A survey of kinetic and mechanistic similarities in living polymerization systems

In the light of recent discoveries in the field of living polymerizations it seems inevitable to reconsider our views on these polymerization systems. This paper surveys the kinetic and mechanistic similarities in living polymerizations, and analyses and compares chain transfer dominated nonliving polymerizations and living systems to conclude on the nature of propagating species, shelflife and livingness. Some recently raised specific problems are also summarized and discussed. It has been found that most of the living polymerizations known to date, such as living anionic, cationic ring opening, group transfer, carbocationic, ring opening metathesis, Ziegler-Natta, free radical and immortal polymerizations, exhibit the characteristics of quasiliving polymerization, i.e., an equilibrium exists between propagating (active) and inactive (dormant) species. On the basis of this finding and a comparison between mechanistic and kinetic models of quasiliving and ideal living polymerizations, it is suggested that the former is the general phenomenon, and ideal living polymerization is a subclass of quasiliving polymerizations.     

Comparison of living polymerization systems

Recent developments in the field of living polymerization are surveyed. Comparison of the available mechanistic and kinetic information is made for living anionic, cationic, free radical, group transfer, ring-opening metathesis, Ziegler-Natta and immortal polymerizations. This evaluation indicates that the majority of living polymerizations involve quasiliving equilibrium between active (propagating) and dormant (nonpropagating) polymer chains. On the basis of the kinetics of a general model for quasiliving and ideal living polymerizations it is concluded that ideal living polymerization is a special subclass of quasiliving polymerizations. Classification of living polymerization systems is also attempted.     

Quasiliving Carbocationic Polymerization. XI. An Interpretation of Solvent Effects by Donor and Acceptor Numbers

Counteranion/solvent interactions (counteranion solvation) profoundly influence each and every elementary step of carbocationic polymerizations and are just as important as the commonly emphasized cation/solvent interactions (cation solvation). Counteranion solvation and carbocation solvation have been characterized by Gutmann' s acceptor number AN and donor number DN, respectively. Analysis of earlier data leads to the conclusion that the effect of monomer concentration on the rate, molecular weight, and molecular weight distribution obtained in cationic olefin polymerizations in “polar” solvents are in fact due to subtle changes in solvent concentration. Indeed, olefin monomers behave as “nonpolar” solvents and by changing the monomer concentration the character of the medium may profoundly change. It is further concluded that quasiliving polymerizations cannot be achieved in batch operations because the conditions that prevail in the initial charge, although possibly suitable for quasiliving polymerizations, must continuously change with the diminishing monomer concentration, i.e., by continuously changing the solvent character of the system. In contrast, in continuous systems initial conditions in the charge suitable for the attainment of living or quasiliving conditions can be maintained even for long periods of time by continuously replenishing the consumed monomer. By the use of these concepts, heretofore unexplained observations made in the course of quasiliving polymerization studies have been accounted for and, beyond this, new insight into solvation phenomena in cationic polymerizations is generated.     

Quasiliving Carbocationic Polymerization. II. The Discovery: The α-Methylstyrene System

A detailed analysis of elementary reactions of carbocationic polymerization culminated in the prediction and subsequent experimental demonstration of quasiliving polymerization. Quasiliving polymers are formed in a system provided that the process of chain termination and chain transfer to monomer are absent or reversible, i.e., the propagating ability of the chain end is maintained throughout the experiment, and the molecular weight increases in proportion to the cumulative amount of monomer added. The chain end can be active (carbocation) or dormant (reactivable polymeric olefin or cation source). Chain transfer is suppressed by keeping the monomer concentration low. Quasiliving polymerizations are maintained by continuous slow feeding of dilute monomer to a charge containing the initiating or propagating species (quasiliving polymerization technique). A comprehensive kinetic scheme has been developed that describes quasiliving polymerization in quantitative terms. Quasiliving polymerization was demonstrated experimentally in the “H₂O”/BCl₃/α-methylstyrene and cumyl chloride/BCl₃/α-methylstyrene systems. M _n versus monomer input plots are linear over wide ranges, indicating quasiliving conditions, and poly(α-methylstyrenes) with M _n > 2 × 10⁵ have been obtained, Molecular weight distributions were found progressively to narrow and dispersion ratios M _w/M _n to decrease.     

Quasiliving Carbocationic Polymerization. I. Classification of Living Polymerizations in Carbocationic Systems

It has been shown that in addition to classical living polymerizations, several other polymerization systems exist that may exhibit partially living so-called quasiliving character. The single requirement for quasiliving polymerization is the absence of irreversible termination. The various possible living systems have been classified by taking into consideration the absence or reversibility of termination and the absence, reversibility, or irreversibility of chain transfer. In regard to chain transfer, both unimolecular and/or bimolecular processes have been considered. A comprehensive examination of all possibilities yielded, in addition to the classical terminationless-transferless living system, five quasiliving systems. Kinetic analysis led to equations defining these systems and to diagnostic techniques useful for the classification and characterization of the mechanism of living carbocationic polymerizations.     

Ring-opening polymerization of cyclobutane adduct of dimethyl 1,1-dicyanoethylene-2,2-dicarboxylate and ethyl vinyl ether

For an extension of the work on the ring-opening polymerizations of cyclobutane adducts of strong donor olefins and strong acceptor olefins yielding novel alternating copolymers of those olefins, the ring-opening polymerization of the cyclobutane adduct 3 of dimethyl 1,1-dicyanoethylene-2,2-dicarboxylate (DDED) and ethyl vinyl ether (EVE) is investigated. Cyclobutane 3 reacted with methanol and acetic acid at ambient temperature to yield the corresponding ring-opened adducts. The polymerizations of 3 were carried out with anionic initiators, tertiary amines, ammonium halides, and Lewis acids, respectively, according to the polymerization methods of the cyclobutane adduct 1 of tetracyanoethylene (TCNE) and EVE. All these polymerization catalysts except for ammonium halides were effective for the polymerization of 3 , yielding alternating copolymers of DDED and EVE. The chain transfer reactions of the polymerization with anionic initiators are also discussed on the basis of a model reaction. © 1997 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 35: 1563–1570, 1997     

Comparison of living polymerization mechanisms. Acrylates and carbocationic polymerization

The concept of a living polymerization is critically discussed. A system ranking various classes of “livingness” is proposed, and the importance of determining the real values of k_tr/k_p and k_t/k_p ratios is expounded. New living systems, including carbocationic polymerization and group transfer polymerization of acrylates are compared with classic ionic systems. The mechanism of propagation and the nature of the true active species are similar in both new and classic polymerizations. The role of various components which improve the “livingness” of the polymerizations is discussed and explained by dynamic equilibration between dormant and active species and suppression of side reactions.     

Free-Radical Ring-Opening Polymerization

Although the ionic ring-opening polymerization of heterocyclic compounds, such as ethylene oxide, tetrahydrofuran, ethylen-imine, β-propiolactone, and caprolactam, as well as the Ziegler-Natta ring-opening of cyclic olefins, such as cyclopentene and norbornene, are well known, free radical ring-opening polymerizations are rather rare. The few examples that are reported in the literature involve cyclopropane derivatives or highly strained bi-cyclic olefins. For example, Takahashi [l] studied the free radical polymerization of vinylcyclopropane and reported that the cyclopropane ring opened to give a polymer containing about 80% 1,5-units and about 20% of undetermined structural units.     

Anionic amphiphilic end‐linked conetworks by the combination of quasiliving carbocationic and group transfer polymerizations

A series of amphiphilic end‐linked conetworks was synthesized by the combination of two “quasiliving” polymerization techniques, quasiliving carbocationic (QLCCP) and group transfer polymerizations (GTP). The hydrophobic monomer was polyisobutylene methacrylate synthesized by the QLCCP of isobutylene and subsequent terminal modification reactions. The hydrophilic monomer was methacrylic acid (MAA) introduced via the polymerization of 2‐tetrahydropyranyl methacrylate followed by acid hydrolysis after (co)network formation. The conetwork syntheses were performed by sequential monomer/crosslinker additions under GTP conditions. All the precursors and the extractables from the conetworks were characterized by gel permeation chromatography and ¹H NMR. The resulting polymer conetworks were investigated in terms of their degree of swelling (DS) in aqueous media and in tetrahydrofuran (THF) over the whole range of ionization of the MAA units and in n‐hexane for uncharged conetworks. The DSs in water increased with the degree of ionization (DI) of the MAA units and the hydrophilic content in the conetwork, whereas the DSs in THF increased with the reduction of the DI of the MAA units. The effective pK of the MAA units in the conetworks increased from 8.4 to 10.5 with decreasing MAA content. These findings can facilitate the design of similar unique conetworks with adjustable swelling behavior and composition‐dependent pK values. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 4289–4301, 2009     

“Charge transfer” polymerization—and the absence thereof!

Mechanisms for “charge‐transfer” spontaneous polymerizations and cycloadditions between electron‐rich olefins and electron‐poor olefins were reviewed. As for propagation, literature proposals involving charge‐transfer complexes were rejected. Instead, alternating copolymerization is ascribed to polar effects in free‐radical reactions. As for spontaneous initiation, literature proposals involving charge‐transfer complexes, with or without proton transfer, were rejected. Instead, the initiating species is postulated to be a tetramethylene zwitterion biradical, which may initiate either ionic homopolymerization or free‐radical copolymerization. A new hypothesis proposes that any interaction that brings vinyl monomers close together may facilitate tetramethylene formation and spontaneous polymerization. These interactions include Coulombic, acid–base, hydrophobic–hydrophilic and templating–tethering interactions. © 2001 John Wiley & Sons, Inc. J Polym Sci Part A: Polym Chem 39: 2069–2077, 2001     

New processes and designed polymers by cationic techniques

The most important recent development in cationic polymerizations is the emergence of living polymerizations leading to a variety of new potentially useful well-defined macromolecules under conventional laboratory conditions. Three requirements have to coexist for living carbocationic polymerization to occur: Controlled initiation controlled chain-transferless propagation and controlled (quasiliving) termination. The first part of this presentation will briefly discuss the road to these three key requirements. The second part will concern practical consequences and select systems. The synthesis of narrow-molecular-weight-distribution (M̄_w/M̄_n = 1.1 - 1.3) tert-chlorine telechelic polyisobutylenes over a wide molecular weight range (M̄_n from ∼1000 to ∼125, 000 g/mole) will be outlined together with recent work on aromatic olefins, e.g., styrene, tert-butylstyrene and p-chlorostyrene. These developments led to the combination of these living systems for the synthesis of block copolymers by sequential monomer addition. Tri- or higher block copolymers comprising glassy outer segments and rubbery inner segments, for example, poly(styrene-b-isobutylene-b-styrene, poly(p-chlorostyrene-b-isobutylene-b-p-chlorostyrene), have been prepared. These new thermoplastic elastomers exhibit phase-separated microstructures and an interesting combination of physical-mechanical properties.     

PHOTOINITIATED CATIONIC POLYMERIZATION OF EPOXY MONOMERS IN THE PRESENCE OF POLY(3,4-EPOXY-1-BUTENE)

ABSTRACT

Hydroxyl-terminated poly(3,4-epoxy-1-butene) (polyEPB) is an interesting and highly useful agent for the acceleration of the photoinitiated cationic ring-opening polymerization of epoxide monomers. Kinetic investigations using real-time infrared spectroscopy have shown that the observed acceleration of the polymerization is due to two independent mechanisms. Crosslinking polymerization of epoxide monomers is accelerated due to an activated monomer mechanism that results in facile chain transfer due to interaction of the terminal hydroxyl groups of polyEPB with the growing oxonium ion chain ends. A second mechanism involving participation of polyEPB in a free radical chain induced decomposition of the onium salt photoinitiator is mainly responsible for the observed acceleration in the rate of polymerization. A large number of polymer-bound carbocationic species are generated by this mechanism that are capable of initiating polymerization of the epoxide monomer.     

Macrocyclic polymerization - a new approach to molecular engineering

2,2-Dibutyl-2-stanna-1,3-dioxacycloalkanes were used as cyclic initiators for the ring-opening polymerization of various lactons. This method exclusively yielded series of macrocyclic polylactones without any competition with linear polymers. Under optimized reaction conditions these macrocyclic polymerizations obey the pattern of “living polymerizations”. The living chain ends allow the syntheses of macrocyclic blockcopolymers. The macrocyclic polylactones react with carboxylic acid chlorides by ring-opening yielding telechelic oligo or polylactones. Furthermore, the tin containing macrocyclic polylactones can be used as difunctional “monomers” for polycondensations with dicarboxylic acid dichlorides.     

Synthesis of novel polymeric materials based on aliphatic polyesters by combination of different controlled polymerization methods

Combination of the living ring-opening polymerization (ROP) of ε-CL and lactides with the “controlled” free radical polymerization of styrene and methacrylic monomers is a versatile strategy for the synthesis of well-defined block and graft copolymers. In this respect, the dual “living” polymerization strategy in which two different functional groups on a single molecule used to initiate the two controlled mechanisms is particularly efficient. Combination of ROP and step-growth polymerization is another versatile methodology for the preparation of a large variety of new materials, e.g. polyimide nanofoams, polyester/silica hybrid materials and star and branched polyesters by dendritic initiation.     

Quasiliving Carbocationic Polymerization. XVII. Synthesis Of Poly (Styrene-β-Isobutylene-β-Styrene)

Poly(styrene-b-isobutylene-b-styrene) has been synthesized by sequential carbocationic polymerization under quasiliving conditions at -90°C. The quasiliving synthesis was effected by first continuously and slowly condensing gaseous isobutylene (IB) to a bifunctional initiating system (p-dicumyl chloride/TiCl₄) dissolved in a hexane-methylene chloride (60:40 v/v) mixture. After the quasiliving polyisobutylene (PIB) sequence had reached a desired molecular weight, styrene (St) was continuously and slowly added to produce the polystyrene (PSt) sequence. The products consisted of the target triblock. However, due to initiation by impurities and possibly to chain transfer to both IB and St, it also contained diblocks and small amounts of homopolymers. While the latter could be removed by selective fractionation, the triblocks and diblocks could not be separated. The mechanism of quasiliving polymerization leading to PIB/PSt blocks is discussed.     

受阻路易斯酸碱对在高分子聚合上的应用

受阻路易斯酸碱对(frustrated Lewis pairs,FLPs)是大位阻的路易斯酸和大位阻的路易斯碱在溶液中受空间位阻因素影响而不能形成配位键所得到的组合。 在这种特殊的组合中,路易斯酸和路易斯碱未能被中和淬灭,依旧保持着的反应活性。 而当H₂等小分子靠近时,FLPs可以将H₂的化学键异裂,进而得到一个阳离子和一个阴离子。 这种独特的反应特性使得FLPs在催化加氢、小分子气体活化、烯烃聚合和开环聚合等方面展现出了一些具有新特性的研究思想和方法。 尤其是在烯烃聚合和开环聚合中,FLPs具有很强的催化活性。 本文简要介绍了FLPs的发展历史及其在小分子活化中的应用,并重点介绍了其在高分子催化领域中的应用。     

Some still unsolved problems in carbocationic polymerization

Some controversial problems in both “conventional” and “living” carbocationic polymerizations are discussed: direct initiation, apparently negative activation energies, effect of hindered bases and of other electron donors.     

Synthetic applications of non-polymerizable monomers in living carbocationic polymerization

The foundation and methodology of using highly reactive but non-polymerizable monomers in living cationic polymerizations is introduced. The chemistry and kinetics of 1,1-diphenylethylene (DPE) addition to living polyisobutylene (PIB) in methyl chloride/n-hexanes 40/60 v/v at −80°C is reported. Monoaddition occurred even when large excess of 1,1-diphenylethylene was used. The methanol quenched polymer of the DPE capped PIB carried -OCH₃ functionality exclusively, suggesting that the diphenyl alkyl chain-ends are completely ionized, which was confirmed by conductivity studies. By in-situ functionalization using soft nucleophiles a variety of functional groups were obtained, most notably ester upon reaction with silyl ketene acetal. It was found that the diphenyl carbenium ion is an efficient initiating species for the polymerization of reactive monomers such as vinyl ethers and α-methylstyrene. The synthesis of PIB based block copolymers was accomplished by sequential monomer addition, using para-methylstyrene, α-methylstyrene or isobutyl vinyl ether as the second monomer. It involved capping with DPE, followed by tailoring the Lewis acidity to the reactivity of the second monomer by the addition of titanium(IV) alkoxide, by replacing the Lewis acid with a weaker one or by the use of a common ion salt. PIB-b-PMMA was obtained by the combination of living cationic and group transfer (GTP) polymerizations.     

Ring-opening polymerization of nitrogen– and phosphorus-containing monomers: Polymerization mechanism and polymer properties

The first part of this paper describes cationic polymerizations of cyclic imino ethers, 2-oxazolines and 5,6-dihydro–4H–1,3–oxazines, which proceed via cyclic onium propagating species or via covalently bonded alkyl halide species. In an extreme case, both ionic and covalent species are present in equilibrium and propagate concurrently. The propagation rate constants due to the respective species were determined. A poly(cyclic imino ether) becomes hydrophilic or lipophilic dependent on the substituent of the monomer. Based on this principle, various types of nonionic polymer surfactants have been prepared, e.g., diblock and triblock copolymers, graft copolymers, and surfactants having one 2-oxazoline chain. The second part is concerned with ring-opening polymerizations of new eight cyclic trivalent phosphorus monomers. These polymerizations produced phosphorus-containing functional polymers such as a chelating resin. ³¹P NMR analyses of polymerization of cyclic phosphinite monomers led to propose a new mechanism of “Electrophilic Ring-Opening Polymerization”.