solvent. The data are fit using a model which provides both the long and short axis radius of gyration (R_(g,2) and R_(g,1), respectively), providing a measure for how the conformation changes as a function of concentration. At low concentrations, a highly anisotropic structure is observed (R_(g,2)/R_(g,1) ≈ 4), becoming more isotropic at higher concentrations (R_(g,2)/R_(g,1) ≈ 1.5). The concentration scaling for both R_(g,2) and the overall Rg is evaluated and compared with predictions in the literature. Coarse-grained molecular dynamics simulations were also conducted to probe the impact of concentration on bottlebrush conformation, showing qualitative agreement with the experimental results.

A series of second-generation dendronized wedge polymers were synthesized by ring-opening metathesis polymerization, and the linear viscoelastic response over a wide range of temperatures was investigated. From 0 to 90 °C the dynamic moduli (G′(ω) and G″(ω)) were determined, and frequency–temperature superposition was used to create master curves that showed behavior from the terminal zone to the glassy regime. An apparent extremely low rubbery plateau of ∼10 kPa was observed in both the dynamic response and in the corresponding van Gurp–Palmen plot. However, further investigation shows that the apparent rubbery plateau is related to the steady-state recoverable compliance, not the onset of entanglements. In addition, these wedge polymers exhibit an extremely low glassy modulus of ∼100 MPa at 0 °C, which is shown to increase at 1 Hz to ∼700 MPa at −80 °C for the wedge polymer 2G-EHW-311. In addition, both small- and wide-angle X-ray scattering patterns were obtained for all of the polymers investigated, and these showed that the polymer molecules adopt an extended cylinder conformation. Furthermore, based on calorimetric measurements, the polymers were found to exhibit two glass transition temperatures, with a 100 K difference between the higher (T_(g,hi) = 26.8 ± 0.7 °C) and lower glass transition temperatures (T_(g,lo) = −76.1 ± 1.1 °C) for the 2G-EHW-311 material. Hence, an intermediate regime extends to well below the T_(g,hi) to T_(g,lo), providing an explanation for the low glassy modulus of ∼100 MPa at 0 °C and its increase to ∼700 MPa when measured at T_(g,hi) – 100 °C and approaching the T_(g,lo).

Ru-based metathesis catalysts employed in cyclopolymerization (CP) of 1,6-heptadiyne derivatives have promoted regioselective α-addition to alkynes, forming various conjugated polyenes containing exclusively five-membered repeat units. Recently, we discovered that a new chelated Ru catalyst could promote regioselective β-addition to produce analogous polyenes containing six-membered rings with moderate to good β-selectivity. Since then, we have focused our research on pursuing more active and β-selective regiocontrol to produce conjugated polymers with excellent β-selectivity, with a much broader range of monomers. Herein, we demonstrate highly β-selective CP by combining a new dithiolate-chelated Ru-based catalyst with weakly coordinating pyridine additives, which significantly enhance the conversion and β-selectivity. An in-depth mechanistic investigation by ^1H NMR revealed a prominent role for the additives, which improve the stability of the propagating carbene.

The ideal of living polymerization has defined research in polymer chemistry over the past 50 years. In this Perspective, we present the case that this concept has enabled the treatment of polymers as organic molecules, rather than impure mixtures of species, and allowed the translation of methods developed by synthetic organic chemists into ever more accessible living and/or controlled polymerization methods. The concurrent development of rapid analytical methods for screening new polymerization methods for living characteristics, chiefly size exclusion chromatography, has greatly aided in the expansion of living polymerization methods.

The alternating copolymerization of cis-cyclooctene and 1-substituted oxanorbornenes with commercially available ruthenium-based olefin metathesis catalysts was investigated. We determined that RuCl2(CHPh)(PCy3)2 performs a standard alternating copolymerization with these monomers. Unexpectedly, another route to the desired alternating copolymer was revealed. The “sequence editing” route involves the initial polymerization of cis-cyclooctene, followed a ring-opening cross metathesis step that introduces the 1-substituted oxanorbornene monomer in the polycyclooctene chain and largely avoids oxanorborene homopolymerization. Selectivity for alternating diads in the polymer exceeds 90%. Polymer molecular weight can be controlled by linear olefin chain-transfer reagents during sequence editing.

A new three-component bio-inspired adhesive was synthesized that is a terpolymer composed of a water-soluble segment, an interfacial adhesion segment, and a cross-linking segment. Strong wet adhesion properties are obtained utilizing a 3,4-dihydroxy-l-phenylalanine (DOPA) moiety. Poly(acrylic acid) provides high water solubility due to strong ionic interactions with water. An acrylic acid N-hydroxysuccinimide ester (NHS) was included in the adhesive polymer to allow rapid cross-linking with thiol-terminated, 3-armed poly(ethylene glycol) cross-linking agents. The thiol terminal poly(ethylene glycol) was designed to be bulky to avoid possible penetration of molecules to the cell and tissue. The NHS and thiol groups react within 30 s to form covalent bonds. This design allows for rapid optimization of properties for specific applications. Lap shear strength tests on wet porcine skin demonstrated a 190% increased value in adhesion strength for adhesives having the DOPA moiety. After cross-linking, adhesion was enhanced by 450% over poly(acrylic acid-co-acrylic acid NHS) and was 240% higher than un-cross-linked poly(acrylic acid-co-acrylic acid NHS-co-N-methacryloyl-3,4-dihydroxyl-l-phenylalanine). Rheology studies show adhesive viscosity drops significantly at high shear rates, demonstrating its potential to be injected via syringe. The cross-linked adhesive displayed much stronger mechanical properties and higher elastic and viscous moduli than an un-cross-linked adhesive model. Furthermore, the cross-linked adhesive has enhanced moduli near body temperature (38 °C) as compared to room temperature (23 °C), increasing the applications as a biomedical adhesive.

We have examined the linear rheological responses of a series of well-defined, dense, regularly branched brush polymers. These narrow molecular weight distribution brush polymers had polynorobornene backbones with degrees of polymerization (DP) of 200, 400, and 800 and polylactide side chains with molecular weight of 1.4 kDa, 4.4 kDa, and 8.7 kDa. The master curves for these brush polymers were obtained by time temperature superposition (TTS) of the dynamic moduli over the range from the glassy region to the terminal flow region. Similar to other long chain branched polymers, these densely branched brush polymers show a sequence of relaxation. Subsequent to the glassy relaxation, two different relaxation processes can be observed for samples with the high molecular weight (4.4 and 8.7 kDa) side chains, corresponding to the relaxation of the side chains and the brush polymer backbone. Influenced by the large volume fraction of high molecular weight side chains, these brush polymers are unentangled. The low...

Graft-through ring-opening metathesis polymerization (ROMP) using ruthenium N-heterocyclic carbene catalysts has enabled the synthesis of bottle-brush polymers with unprecedented ease and control. Here we report the first bivalent-brush polymers; these materials were prepared by graft-through ROMP of drug-loaded polyethylene-glycol (PEG) based macromonomers (MMs). Anticancer drugs doxorubicin (DOX) and camptothecin (CT) were attached to a norbornene-alkyne-PEG MM via a photocleavable linker. ROMP of either or both drug-loaded MMs generated brush homo- and co-polymers with low polydispersities and defined molecular weights. Release of free DOX and CT from these materials was initiated by exposure to 365 nm light. All of the CT and DOX polymers were at least 10-fold more toxic to human cancer cells after photoinitiated drug release while a copolymer carrying both CT and DOX displayed 30-fold increased toxicity upon irradiation. Graft-through ROMP of drug-loaded macromonomers provides a general method for the systematic study of structure-function relationships for stimuli-responsive polymers in biological systems.

Telechelic polyisoprene was synthesized via the ring-opening metathesis polymerization (ROMP) of 1,5-dimethyl-1,5-cyclooctadiene (DMCOD) in the presence of cis-1,4-diacetoxy-2-butene as a chain transfer agent (CTA). This method afforded telechelic polymer in excellent yield, and the acetoxy groups were successfully removed to yield α,ω-hydroxy end-functionalized polyisoprene with potential for subsequent reactions. Efficient, quantitative incorporation of CTA was achieved, and NMR spectroscopy was utilized to confirm the chemical identity of the polymer end groups. Polymerization of discrete DMCOD monomer generated polyisoprene with excellent regioregularity in the polymer backbone. Successful ROMP of sterically challenging DMCOD in the presence of a CTA for chain end-functionalization was borne out through screening of a variety of Ru-based olefin metathesis catalysts.

Novel block copolymers were prepared by combining ROMP and ATRP. Using the fast initiating ruthenium metathesis catalyst (H_2IMes)(Cl)_2(pyr)_2RuCHPh and (Z)-but-2-ene-1,4-diyl bis(2-bromopropanoate) as a terminating agent, three different monotelechelic poly(oxa)norbornenes were synthesized. Complete end-functionalization was shown by carrying out the ATRP of styrene and tert-butyl acrylate from the poly(oxa)norbornenes using a CuBr/PMDETA catalyst system. GPC showed no remaining homopolymer, and all block copolymers were found to have matching theoretical and observed molecular weights and low polydispersities.

The syntheses of regioregular as well as stereoregular ethylene-vinyl alcohol (EVOH) copolymers by ring-opening metathesis polymerization (ROMP) with ruthenium catalysts are reported. Symmetric cyclooctenediol monomers were protected as acetates, carbonates, or acetonides to temporarily increase ring strain as well as impart solubility to the monomer. Polymer molecular weights could be easily controlled by either varying the monomer-to-catalyst ratio or by the addition of a chain transfer agent. Hydrogenation and subsequent deprotection of the ROMP polymers afforded the EVOH materials in high yields, and the structures were confirmed by ^1H NMR and ^(13)C NMR spectroscopies. Thermal properties of the corresponding EVOH copolymers are reported and suggest that differences in diol stereochemistry significantly affect the polymer morphology.

A detailed study of the ring-opening metathesis polymerization of low-strain monomers with ruthenium catalysts is reported. The effects of monomer concentration and catalyst dependence are described for unsubstituted cycloolefins. The ROMP of low-strain olefins with polar substituents is also examined with ruthenium olefin metathesis catalysts, and a predictive model for ROMP feasibility is proposed.

Fluorinated tricyclo[4.2.1.0^(2,5)]non-7-ene-3-carboxylic acid esters are shown to undergo metal-catalyzed addition polymerization. The resulting homopolymers are transparent at 157 nm and demonstrate the utility of these monomers in development of photoresists for 157 nm lithography. Fluorinated tricyclononene (TCN) structures with ester substituents exhibit up to 3 orders of magnitude more transparency at 157 nm than conventional ester-functionalized norbornene structures as determined by gas-phase vacuum-ultraviolet spectroscopy and variable angle spectroscopic ellipsometry. Unlike their fluorinated norbornene counterparts, the fluorinated, ester-functionalized TCN monomers successfully undergo transition-metal-catalyzed addition polymerization to produce polymers with high glass transition temperatures and the etch resistance required for photolithographic resist materials applications. The potential use of fluorinated TCN structures for 157 nm photoresists is demonstrated through the synthesis and characterization of TCN monomers and polymers.

Three metal-catalyzed vinyl addition copolymers derived from partially fluorinated norbornenes and tricyclononenes have been synthesized and evaluated for use in formulating photoresists for 157 nm lithography imaging. The transparency of these polymers at 157 nm, as measured by variable angle spectroscopic ellipsometry (VASE), is greatly improved over their nonfluorinated counterparts. The results of preliminary lithographic evaluations of resists formulated from these polymers alone and with the addition of several new fluorinated dissolution inhibitors are presented. Images as small as 70 nm have been printed in some formulations.

The synthesis of a new methylene−(vinyl alcohol) copolymer, poly((vinyl alcohol)_2-alt-methylene) (MVOH), by ring-opening metathesis polymerization with ruthenium catalysts is reported. Unsaturated cyclic 1,3-diols were protected with a di-tert-butylsilyl group to form strained cyclic olefins. The molecular weights of the polymers were controlled by varying the monomer-to-catalyst ratios or by the addition of a chain transfer agent. Hydrogenation and subsequent deprotection of the ROMP polymers yield the MVOH copolymer structure which was confirmed by ^1H NMR and ^(13)C NMR spectroscopies. Thermal properties of the corresponding MVOH copolymer are reported.

Synthetic norbornenyl polymers with pendent cell adhesive sequences glycine-arginine-glycine-aspartic acid (GRGD) and serine-arginine-asparagine (SRN) were synthesized by ring-opening metathesis polymerization (ROMP) using newly developed ruthenium initiators. Initially, simpler polymers with pendent glycine, alanine, or penta(ethylene glycol) (EO_5) units attached directly or through ethyl and propyl spacers to various norbornenyl backbones were synthesized using Ru CHPh(Cl)_2(PCy_3)_2 (1) as the initiator. The molecular weights, PDI's, polymerization times, yields, and glass transition temperatures were compared for these polymers. As a result of this comparison, poly(5-norbornene-2-carboxyl) was chosen as the backbone for the more complex oligopeptide containing polymers, and norbornene monomers with pendent EO_5 (21), GRGD (24), and SRN (25) units were made. Monomers 21 and 24 were copolymerized to form a poly(norbornene) containing 9.2 mol % GRGD (26a) using 1 as the initiator. However, incorporating larger amounts of GRGD resulted in extremely low yields of polymers that exhibited bimodal molecular weight distributions. Homopolymers and copolymers with larger amounts of GRGD and SRN were synthesized in good yields (32−92%) with monomodal molecular weight distributions using the newly developed, more active, 2,3-dihydroimidazolylidene initiators, Ru CHPh(Cl)_2(PCy_3)(DHIMes) (2) and Ru CH−CH C(CH_3)_2(Cl)_2(PCp_3)(DHIMes) (3). In this way, EO_5 containing copolymers with 49 mol % GRGD (26b), 53 mol % SRN (27b), or 32 mol % GRGD and 21 mol % SRN (28a) were synthesized, as well as copolymer 28b with 53 mol % GRGD and 47 mol % SRN. To alter the presentation of the GRGD, an EO_5 containing copolymer with a propyl spacer between the GRGD and the backbone (30) was also synthesized.

The synthesis of ring-opening metathesis polymerization (ROMP) chain transfer agents (CTA)s bearing methacrylate or epoxide end functionality was accomplished. In the presence of these CTAs, the ROMP of cyclooctadiene (COD) initiated with (PCy_3)_2Cl_2Ru CHPh afforded the respective telechelic poly(butenylene)s with either methacrylate or epoxide end groups. Control over the polymer molecular weight was demonstrated by varying the COD/CTA ratio. Successful cross-linking by thermal or photochemical initiation of the bis(methacrylate)-functionalized telechelic poly(butenylene)s or through acid catalysis of the bis(epoxide)-functionalized telechelic poly(butenylene)s was accomplished. While cross-linking was found to reduce solubility of the polymers, it greatly enhanced their thermal stability.

A variety of polyethers were synthesized by a tandem approach incorporating ring-closing metathesis (RCM) followed by ring-opening metathesis polymerization (ROMP) using RuCl_2(=CHPh)(PCy_3)_2 (1) as an initiator. Unsaturated crown ether monomers, including a 12-crown-4 analogue (3), a benzocrown ether (8), and a benzocrown ether with a pendent phenylalanine methyl ester (9), were synthesized in good yields using a lithium ion as a template and 1 as a catalyst for RCM. Saponification of 9 afforded the benzocrown phenylalanine carboxylic acid monomer 10. The ROMP of 3, 8, and 9 with 1 as an initiator yielded the homopolymers 11, 12e, and 13e, respectively. The relative concentrations of 3 to 1 were varied to produce 11 with a wide range of molecular weights (M_n from 10 900 to 206 300). Hydrogenation of 11 proceeded quantitatively to yield a saturated polyether. Monomers 8 and 9 were copolymerized with 3 to generate polymers 12a−d and 13a−d, respectively. The copolymer composition corresponded to the feed ratio of the monomers. Crown ether 10 was copolymerized with 3 at a low feed ratio to form the corresponding polyether with pendent amino acids.

Discotic liquid crystalline polymers bearing alkoxy-substituted triphenylene moieties in the side-chain were synthesized by ring-opening metathesis polymerization using a well-defined ruthenium initiator. To elucidate the effect of backbone flexibility on the mesomorphism, norbornene and cyclobutene monomers containing triphenylene moieties were synthesized, which yielded the relatively rigid poly(norbornene)s and the more flexible poly(butadiene)s, respectively, after polymerization. To further increase the backbone flexibility, the poly(butadiene)s were hydrogenated using Crabtree's catalyst to yield triphenylene-substituted poly(butane)s. The mesomorphic behavior of the polymers has been investigated by differential scanning calorimetry (DSC) and powder diffraction X-ray scattering (wide angle X-ray scattering, WAXS). All polymers bearing a 2,3,6,7,19-pentakis(decyloxy)triphenylene-based mesogenic unit exhibit enantiotropic discotic hexagonal mesophases, while the pentoxy analogues do not display liquid crystalline behavior. No effect of backbone rigidity on the mesomorphism could be detected.

2,3-Dicarboxybicyclo[2.2.2]octa-2,5,7-triene (barrelene) monomers (4a,b) were synthesized by a novel route and then polymerized by ring-opening metathesis polymerization (ROMP). Complete initiation of the ROMP initiator (5) and living polymerization of monomer 4b were achieved by tuning the activity of 5 with hexafluoro-tert-butanol (HFB) and tetrahydrofuran (THF). The precursor polymers (6a,b) were readily converted to diester-substituted poly(1,4-phenylenevinylenes) (PPVs) (7a,b) by aromatizing the cyclohexadiene rings using 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ). The resulting PPVs were highly luminescent, and polymer 7b, bearing tert-butyl groups on its esters, was soluble in methylene chloride and chloroform. We found that partially oxidizing 6b so that only 80% of the polymer units are aromatized increases both the solubility and photoluminescence quantum yield of 7b. Deprotection of polymer 7b by acid-catalyzed thermolysis of the tert-butyl groups followed by treatment with aqueous base pr...

The polymerization of cyclooctadiene (COD) in the presence of an allylic difunctionalized chain transfer agent (1) by a ruthenium metathesis catalyst (Ru) was accomplished. Deprotection of the resulting acetate end-functionalized polybutadiene was performed, leading to commercially important 1,4-hydroxytelechelic polybutadiene (HTPBD) with number average functionalities close to 2.0. The polymerizations were performed at high monomer concentrations or in the absence of solvent, and the robust nature of Ru allowed the use of high monomer to catalyst ratios (∼0.01 mol % Ru relative to total olefin). Investigation of the metathesis of olefin alcohols with Ru showed that side reactions may complicate the use of these compounds as chain transfer agents in metathesis polymerizations. The kinetics of this Ru-catalyzed ring-opening metathesis polymerization with added chain transfer agent was also investigated.

The effect of backbone flexibility on the mesomorphic behavior of side-chain liquid crystalline polymers synthesized by ring-opening metathesis polymerization was investigated. The synthesis of norbornene and cyclobutene monomers containing a p-nitrostilbene moiety as the mesogenic group and polymerization of these monomers to produce side-chain liquid crystalline polymers with low polydispersities and defined molecular weights was accomplished. The relatively rigid poly(norbornene)s displayed enantiotropic nematic mesomorphism with glass transitions from 44 to 64 °C and isotropization temperatures between 108 and 121 °C, whereas the more flexible poly(butadiene)s showed enantiotropic smectic A mesomorphism with glass transition temperatures from 14 to 31 °C and isotropization temperatures between 74 and 111 °C. A diblock copolymer containing a 1:1 mixture of the poly(norbornene) and poly(butadiene) also exhibited a smectic A mesophase, clearly demonstrating the dominance of the poly(butadiene) backbone. ...

The syntheses of hexyl- and undecyl-substituted benzobarrelenes 10a,b are described. These molecules readily undergo ring-opening metathesis polymerization (ROMP) to generate the corresponding ring...

Norbornene and 7-oxanorbornene derivatives were polymerized by ring-opening metathesis polymerization (ROMP) in a living manner by employing three new well-defined, bimetallic ruthenium catalysts (...

The ROMP of 5-methacryloyl-1-cyclooctene and the copolymerization of this monomer with cyclooctadiene were investigated to produce polymers with cross-linkable side chains. The impact of concentration, monomer to catalyst ratio, and the amount of inhibitor in the polymerization was examined. These polymers were cross-linked through the methacrylate side chains with either thermal or photochemical initiation, and the incorporation of these polymers into poly(methyl methacrylate) (PMMA) to produce AB cross-linked materials was accomplished. A comparison of the physical properties of PMMA and these new materials demonstrated that these materials had higher thermal stability and solvent resistance than pure PMMA.

To address the practical issues of polymer molecular weight determination, the first accurate polymer weight-average molecular weight determination method in diverse living/controlled polymerization via DOSY (diffusion-ordered NMR spectroscopy) is reported. Based on the linear correlation between the logarithm of diffusion coefficient (log D) and the molecular weights (log M_w), external calibration curves were created to give predictions of molecular weights of narrowly dispersed polymers. This method was successfully applied to atom transfer radical polymerization (ATRP), reversible addition–fragmentation chain transfer (RAFT), and ring-opening metathesis polymerization (ROMP), with weight-average molecular weights given by this method closely correlated to those obtained from GPC measurement.

Ring opening metathesis polymerization (ROMP) has been used to prepare conjugated polymers that contain ferrocene moieties as part of their backbones. The conjugated polymer poly-(ferrocenylenedivinylene) (PFDV) and the analogous unconjugated polymer poly(ferrocenylenebutenylene) (PFB) with chain lengths of greater than 10 were found to be insoluble in typical organic solvents. The conductivities of oxidatively doped films of PFDV and PFB were found to be 10 -4 and 10 -5 Ω -l cm -l , respectively. Oxidative doping of the monomer 1,4-(1,1'-ferrocenediyl)-1-butene was found to yield conductivities on the order of 10 -5 Ω -l cm -l , supporting interchain hopping as the dominant mechanism for charge carrier movement through these films. The ROMP of monomers octamethyl-1,4-(1,1'-ferrocenediyl)-1,3-butadiene and 1,4-(1,1'-ferrocenediyl)-1-methoxy-1,3-butadiene was also studied. Although soluble polymer was successfully generated from the latter monomer, no conditions were found under which the octamethyl compound could be polymerized.

The ring-opening metathesis polymerization of cyclooctadiene (COD) in the presence of a difunctional chain transfer agent (CTA) was investigated. The effect of the CTA structure and choice of metathesis catalyst on the overall efficiency of the polymerization reaction is discussed. A separation of the oxygen functionality from the olefin in the CTA by two methylene units significantly enhances the stability of the discrete, tungsten-based metathesis catalyst used for these polymerizations. Furthermore, a more robust well-defined metathesis catalyst based on molybdenum was more effective than the tungsten-based catalyst. The preparation oflow-molecular-weight hydroxytelechelic polybutadienes with one type of regiochemistry, number-average functionalities that approach 2.0, and in high yield was accomplished.

A series of sugar-substituted norbornenes were synthesized in order to determine whether monodisperse glycopolymers might be prepared by ring-opening metathesis polymerization (ROMP) using the ruthenium carbene initiators (R 3 P) 2 Cl 2 Ru=CHCH=CPh 2 (R = phenyl (Ph), 1 ; R = cyclohexyl (Cy), 2). The polymerization of 2-((±)-exo-5-norbornene-2-carboxamido)-2-deoxy-D-glucopyranose (3) and the protected sugar derivatives based on this monomer, 2-((±)-exo-5-norbornene-2-carboxamido)-2-deoxy-1,3,4,6-tetra-O-acetyl-D-glucopyranose (4), 2-((±)-exo-5-norbornene-2-carboxamido)-2-deoxy-1,3,4,6-tetra-O-benzyl-D-glucopyranose (5), 2-((±)-exo-5-norbornene-2-carboxamido)-2-deoxy-1,3,4,6-tetra-O-triethylsilyl-D-glucopyranose (6), and 2-((±)-exo-5-norbornene-2-carboxamido)-2-deoxy-6-O-trityl-D-glucopyranose (7), were investigated. The Cy 3 PRu catalyst, 2, initiated the polymerization of monomers 3-7 ; only the acetate monomer, 4, could be polymerized by the less active Ph 3 PRu catalyst 1. Benzene is a more effective polymerization solvent than methylene chloride for the ether-protected sugar derivatives 5-7, whereas methylene chloride is a better solvent for the acetate monomer, 4, which is prone to gelation. The reaction rates are highly dependent upon the sugar protecting group and range from

In addition to environmental issues associated with polymers possessing easily cleavable links, the selective degradation of copolymers at one of the monomer units is a convenient route to telechelic polymers. Ring-opening metathesis polymerization (ROMP) initiated by (Cy 3 P) 2 Cl 2 Ru=CHR (Cy = cyclohexyl ; Ph = phenyl ; R = CHCPh 2 or Ph) was used for the synthesis of cyclooctadiene (COD)/4,7-dihydro-1,3-dioxepin copolymers. 1,4-Hydroxytelechelic polybutadiene (HTPBD) is obtained upon fragmentation of a copolymer of COD and 4,7-dihydro-2-phenyl-1,3-dioxepin at the benzylidene acetal functionalities by stirring with acids (aqueous HCl/MeOH, aqueous H 2 SO 4 , or trifluoroacetic acid (TFAA)/MeOH in CH 2 Cl 2 ). For the benzylidene acetal the copolymerization reaction may be run in neat monomer with no additional solvent. Reaction quenching, and polymer cleavage, precipitation, and purification may be effected in one step by stirring the viscous reaction product in MeOH containing 10% HCl. A two-step procedure using trifluoroacetic anhydride (TFAA)/acetic acid (AcOH) in CH 2 Cl 2 followed by NaOMe/MeOH in THF was employed for the degradation of a COD/4,7-dihydro-1,3-dioxepin copolymer. Acetal incorporation and thus the molecular weights of the resulting telechelics are dependent upon the feed ratios. The percent methylene acetal incorporated into the COD/acetal copolymer is ∼60% of the amount of acetal in the feed (e.g., for COD/acetal feed = 2 :1, COD/acetal incorporation =∼4 :1). For the less reactive benzylidene acetal, incorporation is 40% of the amount of acetal in the feed. Polydispersities for 1,4-HTPBD prepared from the COD/benzylidene acetal copolymer are typically narrow (∼1.1-1.3). It was also demonstrated that the COD/benzylidene acetal copolymer may be simultaneously hydrogenated and degraded in one step using p-toluenesulfonohydrazide to produce hydroxytelechelic polyethylene (HTPE).

The ring-opening metathesis polymerization (ROMP) of a variety of 5-substituted cyclooctenes by the well-defined metathesis catalyst (PCy 3 ) 2 Cl 2 RUCHCHCPh 2 (1) was accomplished. Direct polymerization of functionalized monomers allowed the incorporation of alcohol, ketone, ester, and bromine functionality along the polymer backbone. The polymers were obtained in moderate to good yields. The attempted polymerization of epoxy- and cyano-substituted cyclooctenes by 1 failed. Structures of the polymers were confirmed by IR, 1 H NMR, and 13 C NMR spectroscopies. The molecular weight of acetate-containing polymer was controlled by varying the monomer to catalyst ratio and by the addition of a chain transfer agent to the polymerization solution. Hydrogenation of the acetate derivative gave the corresponding ethylene/vinyl copolymer. The thermal properties of the polymers are reported.

Block copolymers (5) of silicon-containing norbornene derivatives were prepared using living ring-opening metathesis polymerization (ROMP) with a well-defined ruthenium carbene complex [1, (R 3 P) 2 Cl 2 Ru=CHCH=CPh 2 ; 1a, R = phenyl ; 1b, R = cyclohexyl]. For example, the living polymerization of norbornene 2a (NBE) by 1a was carried out at room temperature in a mixture of methylene chloride and 1,2-dichloroethane. After 3 h, the polymer isolated had a narrow molecular weight distribution (MWD) (M n = 58 000, M w /M n = 1.20). On addition of a silicon-containing norbornene derivative (4), polymerization immediately ensued at 50 °C from the living ends of 3 and afforded a block copolymer 5a. The MWD curve of the resulting block polymer 5a shifted toward the higher molecular weight region, and the polydispersity remained narrow (M n = 129 000, M w /M n = 1.25). The segment ratio (m :n = 1.86) of 5a determined by 1 H NMR was in good agreement with the calculated value (m :n = 1.78). Silicon-containing block copolymers with polar functional groups were also synthesized by ROMP with 1b.

Monomers 3-methylcyclobutene (1) and 3,3-dimethylcyclobutene (2) can be polymerized by well-defined alkylidene complexes of the type M(CHC(CH 3 ) 3 )(NAr)(OC(CH 3 ) n (CF 3 ) 3-n ) 2 (where Ar=2,6-diisopropylphenyl; M=Mo or W; n=2 or 3) to give polymers, which upon hydrogenation are equivalent to alternating copolymers of poly(ethylene-alt-propylene) and poly(ethylene-alt-isobutylene), respectively. Monomer 2 can be polymerized in a highly regioselective fashion to give polymers with exclusively trans and head-to-tail configuration. The high regioselectivity of the polymerization is attributed to the steric directing effect of the bulky substituent on the 3-position of the cyclobutene as the monomer approaches the propagating alkylidenes. In the presence of PPhMe 2 narrowly dispersed poly(3-methylcyclobutene) can be obtained

A combined computational and theoretical method is developed to predict the equilibrium ring-chain distribution of the products of ring-opening metathesis polymerization (ROMP). In ring-chain equilibria, the free energy change of the reaction includes an entropic cost associated with forming cyclic rather than linear products and an enthalpic cost if ring strain is significant (i.e., for small rings). The entropy change is determined using statistical mechanics (based on the Jacobson-Stockmayer theory). The enthalpy change is computed with a Monte Carlo configurational search using molecular mechanics (MM3). Using the resulting equilibrium constants, the calculated distribution of cyclic and linear products is in good agreement with experiment. To characterize the polymerizability of cycloolefins, we compute the critical concentration [M] c, ∞, defined as the total monomer lost to cyclic products.

The number-average degree of polymerization and polydispersity index of living polymerization in the presence of chain-transfer agents are calculated numerically and analytically to determine how the molecular weight of the polymer can be regulated. The solutions reveal that un-steady-state polymerization exists in certain cases. The Mayo plot is nearly linear, but (as opposed to a nonliving system) its slope is not equal to k tr /k p in general (where k i , k p , and k tr are the specific rate constants of initiation, propagation, and chain transfer, respectively), the former differing from the latter by about 0.5-1 order of magnitude when k tr k p = 0.01-1.0. Plots of the slopes of Mayo plot versus k tr k p for different values of k p /k i reveal that only when k tr is equal to or greater than k p by an order of magnitude can the molecular weight be effectively controlled by addition of chain-transfer agent. A sufficient amount of chain-transfer agent is a necessary but not a sufficient condition to ensure monomodal molecular weight distribution. An analytical expression for the number-average degree of polymerization of the dead chains when k tr < k p has been derived and shows excellent agreement with numerical results. An analytical expression relating the slope of the Mayo plot to k tr /k p has also been obtained. These equations hold exactly when the concentration of the catalyst is much less than that of the chain-transfer agent. An experimental investigation of the kinetics of ring-opening metathesis polymerization (ROMP) of norbornene by Mo(=CHCMe 2 Ph)(NAr)(OCMe 3 ) 2 (Ar = 2,6-diisopropylphenyl) (1) in the presence of neohexene suggests that this ROMP system is adequately described by a relatively simple polymerization scheme. As measured from NMR spectroscopy, the specific rate constants of initiation, propagation, and chain transfer at 22° C are 0.57, 17, and 0.00003 M -1 s -1 , respectively.

Near monodispersed linear polyethylene can be synthesized y hydrogenation of polybutadiene prepared by the living ring-opening metathesis polymerization (ROMP) of cyclobutene. The polyethylene is highly crystalline, as determined by powder X-ray diffraction, and defect free, as shown by spectroscopic data. Living polycyclobutene can also react with anthracene dialdehyde to give a labeled polymer and with norbornene to produce a block copolymer.

Ring-opening metathesis polymerization (ROMP) of exo,exo-5,6-bis(methoxymethyl)-1-oxabicyclo[2.2.1]hept-2-ene (3) catalyzed by Ru II (H 2 O) 6 (tos) 2 (tos= p-toluenesulfonate) (1) in the presence of terminal acyclic olefins such as 3-buten-1-ol and methyl acrylate affords low molecular weight polymeric materials. Analysis of these oligomer samples by one- and two-dimensional 1 H NMR spectroscopy identifies them as resulting from a true metathesis chain transfer mechanism. These data and the identification of the ring-opened monomer units with alkylidene end groups corresponding to the acyclic chain transfer agent by high resolution mass spectrometry confirm this study as the first example of metathesis of an acyclic olefin by 1

Ring-opening metathesis polymerization (ROMP) has evolved into a valuable tool for the polymer chemist. The polymerization is generally mild and well controlled, and a large pool of readily available cyclic olefins (monomers) can be polymerized to nearly any size or shape. Employing the Ru-based initiator 1 (or its more active derivative 2) permits incorporation of high degrees of functionality and affords polymers with novel mechanical, electronic, and more recently biological properties. However, the polydispersity indices (PDIs) of the polymers obtained from initiator 1 are generally broad (between 1.3 and 1.5), which arises from an unfavorable rate of initiation (k_i) relative to propagation (k_p) as well as considerable secondary metathesis (“backbiting”). This creates difficulties when attempting to accurately predict polymer molecular weight a priori or when preparing well-defined block copolymers (where complete initiation is necessary). A recent disclosure from Gibson and co-workers revealed that the initiation efficiency of 1 was enhanced when the PCy_3 ligands were substituted with Cy_2PCH_2Si(CH_3)_3. When the resulting complex (3) was used to initiate the ROMP of norbornene derivatives (similar to 4), the k_i/k_p was found to be 4.35 (up from 0.06 when initiated with 1), and the resulting polymers were nearly monodispersed (PDIs ∼ 1.1). The enhanced initiation was attributed to a combination of the lower basicity and smaller size of Cy_2PCH_2Si(CH_3)_3 (relative to PCy_3) which respectively helped facilitate phosphine dissociation (a key step in Ru-based ROMP, see below) and increase monomer accessibility.

The microstructures of polymers prepared from ring-opening metathesis polymerization (ROMP) of 7-oxanorborene (7-oxabicyclo[2.2.11 hept-2-ene) (3), endo-5-(methoxymethyl)-7-oxanorbornene (2), and exo,exo-5,6-bis(methoxymethyl)-7-oxanorbornene (1) with four metathesis catalysts were studied by ^1H and ^(13)C NMR spectroscopy. Tacticities were determined by hydrogenation of the polymers (which removed double-bond isomerism) and also by ROMP of (S)-endo-5-(methoxymethyl)-7-oxanorbornene (2S)(which related head-tail isomerism with tacticity). Polymers prepared from W(CH-t-Bu)(NAr)(OCMe(CF_3)_2)_2 (Ar = 2,6-diisopropylphenyl) (6) have all cis double bonds and are highly syndiotactic, while those from RuCl_3•3H_2O (8) and [RCl(µ-Cl)(η^3:η^3-C_(10)H_(16)]_2 (C_(10)H_(16)) = 2,7-dimethyloctadienediyl) (9) have a high trans double-bond content and are highly isotactic. Polymers prepared from [Ru(H_2O_6](tosylate)_2 (7) exhibit roughly equal amounts of cis and trans double bonds that are randomly distributed in the polymer chain and are atactic. The apparent correlation between double-bond isomerism and tacticity for a classical hexacoordinated metal center is consistent with the model proposed by Ivin and co-workers.

The ring-opening metathesis polymerization (ROMP) of exo-N-methyl-7-oxabicyclo[2.2.1]-hept-5-ene-2,3-dicarboximide (1) was accomplished in aqueous solution under mild conditions using the simple ruthenium(II) coordination complex Ru^(II)(H_2O)_6(tos)_2 (tos = p-toluenesulfonate) (2). The polymerization yield was nearly quantitative. The resultant high molecular weight polymer (poly-1) exhibited a glass transition temperature (T_g) of 225 °C and was thermally stable up to 402 °C. The thermal and radiative stabilities of poly-1 were further investigated. Poly-1 was hydrogenated to yield its saturated analogue, which had a lower T_g (170 °C) but a higher temperature of decomposition (430 °C). Complex 2 was deactivated by coordination of free maleimides formed during the polymerization of carboximide-functionalized monomers capable of undergoing retro Diels-Alder chemistry. The scope of this polymerization was therefore limited to carboximide-functionalized monomers that are stable toward retro Diels-Alder reactions at the polymerization temperature. Both ruthenium (II)-1 and ruthenium(II)-N-methylmaleimide olefin complexes were identified.

Over the past several years there has been an increasing interest in the synthesis of polymers with low glass transition temperatures and reactive functional groups at each end of a given polymeric chain. They have found use as agents capable of modifying the thermal and mechanical properties of condensation polymers, in the formation of polymeric networks, and as components in the synthesis of block copolymers. Telechelic hydroxyl, amino, and carboxyl poly(butadiene)s (PBD)s are excellent examples of such polymers that have been employed in the above applications. While these telechelic polymers can be synthesized through the radical or anionic polymerization of 1,3-butadiene using the appropriate initiating or terminating agents, there are several drawbacks. The average number of functional groups per polymer chain (F_n) can deviate greatly from 2 and depends heavily on experimental conditions, which are often demanding. Furthermore, varying amounts of 1,2-linkages are introduced during the polymerization, limiting the elastomeric potential of the resulting polymer.

The synthesis of poly(styrene)-b-poly(butadiene)-b-poly(styrene) (SBS) and poly(methyl methacrylate)-b-poly(butadiene)-b-poly(methyl methacrylate) (MBM) triblock copolymers with poly(butadiene) (PBD) segments containing 100% 1,4-microstructure is described. Bis(allyl chloride) and bis(2-bromopropionate) terminated telechelic PBD's were synthesized by the ring-opening metathesis polymerization (ROMP) of 1,5-cyclooctadiene in the presence of the corresponding difunctional chain transfer agents. These telechelic PBDs were subsequently used as difunctional macroinitiators for the heterogeneous atom transfer radical polymerization (ATRP) of styrene and methyl methacrylate to form SBS and MBM triblock copolymers. Triblock structure was confirmed by selective PBD degradation. In addition, the tandem ROMP−ATRP approach was successfully extended to a “one-pot” synthesis.

Ru II (H 2 O) 6 (tos) 2 (tos= p-toluenesulfonate) is an active catalyst for ring-opening metathesis polymerization (ROMP) of norbornene and 7-oxanorbornene derivatives in protic media. The activity of this catalyst can be extended to less reactive monomers, such as cyclooctene and cyclopentene, by the addition of ethyl diazoacetate to serve as an initiator. Due to the polymer/ring equilibrium for this system, the yield of polymer is low. The GPC trace of the product is distinctly bimodal, consisting of a mixture of cyclic oligomers and higher molecular weight polymer. These results show that the difficulty of polymerizing unstrained cyclic olefins in the original ruthenium systems lies in the initiation step and not in the propagation

Living polymerizations using ring-opening metathesis catalysts (ROMP) have allowed the synthesis of polymers with controlled structure and properties. In the past few years, efforts have been focused on developing novel transition-metal alkylidene complexes that catalyze this reaction and provide either access to new product or easier synthesis. All of the catalysts developed to date are high oxidation state organometallic complexes, which exhibit limited tolerance to polar functional groups, and only catalyze the living polymerization of norbornene and its derivatives. Recently, we have synthesized a ruthenium alkylidene complex that exhibits exceptional tolerance to a variety of polar functional groups and also catalyzes the living ROMP of norbornene. Because we were interested in elucidating the details of this living polymerization, we investigated the ROMP of highly reactive monomer bicyclo[3.2.0] heptene (I), catalyzed by the late-transitionmetal complex (PPh_3)_2(Cl)_2Ru(=CHCH=CPh_2) (II; eq 1). We hope that an investigation of this system will provide us with a better understanding of the mechanism of polymerization catalyzed by late-transition-metal alkylidene complexes and eventually allow us to control molecular weight in the preparation of functionalized polymers with potential uses in biological applications.

The ring opening metathesis polymerization of 1,5-cyclooctadiene catalyzed by the tungstene complex W(CHAr)(NPh)[OCCH 3 (CF 3 ) 2 ] n (THF) leads to a telechelic polymer having a repeat unit identical to that of 1,4-polybutadiene