Reversible Chain Transfer Research Articles

Dialkoxy-Substituted, C1-Symmetric Metallocenes: Synthesis and Catalytic Behavior in the Propylene Polymerization Reaction

The synthesis of a series of C1-symmetric metallocene complexes rac-[1-(5,6-dialkoxy-2-methyl- 1-η5-indenyl)-2-(9-η5-fluorenyl)ethane]zirconium dichlorides (alkyl: n-butyl, n-hexyl, n-octyl, n-decyl) is described. These complexes are versatile catalysts in the polymerization of propylene after in situ activation with triisobutylaluminum (TIBA) and Ph3C[B(C6F5)4] in toluene and heptane solution. All catalysts show higher solubility and improved polymerization properties in industrially used hydrocarbon solvents (e.g. heptane). However, the molecular weights and isotacticity values of the resulting polypropylene materials are decreased compared to the ethoxy-bridged analogue rac- [1-(5,6-ethylenedioxy-2-methyl-η5-indenyl)-2-(9-η5-fluorenyl)ethane]zirconium dichloride. A possible explanation is based on enhanced interaction of the active catalyst centers with Al(III) scavenger molecules even at low Al : Zr ratios, leading to reversible chain transfer.

Microemulsion polymerization for producing fluorinated structured materials

AbstractWhen perfluoropolyether microemulsions are used during polymerization of fluoropolymers, the structure of the reaction environment can be strictly controlled. In particular, the number and size of polymer particles in latexes can be set freely, yielding a number of advantages.First, as a result of radical segregation, terminations can be decreased without reducing polymerization rate: in this way high molecular weights are easily obtained also with poorly reactive monomers.Moreover, in combination with a reversible chain transfer mechanism based on iodine, particle segregation allows establishing pseudo‐living polymerization conditions. In this situation formation of long branches in the polymer can be controlled by using bifunctional molecules that are able to link two different polymer chains to each other during polymerization. This is the so‐called “branching and pseudo‐living” technology.Finally, by co‐coagulating latexes of different polymers prepared by microemulsion polymerization, very small particles and, thus, high interface areas are generated. In this way properties of different polymers, such as fluoroelastomers and fluorinated semicrystalline polymers, are matched effectively, generating new nanocomposite materials that exhibit outstanding properties.In this paper these results are reported and an overview of some novel sophisticated fluoropolymers obtained in microemulsion is given.

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Oxygen-Containing, Asymmetric “Dual-Side” Zirconocenes: Investigations on a Reversible Chain Transfer to Aluminum

A series of new oxygen-substituted, asymmetric zirconocene dichlorides (rac-{1-[5,6-(ethylenedioxy)-2-methyl-1-η5-indenyl]-2-(9-η5-fluorenyl)ethane}zirconium dichloride (3a), rac{1-[5,6-(ethylenedioxy)-2-methyl-1-η5-indenyl]-2-(9-η5-fluorenyl)ethane}dimethylzirconium (3b) and rac-{[5,6-ethylenedioxy)-2-methyl-1-η5-indenyl](9-η5-fluorenyl)dimethylsilane}zirconium dichloride (3c)} have been prepared and their polymerization behavior was investigated in dependence of monomer concentration, temperature and catalyst activation. The presence of oxygen substituents on the indenyl ring results in a strong increase of polymerization activities and also of polymer molecular weights with decreasing Al/Zr ratio. Significantly higher molecular masses and activities were found for the dimethyl complex 3b after activation with [(C6H5)3C+][(C6F5)4B−] deriving from the absence of chain transfer to aluminum and higher concentration of active catalyst species. The mechanism of stereoerror formation of the oxygen-containing C1-symmetric catalyst was investigated by deuterium labeling studies on propene monomers. The results are discussed on the basis of a reversible chain transfer to aluminum.

Tailored polymer architectures by reversible addition-frasmentation chain transfer

Living radical polymerization methods that allow the preparation of polymers with predetermined molecular weight, narrow molecular weight distribution and tailored architecture (e.g. block, graft, stars) are offering a vast range of new and advanced materials. With applications ranging from surfactants, dispersants, coatings and adhesives, to microelectronics, membranes, drug delivery, and biomaterials they have the potential of revolutionizing a large part of the polymer industry.

Living polymerization with reversible chain transfer and reversible deactivation: The case of cyclic esters

Polymerization of cyclic esters at the properly chosen conditions can be treated as living polymerization, in agreement with the tentative definition of the Nomenclature Commission of IUPAC (Macromolecular Division) requiring that no irreversible transfer or irreversible termination take place. For these processes the most kinetic or structural (end group) studies do not reveal any deviation. However, since in these polymerizations reversible transfer to backbones of macromolecules and/or reversible deactivation take place, the molar mass distribution can be Poissonian only at certain conditions. These processes have been studied quantitatively and the corresponding rate constants were determined. Thus, the importance of these processes could be established by comparing the rate constants of transfer and/or deactivation with rate constants of propagation. In this way, polymerizations of cyclic esters were used to illustrate the meaning and scope of the definition of “living polymerization”, a process from which irreversible transfer and deactivation are absent and in which living polymers are formed, i. e. composed of macromolecules that do not irreversibly loose their ability to grow.

Synthesis of 3-[tris(trimethylsilyloxy)silyl]propyl methacrylate macromers using catalytic chain transfer polymerization: a kinetic and mechanistic study

Macromers of 3-[tris(trimethylsilyloxy)silyl]-propyl methacrylate (TRIS) were synthesised using catalytic chain transfer polymerization, and the kinetic parameters governing the reaction were evaluated. A study on the radical solution polymerization of TRIS in the presence of the catalytic chain transfer agent bis[(difluoroboryl)dimethylglyoximato]cobalt(II) (COBF) at 60 degrees C was conducted. Using appropriate Mark-Houwink-Kuhn-Sakurada constants for polyTRIS, the chain transfer constant (C-s) for COBF was found to be similar to 1400 in toluene solution. This low Cs value, as compared to the value reported for methyl methacrylate polymerization (similar to 3.5 . 10(4)), is only partly explained by a diffusion-controlled chain transfer reaction in the methacrylate series of monomers. A study on the influence of conversion on the molecular weight distribution indicated significant broadening and bimodality, consistent with reversible catalyst poisoning and chain transfer to the macromers. High oxygen solubility in TRIS is hypothesised to play a role in the reversible catalyst poisoning making it difficult to obtain a controlled reaction under normal free-radical polymerization reaction conditions.

Living Free-Radical Polymerization by Reversible Addition−Fragmentation Chain Transfer: The RAFT Process

mechanism involves Reversible Addition-Fragmentation chain Transfer, and we have designated the process the RAFT polymerization. What distinguishes RAFT polymerization from all other methods of controlled/living free-radical polymerization is that it can be used with a wide range of monomers and reaction conditions and in each case it provides controlled molecular weight polymers with very narrow polydispersities (usually <1.2; sometimes <1.1). Living polymerization processes offer many benefits. These include the ability to control molecular weight and polydispersity and to prepare block copolymers and other polymers of complex architecturesmaterials which are not readily synthesized using other methodologies. Therefore, one can understand the current drive to develop a truly effective process which would combine the virtues of living polymerization with versatility and convenience of free-radical polymerization.2-4 However, existing processes described under the banner “living free-radical polymerization” suffer from a number of disadvantages. In particular, they may be applicable to only a limited range of monomers, require reagents that are expensive or difficult to remove, require special polymerization conditions (e.g. high reaction temperatures), and/or show sensitivity to acid or protic monomers. These factors have provided the impetus to search for new and better methods. There are three principal mechanisms that have been put forward to achieve living free-radical polymerization.2,5 The first is polymerization with reversible termination by coupling. Currently, the best example in this class is alkoxyamine-initiated or nitroxidemediated polymerization as first described by Rizzardo et al.6,7 and recently exploited by a number of groups in syntheses of narrow polydispersity polystyrene and related materials.4,8 The second mechanism is radical polymerization with reversible termination by ligand transfer to a metal complex (usually abbreviated as ATRP).9,10 This method has been successfully applied to the polymerization of various acrylic and styrenic monomers. The third mechanism for achieving living character is free-radical polymerization with reversible chain transfer (also termed degenerative chain transfer2). A simplified mechanism for this process is shown in

Polymerization of Cyclopentene Using Metallocene Catalysts: Competitive Cis- and Trans-1,3 Insertion Mechanisms

Hydrooligomerization or polymerization of cyclopentene using racemic 1,2-ethylenebis(η5-indenyl)zirconium dichloride (1) and methylaluminoxane leads to the production of poly(cis-1,3-cyclopentane). Analogous reactions using racemic 1,2-ethylenebis(η5-tetrahydroindenyl)zirconium dichloride (2) leads to the production of oligomers in which cyclopentene is incorporated in a cis- or trans-1,3 manner. Two plausible mechanisms for trans-1,3 insertion are presented which involve reversible β-hydrogen elimination reactions of unsaturated oligomers or direct interconversion of olefin hydride complexes via the intermediacy of a σ-CH complex. Copolymerization of cyclopentene and 3-cyclopentylcyclopentene-2-d (9) reveals that the former process can occur but is not competitive with simple copolymerization; this, in combination with other observations, suggests that reversible chain transfer is unlikely to account for trans-1,3 insertion. Polymerization of cyclopentene-d8 led to significantly different, instantaneous ...

Triallylstannyllithium-allyllithium as an initiator for the anionic reversible chain transfer polymerization of 1,3-butadiene

The anionic polymerization of 1,3-butadiene using a novel metalloidal anion initiator, triallylstannyllithium (TALi)-allyllithium (ALi), was studied. The TALi-ALi initiated anionic polymerization of 1,3-butadiene gave the star polymer along with the linear polybutadiene (PBD). The star polymer consisted of three PBD branches and a central tin atom. What is striking is a fact that the number-average molecular weights (M n ) and molecular weight distribution of three PBD branches and linear PBD were almost identical. A reversible chain transfer polymerization mechanism, which includes the equilibrium between tri(macroallyl)-stannyllithium and macroallylic anion, is proposed.

Experimental Criteria for Living Polymerizations

AbstractSeven experimental criteria for living polymerizations are critically reviewed. The limitations of the use of linear plots of M̄n (or X̄;n) versus conversion and linear plots of the number of polymer chains versus conversion are illustrated by an example of alkyllithium‐initiated polymerization of styrene by incremental additions of styrene with deliberate termination (5% per incremental monomer addition). The experimental criteria of using narrow molecular weight distribution and the formation of block copolymers are illustrated with practical examples and size exclusion chromatography data. The use of the terms ‘living polymerization with reversible termination’ and ‘living polymerization with reversible chain transfer’ are proposed.

Kinetic Simulation of Polymerization Involving Termination by Reversible Chain Transfer

Numerical integration has been used as a means of simulating the title polymerization so that the variation of molecular weight and molecular weight distribution with reaction time (conversion) can be evaluated. The time/conversion dependence of the polydispersity (R) has been evaluated as a function of the relative magnitude of the rate constants associated with the initiation/termination equilibria . It is shown that the short term behaviour of R and the rate of approach to the value (R = 4/3) predicted by the steady state treatment have a marked dependence on the choice of rate constants.

Monte Carlo simulation of polymerization with reversible chain transfer

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTMonte Carlo simulation of polymerization with reversible chain transferKenneth F. O'DriscollCite this: Macromolecules 1985, 18, 7, 1508–1510Publication Date (Print):July 1, 1985Publication History Published online1 May 2002Published inissue 1 July 1985https://doi.org/10.1021/ma00149a025RIGHTS & PERMISSIONSArticle Views40Altmetric-Citations1LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (371 KB) Get e-Alerts Get e-Alerts

Quasiliving Carbocationic Polymerization. X. Molecular Weight Averages and Polydispersity

Abstract In quasiliving polymerizations with reversible chain transfer (QL0R systems), polymers with narrow molecular weight distribution can be obtained, It has been shown that while in true living systems (L00) R = 1, and in quasiliving systems with irreversible chain transfer (QL01) R = 2 is the limiting value of polydispersity, in QL0R systems r = 4/3 is the polydispersity to which the distribution of the polymer tends with increasing polymerization time. This limit is independent of the rate of reinitiation; the course of the R vs t curves is, however, determined by the various rate constants.

Reactivity of condensed‐ring hydrocarbons as polymerization retarders

AbstractSeventeen condensed‐ring hydrocarbons have been investigated as retarders or inhibitors of styrene and methyl methacrylate polymerization. Rates were measured dilatometrically. Nine of the compounds tested retarded or inhibited the polymerization of styrene, but methyl methacrylate appears not to be affected by this class of substances. Styrene data obtained are examined in the light of Fukui's “frontier electron” formalism for correlating structure and reactivity. It is concluded that the principal locus of radical attack is at the meso carbon atoms, Fukui's (Ss) region, and that a substantial value of the (Ss) density index is required for retardation, although when the molecule also possesses an appreciable (Sp) index, a synergistic action is noted. The point is made that since condensed‐ring hydrocarbons do not have “functional groups” in the sense that quinones and other retarders do, the effect of monomer reactivity is likely to be of added importance in determining retarder efficiency. The lack of retardation effects in polymerizing methyl methacrylate is interpreted from this standpoint. A reverse chaintransfer reaction is tentatively proposed to account for the observed behavior.