Skewered reactions in free‐radical crosslinking (co)polymerizations of liquid polybutadiene as internal olefinic multivinyl crosslinker

As an extension of our continuing studies concerned with the mechanistic discussion of network formation in the free‐radical crosslinking (co)polymerization of multivinyl monomers, this work refers to the skewered reactions in the crosslinking (co)polymerizations of liquid polybutadiene rubber (LBR) as an internal olefinic multivinyl monomer or crosslinker, especially focused on the competitive occurrence of both addition or skewered reaction to internal carbon–carbon (CC) double bonds and abstraction reaction of allylic hydrogens in LBR by growing polymer radical. Thus, LBR is regarded as an internal olefinic multiallyl monomer‐linked allyl groups (? CH?CH? CH₂? ) with methylene units (? CH₂? ). First, gelation in the polymerization of LBR was explored in detail, especially at elevated temperatures. The occurrence of intermolecular crosslinking was easier in the order LBR > LBR containing 20 mol % of 1,2‐structural units > liquid polyisoprene rubber. Then, we pursued the polymerization of LBR using dicumyl peroxide (DCPO) as typical organic peroxide used at elevated temperatures. The primary cumyloxy radical generated by the thermal decomposition of DCPO may add to CC double bond or abstract allylic hydrogen or undergo β‐scission to generate a secondary methyl radical. The initiation by the cumyloxy radical was omitted. The ratio of allylic hydrogen abstraction to β‐scission reaction was estimated; thus, only 39% of cumyloxy radical was used for the allylic hydrogen abstraction reaction. The addition of methyl radical to CC double bond was clearly observed. Finally, we pursued the intermolecular and intramolecular skewered reactions in free‐radical crosslinking LBR/vinyl pivalate copolymerizations. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012     

Structural Defects in Poly(vinyl chloride) and the Mechanism of Vinyl Chloride Polymerization: Comments on Recent Studies

Investigations in the title areas within the past ten years are summarized and critiqued. The polymerizations studied were performed by conventional free-radical methods. A new mechanism, not yet confirmed, is suggested to explain a reported enhancement in the chloromethyl branch concentration of poly(vinyl chloride) (PVC) prepared at high conversions of monomer. This mechanism involves an intramolecular 1,5 hydrogen shift in a 1,3,5,6-tetrachlorohexyl radical. Evidence showing that most of the internal double bonds in PVC are not formed via intermolecular H abstraction from internal monomer units is tentatively rationalized, in part, by hydrogen transfer via at least one cyclic transition state containing more than eight members. The absence of free chlorine atoms from polymerizations of vinyl chloride (VC) is reaffirmed, and the copolymerization of VC with the chloroallylic chain ends of PVC is argued to be insignificant. New information in the literature does not invalidate the currently accepted mechanism of vinyl chloride polymerization.     

Polymerization photoinitiated by carbonyl compounds. VI. Mechanism of benzil photoinitiation

The mechanism of the photoinitiation of the vinyl polymerization sensitized by benzil and 4,4′-dimethoxybenzil was studied. The monomers considered were methacrylic acid esters and styrene derivatives. All these monomers are efficient quenchers of the excited triplet benzil. However, the initiation efficiency of the benzil is important only when styrene derivatives are employed as monomers. The main polymerization process follows a simple free radical mechanism. The initiation step is a consequence of the interaction (triplet benzil–monomer double bond) through a charge transfer complex.     

ESR studies of vinyl polymerization. II. Initial reactions of vinyl esters with redox systems in aqueous media

ESR measurements of transient radicals during redox polymerization of various vinyl esters in aqueous solutions have been made by using the rapid-mixing flow method. The initiation was by means of hydroxyl and amino radicals from the systems titanous chloride-hydrogen peroxide and titanous chloride-hydroxylamine, respectively. The well resolved hyperfine structures obtained at monomer concentrations of about 0.05 mole/1. are unambiguously assigned to the monomer radicals formed by addition of initiator radicals to monomers. At higher monomer concentrations, additional weak signals attributed to the growing polymer radicals were observed. The effect of reaction conditions on the signal intensity has been studied in particular for vinyl acetate. The coupling constants of monomer radicals from various vinyl esters (acetate, propionate, butyrate, crotonate, and isopropenyl acetate) were obtained and the spin densities calculated. From the ESR spectra, the monomer radicals have a conformation with the substituent R (R = HO or NH₂) of R? CH₂? CH(OCOR′) locked in a position above or below the radical plane. This is tentatively interpreted as due to formation of intramolecular hydrogen bonds to ring structures or complexes with titanium ions. In addition, hydrogen abstraction reactions of some model compounds for poly(vinyl acetate) have been briefly studied in relation to chain transfer and grafting reactions.     

New Phenomena in Organometallic‐Mediated Radical Polymerization (OMRP) and Perspectives for Control of Less Active Monomers

The impact of reversible bond formation between a growing radical chain and a metal complex (organometallic‐mediated radical polymerization (OMRP) equilibrium) to generate an organometallic intermediate/dormant species is analyzed with emphasis on the interplay between this and other one‐electron processes involving the metal complex, which include halogen transfer in atom transfer radical polymerization (ATRP), hydrogen‐atom transfer in catalytic chain transfer (CCT), and catalytic radical termination (CRT). The challenges facing the controlled polymerization of “less active monomers” (LAMs) are outlined and, after reviewing the recent achievements of OMRP in this area, the perspectives of this technique are analyzed.     

Carbopalladation of allenic hydrocarbons. : A new way to functionalized styrenes and 1,3-butadienes.

The palladium-catalyzed coupling reaction of allenes, vinyl or aryl halides and stabilized carbanions is described : π-allyl palladium complexes are formed by addition of a vinyl or an aryl-palladium complex to an allenic hydrocarbon and trapped by the sodium enolate of diethyl malonate giving rise with good yields to β-butadienyl or β-styryl malonates. With monoalkyi allenes, the reaction is regiospecific with attack of the nucleophile on the unsubstituted carbon of the Intermediate π-allyl complex and in many cases highly stereoselective with the predominant formation of the E configuration for the trisubstituted double bond of the diene. This configuration was demonstrated by ¹H NMR using NOE difference spectroscopy.     

芳香叔胺引发烯类单体聚合研究——单体结构与聚合的关系

不同烯类单体在芳香叔胺存在下的聚合机构不一样。甲基丙烯酸甲酯等有α-甲基的烯类单体在不照光的条件下即可被芳香叔胺引发聚合,其聚合机构认为是首先α—甲基被胺-氧复合物氧化,生成单体过氧化物。再与胺形成氧化还原体系,分解产生自由基。 丙烯酸酯,丙烯腈等没有α-甲基的单体,只有光照时才被芳香叔胺氧化聚合,不光照时完全不聚合。这是因为这些单体不被胺-氧复合物氧化。光照下聚合的机构认为是因光的激发,这些单体与芳香叔胺形成电子转移激发络合物,再分解产生自由基。 研究了单体结构,胺结构对光聚合速度的影响。不同单体的活性次序是: AN>MA>VA>St 不同芳香叔胺的活性次序是: DMT>DMA>DMB>DNA 表明单体双键电子云密度越小,芳香叔胺氮原子上电子云密度越大,越容易形成激发态电子转移络合物,从而越容易聚合。     

5-Vinylpyrimidines. Synthesis via organopalladium intermediates

Reactions of 1,3-dimethyl-5-iodouracil or 2,4-dimethoxy-5-iodopyrimidine with vinyl acetate in the presence of a catalytic amount of diacetato-bis (triphenylphosphine) palladium (II) resulted in good yields of the corresponding 5-vinylpyrimidines. The reactions are viewed as resulting from regioselective addition of an initially formed 5-pyrimidinyl palladium species to the double bond of vinyl acetate followed by elimination of a palladium acetate with regeneration of the double bond and formation of the 5-vinylpyrimidine product.     

Key Role of Intramolecular Metal Chelation and Hydrogen Bonding in the Cobalt‐Mediated Radical Polymerization of N‐Vinyl Amides

This work reveals the preponderance of an intramolecular metal chelation phenomenon in a controlled radical polymerization system involving the reversible trapping of the radical chains by a cobalt complex bis(acetylacetonato)cobalt(II). The cobalt‐mediated radical polymerization (CMRP) of a series of N‐vinyl amides was considered with the aim of studying the effect of the cobalt chelation by the amide moiety of the last monomer unit of the chain. The latter reinforces the cobalt? polymer bond in the order N‐vinylpyrrolidone<N‐vinyl caprolactam<N‐methyl‐N‐vinyl acetamide, and is responsible for the optimal control of the polymerizations observed for the last two monomers. Such a double linkage between the controlling agent and the polymer, through a covalent bond and a dative bond, is unique in the field of controlled radical polymerization and represents a powerful opportunity to fine tune the equilibrium between latent and free radicals. Possible hydrogen bond formation is also taken into account in the case of N‐vinyl acetamide and N‐vinyl formamide. These results are essential for understanding the factors influencing Co? C bond strength in general, and the CMRP mechanism in particular, but also for developing a powerful platform for the synthesis of new precision poly(N‐vinyl amide) materials, which are an important class of polymers that sustain numerous applications today.     

Side Reactions in Aqueous Phase Polymerization of N‐Vinyl‐Pyrrolidone as Possible Source for Fouling

An improved kinetic model for the radical polymerization of N‐vinyl‐pyrrolidone (NVP) in aqueous medium is developed. Quantum chemical simulations reveal that the transfer to polymer is of minor importance whereas the transfer to monomer by hydrogen abstraction in 3‐position of the pyrrolidone ring leads to a radical with a double bond which initiates a new chain bearing a terminal double bond (TDB). The resulting dead chains with one, two, or more TDB are the main source for a strong increase of molar mass in batch reactors at high conversion due to long chain branching and crosslinking. This can be a source for gel formation and fouling in continuous reactors.     

Catalytic and coordination facets of single-site non-metallocene organometallic catalysts with N-heterocyclic scaffolds employed in olefin polymerization

This review discusses the principles underlying mononucleating N-heterocyclic ligand design, selectivity of metal centers, preparation of organometallic catalysts with a N-heterocyclic backbone, and their catalytic activity in olefin oligo/polymerization. A vast number of N-heterocyclic organometallic compounds have been applied for the polymerization on account of their modest cost, low toxicity, and the large availability of transition metals in stable and variable oxidation states, which makes them versatile precursors for these reactions. The main points of focus in this review are the key advances made over more the past 25 years in the design and development of non-metallocene single-site organometallic catalysts bearing different N-heterocyclic scaffolds as a backbone. These catalysts are applied as precursors for the transformation of ethylene, higher α-olefins, and cyclic olefins into oligo/polymers. Emphasis is placed on the architecture of ligand peripheries for tuning the formed polymer properties and the consequences on product formation of different alkyl or aryl substituents directly attached to the metal center in a N-heterocyclic ligand system.     

Mechanism of activation of a hafnium pyridyl-amide olefin polymerization catalyst: ligand modification by monomer

We have investigated the olefin polymerization mechanism of hafnium catalysts supported by a pyridyl-amide ligand with an ortho-metalated naphthyl group. Ethylene-alpha-olefin copolymers from these catalysts have broad molecular weight distributions that can be fit to a bimodal distribution. We propose a unique mechanism to explain this behavior involving monomer modification of the catalyst, which generates multiple catalyst species when multiple monomers are present. More specifically, we present evidence that the hafnium alkyl cation initially undergoes monomer insertion into the Hf-naphthyl bond, which permanently modifies the ligand to generate new highly active olefin polymerization catalysts. Under ethylene/octene copolymerization conditions, a plurality of new catalysts is formed in relative proportion to the respective monomer concentrations. Due to the asymmetry of the metal complex, two "ethylene-inserted" and eight "octene-inserted" isomers are possible, but it is a useful approximation to consider only one of each in the polymerization behavior. Consequently, gel permeation chromatography data for the polymers can be fit to a bimodal distribution having a continuous shift from a predominantly low molecular weight fraction to predominantly higher molecular weight fraction as [octene]/[ethylene] is increased. Theoretical calculations show that such insertions into the Hf-aryl bond have lower barriers than corresponding insertions into the Hf-alkyl bond. The driving forces for this insertion into the Hf-aryl bond include elimination of an eclipsing H-H interaction and formation of a stabilizing Hf-arene interaction. These new "monomer-inserted catalysts" have no beta-agostic interaction, very weak olefin binding, and olefin-insertion transition states which differ on the two sides by more than 4 kcal/mol. Thus, the barrier to site epimerization is very low and high polymerization rates are possible even when the chain wags prior to every insertion. Experimental evidence for aryl-insertion products is obtained from reactions of ethylene (13C2H4 NMR studies) or 4-methyl-1-pentene (4M1P) using relatively low monomer/catalyst ratios. Quantitative generation of monomer-inserted products is complicated by slow initiation kinetics followed by fast polymerization kinetics. However, NMR evidence for reaction with 13C2H4 was observed in situ at low temperature, and the attachment of monomer to ligand was confirmed by GC/MS and 13C NMR after quenching. Furthermore, a 4M1P-appended ligand was isolated from a polymerization reaction (50:1 monomer:catalyst) by column chromatography followed by multiple recrystallizations. One isomer was characterized by X-ray crystallography, which unequivocally shows a 4-methylpentyl substituent at the 2-position of the naphthyl, consistent with 1,2-insertion into the Hf-aryl bond. NMR suggests a second diastereomer (not isolated) is formed from a 1,2-insertion of opposite stereoselectivity.     

Recent developments in photoinitiated radical polymerization

This article presents the progress made in the development of high-speed photocurable resins and reports the performance of some novel radical-type photoinitiators, acrylate monomers and telechelic oligomers. The polymerization kinetics has been studied by real-time infrared spectroscopy, which records conversion versus time curves for reactions occurring in a fraction of a second. Phosphine oxides are among the most efficient photoinitiators and proved to be particularly well suited for the photocuring of pigmented systems and for solar-assisted polymerization. Acrylate monomers containing a heterocyclic oxygen in their structural unit exhibit an unexpectedly high reactivity. The introduction of an amino group in the chain of a telechelic acrylate-polyester causes a substantial acceleration of the polymerization process. In both cases, the increase in reactivity was attributed to the presence of labile hydrogen atoms which favor chain transfer reactions. The copolymerization of donor-acceptor monomer systems, like vinyl ether-maleate or vinyl ether maleimide, was shown to proceed readily upon UV irradiation, even in the absence of added photoinitiator. Light-induced polymerization was also used to crosslink rapidly polymers functionalized with acrylate or vinyl double bonds, namely acrylated polyisoprene and a styrene-butadiene block copolymer. The addition in small amounts (1 wt%) of a trifunctional thiol was found to speed up drastically the crosslinking polymerization, causing insolubilization of the thermoplastic elastomer to occur after a 0.1 s exposure.     

Mapping the characteristics of the radical ring‐opening polymerization of a cyclic ketene acetal towards the creation of a functionalized polyester

Radical ring‐opening polymerization of cyclic ketene acetals is a means to achieve novel types of aliphatic polyesters. 2‐methylene‐1,3‐dioxe‐5‐pene is a seven‐membered cyclic ketene acetal containing an unsaturation in the 5‐position in the ring structure. The double bond functionality enables further reactions subsequent to polymerization. The monomer 2‐methylene‐1,3‐dioxe‐5‐pene was synthesized and polymerized in bulk by free radical polymerization at different temperatures, to determine the structure of the products and propose a reaction mechanism. The reaction mechanism is dependent on the reaction temperature. At higher temperatures, ring‐opening takes place to a great extent followed by a new cyclization process to form the stable five‐membered cyclic ester 3‐vinyl‐1,4‐butyrolactone as the main reaction product. Thereby, propagation is suppressed and only small amounts of other oligomeric products are formed. At lower temperatures, the cyclic ester formation is reduced and oligomeric products containing both ring‐opened and ring‐retained repeating units are produced at higher yield. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 4587–4601, 2009     

Oxidation of benzene with hydrogen peroxide catalyzed with ferrocene in the presence of pyrazine carboxylic acid

It is found that ferrocene in the presence of small amounts of pyrazine carboxylic acid (PCA) effectively catalyzes the oxidation of benzene to phenol with hydrogen peroxide. Two main differences upon the oxidation of two different substrates, i.e., cyclohexane and benzene, with the same H₂O₂-ferrocene-PCA catalytic system are revealed: the rates of benzene oxidation and hydrogen peroxide decomposition are several times lower than the rate of cyclohexane oxidation at close concentrations of both substrates, and the rate constant ratios for the reactions of oxidizing particles with benzene and acetonitrile are significantly lower than would be expected for reactions involving free hydroxyl radicals. The overall rate of hydrogen peroxide decomposition, including both the catalase and oxidase routes, is lower in the presence of benzene than in the presence of cyclohexane. It is suggested on the grounds of these data that a catalytically active particle different from the one generated in the absence of benzene is formed in the presence of benzene. This particle catalyzes hydrogen peroxide decomposition less efficiently than the initial complex and generates a dissimilar oxidizing particle that exhibits higher selectivity. It is shown that reactivity of the system at higher concentrations of benzene differs from that of an initial system not containing an aromatic component with the capability of π-coordination with metal ions.     

Cyclocarbopalladation involving an unusual 1,5-palladium vinyl to aryl shift as termination step: theoretical study of the mechanism

A DFT/B3LYP model study has been carried out on the cyclocarbopalladation and on an unusual 1,5 vinyl to aryl palladium shift which are the two first steps of a cyclocarbopalladation-Stille coupling tandem reaction of various gamma-bromopropargylic-1,2 diols with alkenyls or alkynyl stannanes catalyzed by Pd(PPh(3))(4). From the calculations, the active intermediates in the catalytic process appear to bear a single phosphine ligand, the palladium(II) center keeping in all cases a square-planar coordination pattern either through intramolecular binding of the triple bond or via an intramolecular Pd...C(phenyl) interaction. The computation of the various transition states and intermediates for the 1,5 vinyl to aryl palladium shift reveals that the intimate mechanism of this pathway corresponds to a one-step hydrogen transfer between the two negatively charged carbon atoms of the vinyl and phenyl groups. A two-step pathway involving a Pd(IV) intermediate is not likely to occur. This conclusion may apply to other 1,n-palladium shifts which have been experimentally observed in various organometallic transformations.     

Theoretical study of chain transfer in anionic polymerization. Transfer by the mechanism of side-group substitution

A new theoretical consideration of chain transfer to monomer in the anionic polymerization of hydrocarbon monomers is presented. It is shown that the kinetic scheme used in theoretical studies reported previously contradicts the widespread views on the chemical mechanism of carbanionic reactions. It is suggested that the most probable path of the transfer reaction is the proton abstraction from the side group of the monomer; the terminal double bond of the monomer molecule remains unchanged, and therefore the intermediate species can participate in succeeding reactions as a macromonomer. The molecular characteristics of polymer formed in processes with monomer transfer by side-group substitution are determined. At high conversion, the polymer formed in such a process is shown to possess a number-average degree of polymerization, P̄_n, approaching the theoretical value for living polymers, and a P̄_w exceeding it the more the higher the intensity of transfer. Furthermore, it shows a broad molecular weight distribution and a fairly noticeable degree of branching. These results considerably differ from those previously reported.     

On the mechanism of the polymerization of poly(vinyl chloride)

The polymerization of poly(vinyl chloride) (PVC), head-to-head macroradical plays an essential role in the formation of branches and the appearance of interruption processes that lead to labile unsaturated end groups. A reaction mechanism is suggested to explain these processes and the method of formation of internal double bonds as a result of transfer to polymer. The transfer to monomer can occur only by atom acceptance by the macroradical and not by the monomer giving up an atom.     

Revised patterns of reactivity scheme. VII. Revised patterns scheme and its relationship to carbon‐13 NMR spectra

The electron density on a carbon atom determines the ¹³C chemical shift observed in the NMR spectrum. In a vinyl monomer, the same electron density must contribute strongly to polar effects involved in the addition of a radical to that monomer and possibly in the addition (to a double bond) of a radical terminated by a unit derived from that monomer. It is shown that the expected correlation exists when the polar effects in polymerization reactions are represented by the parameters of the revised patterns scheme. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 4336–4342, 2000     

Cyclopalladated Ferrocenylimine: Highly Active Catalyst for the Suzuki Reaction of Arylboronic Acids with Arylbromides and Arylchlorides

Carbon-carbon bond formation reactions are some of the most important processes in chemistry, that provide key steps in the building of more complex molecules from simple precursors. Many organometallic reagents have been successfully used in catalytic carbon-carbon bond formation. [1]