Growing Chain End Research Articles

Surface modification and lamination of polymers by photografting

This paper describes experimental processes for UV initiated “surface photografting” of polymers: the batch process with vapor-phase transfer of initiator and monomer and the continuous process with liquid-phase transfer of initiator and monomer in a thin layer of solution on the surface of the substrate. The grafting effiiency is about 80% and the resulting grafted layers are very thin (less than 10 nm). The mechanism of “surface photografting” is discussed and proposed to be based on excitation of benzophenone (BP) as initiator to an S 2 state by absorption of far UV irradiation (≈250 nm) and transition to a “superheated” “BPS 1” state, which transfers its excess energy to CH groups at the substrate surface and forms BP triplet states (T 1 and T 2). The excited CH groups and the BPT 1 and BPT 2 states react by hydrogen abstraction and form radicals at the substrate surface. These radicals add monomer to grafted chains. “ Surface photografting” is developed as a method for “ photoinitiated lamination” of plastic films. A thin layer of initiator and monomer between two films is grafted to the two film surfaces by UV irradiation. Short linear grafted chains are formed which by continued UV-irradiation grow to branched chains which eventually fill the space between the two substrate films. The chain ends are terminated by addition of ketyl radicals from the benzophenone. Some growing chain ends may terminate by radical combination. With addition of a small amount of a multifunctional monomer, a network is formed which is grafted to the two film surfaces by main valence bonds. The “ bulk surface photografting” gives laminate of high mechanical strength. When a functional film is grafted between two carrier films, laminates of selected and high barrier properties are formed which are of interest for application as packageing materials.

Ring-opening polymerization of heterocycles bearing substituted and unsubstituted exo-methylene groups - a review

Monocyclic monomers bearing exo-methylene groups are likely to undergo ring-opening polymerization under the conditions of radical or cationic initiation. Several structural features will determine the course of reaction after initiation by species that exclusively attack the exo-methylene functionality. Reviewing some of the most common classes of monocyclic heterocycles it becomes obvious that a monomer has to meet certain structural preconditions that makes them suitable for isomerizations. Among the considered heterocycles there are some types of monomers possessing an outstanding ring-opening behavior following polymerization mechanisms free from side-reactions. The combination of strain release due to ring-opening isomerization, an energy decrease in the transition state caused by the formation of carbonyl-functionalities and an appropriate stabilization of the growing chain end by steric and electronic effects may lead to clear ring-opening mechanisms. However, this property remains restricted to only some special monomers with simple aromatic substituents being attached to the cycle. Hence a variety of cyclic ketene acetals, enolethers, (meth)acrylates, carbonates, lactones and lactames have been chosen in order to demonstrate both the possibilities and limits of ring-opening.

Controlled Anionic Polymerization of tert-Butyl Acrylate with Diphenylmethyl Anions in the Presence of Dialkylzinc

The anionic polymerization of tert-butyl acrylate (tBA) was carried out with a binary initiator system prepared from diphenylmethyllithium, -potassium, or -cesium (Ph2CHM) and dimethylzinc or diethylzinc in THF at −78 °C. In the absence of dialkylzinc (R2Zn), the poly(tBA)s produced with Ph2CHM possessed ill-controlled molecular weights and broad molecular weight distributions (MWDs). On the other hand, poly(tBA)s having predicted molecular weights based on the molar ratio of monomer to initiators and narrow MWDs (Mw/Mn < 1.15) were obtained in quantitative yields by the initiation with Ph2CHK or Ph2CHCs in the presence of 10−20-fold excess R2Zn, although the polymers obtained with Ph2CHLi/R2Zn still possessed broad MWDs (Mw/Mn = 2). The growing chain end of poly(tBA) associated with cesium counterion is completely stable to reinitiate the further polymerization of tBA in the presence of R2Zn at −78 °C for 1 h, but 56% deactivation occurred after 24 h. It is thus demonstrated that Ph2CHK or Ph2CHCs in conjunction with R2Zn induces the controlled anionic polymerization of tBA in THF at −78 °C. By contrast, addition of R2Zn to the polymerization systems of methyl, ethyl, and isopropyl acrylates showed little effects on the improvement of polymer yields and the molecular weight controls. Well-defined block copolymers, poly(styrene-b-tBA), poly(methyl methacrylate-b-tBA), and poly(tBA-b-N,N-diethylacrylamide), were also anionically prepared by the sequential addition of styrene, methyl methacrylate, or N,N-diethylacrylamide with tBA in the presence of R2Zn.

Radical polymerizations of methyl methacrylate initiated by methyl 2-[(4-diphenylmethylene)-2,5-cyclohexadienyl]-2-methyl-propanoate: A model system for so-called ‘quasi-living’ polymerizations of methyl methacrylate initiated by phenylazotriphenylmethane

The kinetics of radical polymerizations of methyl methacrylate initiated by methyl 2-[(4-diphenylmethylene)-2,5-cyclohexadienyl]-2-methyl-propanoate, a model for the growing chain end in ‘quasi-living’ radical polymerizations of methyl methacrylate mediated by trityl radicals, conventionally generated from phenylazotriphenylmethane, have been investigated. It is shown that the polymerizations have many of the characteristics of living systems, including the ability for polymer chains recovered from the polymerizations to reinitiate polymerization when added to fresh monomer. However, molecular weight distributions of polymers broaden considerably with conversion, suggesting significant termination, probably via transfer to trityl radicals and/or conventional bimolecular termination. Also, the polymers recovered from reinitiation experiments have molecular weights which are too high to be consistent with simple reinitiation via regeneration of growing radicals; rather it seems probable that reinitiation occurs via breakdown of a peroxidic species introduced during work-up of the original polymer.

Living radical polymerization of methylstyrenes by a stable nitroxyl radical, and stability of the aminoxy chain end

Radical polymerization of 2-, 3-, and 4-methylstyrenes (MeSts) was investigated with benzoyl peroxide (BPO) as an initiator, in the presence of 4-methoxy-2,2,6,6-tetramethylpiperidine-1-oxyl (MTEMPO). The polymerization was performed in bulk for 3.5 h at 95°C, and then continued for a defined time at 125°C, to give the corresponding poly(MeSt)s with narrow polydispersity in high yield. It was found that the polymerization proceeded in accordance with a living mechanism, because the molecular weight of the resulting polymers was proportional to the conversion, and to the reciprocal of the initial concentration of MTEMPO. It was found that steric hindrance between the methyl group of 2-MeSt, and the tetramethyl ones of MTEMPO, significantly contributed to the rate of polymerization, and to the stability of the growing polymer chain end. The stability decreased in the order of 2- > 3- > 4-MeSt, by occurrence of decomposition, which was caused by disproportionation of the growing chain end. However, the steric hindrance had no effect on the tacticity of the resulting polymer. © 1998 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 36, 269–276, 1998

Dispersion Polymerizations of Styrene in Carbon Dioxide Stabilized with Poly(styrene-b-dimethylsiloxane)

Herein we report the use of poly(styrene-b-dimethylsiloxane) block copolymers as steric stabilizers for dispersion polymerizations of styrene in a carbon dioxide continuous phase. It was demonstrated that the anchor-soluble balance (ASB) of the stabilizer has a dramatic effect on both the progress of the reaction and the morphology of the resulting polystyrene (PS) colloid. When a stabilizer with an appropriate ASB is employed, the initial concentrations of styrene, stabilizer, and helium have substantial effects on the resulting size and dispersity of the colloidal particles. Dispersion polymerizations carried out under a series of different pressures (143−439 bar) resulted in variations in the conversion, molecular weight, and PS particle diameter, suggesting that the reaction is extremely sensitive to the density of the continuous phase. An investigation of the progress of the reaction as a function of time at two different temperatures revealed that the average molecular weight increases gradually over the course of the reaction. Finally, control reactions in heptane solvent were not as successful as those conducted in CO2 suggesting that the plasticization of the polymer particles by CO2 facilitates diffusion of monomer to the growing chain ends.

Molecular Motion of Polyethylene Chain Ends Tethered to a Fresh Surface of Poly(tetrafluoroethylene) in Vacuo

The molecular motion of polyethylene (PE) chain ends tethered to a fresh surface of poly(tetrafluoroethylene) (PTFE) in vacuo was studied. The tethered PE chain ends were produced by a block copolymerization of PTFE with ethylene in vacuo at 77 K. Each of the tethered PE chains has an unpaired electron at the growing chain end. The mobility of the tethered PE chain ends was observed in the range of 2.8−95 K by an electron spin resonance (ESR) spectrometer with the unpaired electron as a probe. The inter exchange motion between two α protons at the chain ends probably occurs at 2.8 K. The site exchange motion between two conformations at the chain ends occurs above 7 K and is more clearly observed at 15 K. Moreover, the torsional oscillation of the β proton occurs above 30 K. Furthermore, the rotation of the chain end about the chain axis of PE probably occurs at 95 K. The assignment of molecular motion modes was based on spectral simulations. The high mobility of the tethered PE chain ends is attributed to (1) a large space around the chains; (2) the presence of the vacuum; (3) the lack of chain aggregation, because the concentration of the chain ends is low and they are tethered; and (4) the immiscibility of PE and PTFE. The ends of PE chains tethered on the PTFE in vacuo probably behave as isolated PE chain ends in vacuo, and may reveal the mobility intrinsic to PE chain ends.

γ‐Ray‐induced post‐polymerization of a mesogenic methacrylate in the crystalline, liquid crystalline and melt phase: Reaction kinetics and molecular weight distribution

AbstractThe γ‐ray‐induced post‐polymerization of mesogenic 4‐methoxy‐4′‐(6‐methacryloyloxyhexyloxy)biphenyl (1) was investigated. Reaction kinetics and molecular weight distribution of the polymer were studied. Molecular weights and weight distributions were analyzed using size exclusion chromatography. Especially, effects of radiation dose, polymerization time and temperature were investigated. Post‐polymerization was found in the crystalline, liquid crystalline and melt phase. Rate of polymerization and limiting conversion to polymer increase with radiation dose, polymerization time and temperature. Samples exposed to a low γ‐ray dose of 5 kGy and subsequently polymerized in the crystalline state at 20°C, for example, only reach a limiting conversion of about 7% within 100 h, while samples exposed to a high dose of 20 kGy and polymerized in the melt at 79°C reach a limiting conversion of 85.1% within 90 s. Molecular weights increase with polymerization temperature and reach values comparable with in‐source polymerization. Different from in‐source polymerization, they are additionally affected by the parameters radiation dose and polymerization time. High molecular weights are obtained, if samples are exposed to a low radiation dose and isothermally polymerized at high temperature for a long time, while low molecular weights result from high radiation dose and short time of polymerization at low temperature, e.g., at 20°C. By carefully adjusting the reaction parameters, molecular weights can be reliably tailored in the range from 2 × 104 to 6 × 105. Polymerization at room temperature leads to a normal molecular weight distribution, while distributions are broader when the polymerization is carried out in the liquid crystalline or melt phase. The origin of the different reaction rates and molecular weights obtained under the various conditions is discussed in terms of the mobility of growing chain ends and residual monomer, which influences chain growth and termination and thus the kinetic chain length.

γ‐ray polymerization of a mesogenic methacrylate in the crystalline, liquid crystalline and liquid state: kinetics, molecular weight distribution and conversion‐dependent phase behaviour

AbstractThe γ‐ray polymerization of mesogenic 4‐methoxy‐4′‐(6‐methacryloyloxyhexyloxy)biphenyl (1) was investigated. 1 can be polymerized in the crystalline, in the liquid crystalline and in the molten state. The rate of polymerization increases from the crystalline to the melt phase. In order to reach 90% conversion to polymer in melt, smectic and crystalline phase, radiation doses of 10, 40 and more than 120 kGy are necessary. Melt polymerization leads to a weight‐average molecular weight of about 1 × 106 independent of the conversion to polymer. In the smectic and crystalline phase the corresponding molecular weights are about 3.8 × 105 and 4.1 × 104 at low conversion, and shifted to 2.6 × 105 and 3.9 × 104 at high conversion. The polydispersity generally increases with the γ‐ray dose and the reaction temperature. The origin of the different reaction rates and molecular weight obtained in the various phases is discussed in terms of the mobility of growing chain ends and residual monomer, which influences chain growth and termination and thus the kinetic chain length. Differential scanning calorimetry (DSC) of partially polymerized samples was used to describe the conversion‐dependent phase behaviour. Optical microscopy showed that the melt polymerization at 78.5°C proceeds under phase separation into a liquid, monomer‐rich and a solid, polymer‐rich phase. As a consequence, liquid monomer samples solidify upon γ‐irradiation once a higher conversion to polymer is reached.

Syndiotactic‐specific polymerization of propene with a Ni‐based catalyst

AbstractPolymerization of propene at −78°C in the presence of a homogeneous catalytic system based on a Ni(II) diimine derivative and methylaluminoxane affords prevailingly syndiotactic crystalline poly(propylene) (rr triad content ≈80%). 13C NMR analysis of the polymer microstructure indicates that the stereochemistry of the monomer insertion is controlled by the configuration of the methine carbon of the growing chain end (unlike‐1,3 asymmetric induction). This is the first example of stereospecific homopolymerization of propene promoted by a late transition metal catalyst.

The Pivotal Role of Excess Nitroxide Radical in Living Free Radical Polymerizations with Narrow Polydispersity

The pivotal role of the nitroxide concentration in bulk living polymerization of styrene was studied between 115 and 135 °C, using in situ electron spin resonance spectroscopy (ESR) to follow the concentration of the TEMPO stable free radical during the polymerization. Molecular weight and conversion were also followed on the same reaction mixtures using gel permeation chromatography and thermogravimetric analysis, respectively. While molecular weights were linear with conversion, to high conversion, there was an increase in the polymerization rate with time: nonideal behavior for a living polymerization. However, the TEMPO concentration also shows a slow decay as polymerization proceeds. Using the current mechanistic model, which predicts a polymerization rate inversely proportional to TEMPO concentration, this changing concentration was incorporated into the kinetic analysis. Except for low conversion in the lowest temperature polymerization, correction for the TEMPO concentration resulted in ideal, constant polymerization rate constants. While increasing the initial TEMPO concentration decreases the rate of polymerization dramatically, the corrected rate is independent of initial TEMPO concentration, again consistent with the current mechanism. From these corrected polymerization rates, the activation energy for the release of TEMPO from the growing chain end was estimated as 82 kJ/mol, considerably less than the previously observed value of 130 kJ/mol for the release of TEMPO from styrene 1-mers. Using TEMPO as a probe of irreversible chain termination, ESR shows that irreversible chain termination up to 75% conversion is limited to less than 2 chains in a hundred. It is concluded that the TEMPO-mediated polymerization is a living polymerization under the conditions of this study. To aid in the understanding of these living polymerizations that are based on reversible termination, a new term has been defined, the germination efficiency, which describes the yield of living chains in terms of the reversible terminating agent.

Asymmetric Polymerization and Oligomerization of 3-Phenylpropanal with Grignard Reagent−(−)-Sparteine Complexes with Termination by Tishchenko Reaction

3-Phenylpropanal (3-PPA) was polymerized with Grignard reagent−(−)-sparteine complexes, such as ethylmagnesium bromide−(−)-sparteine (EtMgBr−Sp) and n-octylmagnesium bromide−(−)-sparteine (OctMgBr-SP) complexes, in toluene at low temperature. The poly(3-PPA) obtained showed optical activity with negative rotation ([α]25365 −33° to −56°), which may be based on a predominant one-handed helical conformation of the main chain, and also exhibited a signal due to an ester carbonyl group at 1738 cm-1 in its IR spectrum. This means that the polymer has an ester group at the ω-end which is formed by a Tishchenko-type termination reaction between the growing chain end and the 3-PPA monomer. The poly(3-PPA) is stable at room temperature, whereas the poly(3-PPA) obtained with EtMgBr alone at −78 °C degrades slowly even in the solid state to 3-PPA monomer at room temperature. 1H and 13C NMR spectra of the poly(3-PPA) showed rather sharp resonances, suggesting that the polymer may be stereoregular, although the tactici...

Synthesis of monodisperse polyamides by living anionic polymerization of β‐lactams in the mixture of N,N‐dimethylacetamide and lithium chloride

AbstractLiving anionic polymerization of two kinds of methyl‐substituted β‐lactams, 3,3‐dimethyl‐, and 4,4‐dimethyl‐2‐azetidinones, was attained at 25°C in N,N‐dimethylacetamide containing 5 wt % of lithium chloride and proceeded in a homogeneous phase quantitatively. The resulting polyamides were found to have a narrow molecular weight distribution from the gel permeation chromatography. The number average molecular weights estimated from the peak intensities in the 1H‐NMR spectra were almost equal to those calculated from the mol ratio of the consumed lactam to the corresponding N‐benzoyl derivative used as an activator. In addition, the acyllactam‐type growing chain ends were modified quantitatively by the reaction with benzylamine after the polymerization. © 1995 John Wiley &amp; Sons, Inc.

Mechanism of syndiotactic‐specific polymerization of styrene

AbstractPerusal of literature data and some new results concerning syndiotactic‐specific polymerization of styrene suggest a reaction mechanism accounting for the steric control. Key features of the proposed mechanism are stereorigid ηn coordination of the growing chain end and diastereoselective coordination of the monomer imposed by direct interactions with the ancillary ligand of the metal complex, a pseudotetrahedral chiral Ti(III) or Zr(III) cation, which inverts its configuration after every syndiospecific insertion step.

The features of syndiotactic polypropylene

AbstractThe mechanism of stereoselectivity of propylene insertion in propylene‐ethylene copolymerization on a CS symmetrical zirconium complex i‐Pr(Cp) (Flu) ZrCl2 catalyst is discussed. Calculation results indicate that not only the β‐carbon in the growing chain end of the polymer but also the substituent of the β‐carbon play an important role in the selectivity of the prochiral face of the next‐coming propylene monomer. The stereoregularity of propylene units connected to an ethylene unit (PPE) in propylene‐ethylene copolymer was observed to be lower than that in propylene sequences (PPP) in the 13C NMR spectrum, which supports the calculation results. Furthermore, the structure and properties of propylene‐olefin (ethylene, 1‐butene, 1‐pentene, 1‐hexene, and 4‐methyl‐1‐pentene) copolymers prepared with the i‐Pr(Cp) (Flu) ZrCl2 catalyst system were studied. Propylene‐1‐butene copolymer exhibits peculiarly lower melting point depression because 1‐butene units enter into the unit cell of the crystal structure of syndiotactic polypropylene.

On the Anionic Polymerization of (Dialkylamino)isoprenes. 2. Influence of the Tertiary Amino Group on the Polymer Microstructure

The anionic polymerization of 5-(N,N-dialkylamino)isoprenes carrying various alkyl substituents was studied in nonpolar (benzene, hexane) and polar solvents (tetrahydrofuran, dioxane, triethylamine). The microstructure is found to be strongly dependent on the bulkiness of the tertiary amino group. All polymerizations in nonpolar solvents occur by a 4,1-addition of the monomer to the anionic chain end. In THF no polymerization occurs. The results are discussed with respect to the stereoelectronic situation at the growing chain end. For monomers with the most bully branched side groups (isopropyl, isobutyl, and 2,6-cis-dimethylpiperidyl) the addition is not only regioselective but also stereoselective, giving rise to the formation of stereoregular polymers with cis-4,1-repeating units

Binary copolymerizations of styrene and conjugated diolefins in the presence of cyclopentadienyltitanium trichloride‐methylaluminoxane

AbstractBinary copolymerizations of styrene, isoprene, butadiene and 4‐methyl‐1,3‐pentadiene, in the presence of the catalyst cyclopentadienyltitanium trichloride‐methylaluminoxane (CpTiCl3‐MAO), are investigated. The reactivity ratios as well as the reactivities of the different monomers in homopolymerization are tentatively correlated to a possible polymerization mechanism. A qualitative explanation of the reactivity of different monomers in homo‐ and copolymerization is achieved by considering the nucleophilicity of the monomers and the electrophilicity of the active species. The latter is affected by the nature of the bond between the active Ti and the last unit of the growing chain end. The block copolymerization of ethylene with styrene is also tentatively explained by considering that the more the active species are electrophilic the less they should be selective toward monomers of different nucleophilicity.

Molecular architecture control in acrylic polymers by group transfer polymerization

AbstractGroup transfer polymerization (GTP)a of acrylic monomers is mediated by a trialkylsilyl‐capped growing chain end. It operates at room temperature and above and permits a high degree of molecular architecture control. Block, comb, graft, star, and loop polymers are readily synthesized. It appears that GTP is an associative process under mild nucleophilic catalysis and a dissociative process under strong nucleophilic catalysis.

Mechanism of GTP: Can all of the available data be accommodated?

AbstractGroup transfer polymerization (GTP)a of acrylic monomers is a living system mediated by a trialkylsilyl capped growing chain end. The fact that it operates at temperatures as high as 100° C differentiates GTP from living anionic polymerization, which at best operates at 25° C for hindered methacrylates. To accommodate all of the mechanistic data available it appears that at least two mechanistic pathways are required: an associative process for mild nucleophilic catalysts and a dissociative process for strong nucleophilic catalysts.

Molecular engineering of liquid crystalline polymers by living polymerization

This paper describes the synthesis and characterization of poly{3-[4-cyano-4′-biphenyl)oxy]propyl vinyl ether} [poly(6-3)] macromonomers obtained by the functionalization of the growing chain end of the corresponding living polymer obtained by the initiation of the cationic polymerization of 3-[4-cyano-4′-biphenyl)oxy]propyl vinyl ether with CF3SO3H/S(CH3)2, with 2-hydroxy ethyl methacrylate [poly(6-3)-I], 2-[2-(2-allyloxyethoxy)ethoxy]ethanol [poly(6-3)-II] and 10-undecen-1-ol [poly(6-3)-III].