Recent developments in living cationic polymerization

Scope and limitation of the vinyl ether polymerization initiated by NR₄ClO₄(KClO₄;LiClO₄)/CH₃CHI-OR was discussed. Besides isobutyl vinyl ether (IBVE), N-vinylcarbazole (NVC) and 2-chloroethyl vinyl ether (CEVE) were initiated by NR₄ClO₄/CH₃CHI-OR. These polymerizations exhibited the characteristics of a living polymerization. However, in order to observe narrow molar mass distribution NVC was initiated with CH₃CHI-OR, without salts. Block copolymers were synthesized by the method of sequential monomer addition (NVC, IBVE, CEVE). The PCEVE segment was modified by nucleophilic substitution, which allowed the synthesis of amphiphilic block copolymers.     

Synthetic applications of non-polymerizable monomers in living carbocationic polymerization; recent developments

Recent developments using non-(homo)polymerizable monomers, for the synthesis of functional polymers and block copolymers by living cationic polymerization are discussed. The preparation of block copolymers based on IB and αMeSt or IBVE are reported using DPE capping followed by Lewis acidity moderation. The use of non-polymerizable diolefins such as bis (diphenylethylenes) for in situ coupling of living chains is also discussed. Depending on the structure of the diolefin mono- or diaddition is observed.     

Multiarm-star polyisobutylenes by living carbocationic polymerization

This article describes the synthesis of high molecular weight multiarm-star branched polyisobutylenes by living polymerization, using multifunctional initiators, and their initial characterization. First, macrointiators carrying tert-hydroxy function-alities were synthesized by the radical copolymerization of 4-(1-hydroxy-1-methylethyl)-styrene with styrene. This copolymerization system was found to be ideal with r₁ ≡ r₂ ≡ 1. Selected macroinitiators with average functionalities of 8–73 were then used to synthesize the star-branched polyisobutylenes. Polymers with molecular weights up to M̄_n = 400,000 were obtained within 30–60-min reaction times, while under similar conditions the monofunctional 2-chloro-2,4,4-trimethylpentane initiator yielded M̄_n ≈ 10,000 in 20 min. This can be viewed as an indirect proof that simultaneous multiple initiation took place with the macroinitiators. Under controlled conditions a branchedpolyisobutylene with M̄_n = 375,000 and MWD = 1.2, and theoretically calculated 23 arms, with no detectable side products was obtained under living conditions in 60 min; the molecular weight of this polymer increased linearly with time. The branched structure of the polymers were demonstrated by SEC-LLS analysis and core destruction of selected samples. © 1998 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 36: 85–92, 1998     

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.     

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.     

Recent developments in nickel-catalyzed intermolecular dicarbofunctionalization of alkenes

Nickel-catalyzed three-component alkene difunctionalization has rapidly emerged as a powerful tool for forging two C–C bonds in a single reaction. Building upon the powerful modes of bond construction in traditional two-component cross-coupling, various research groups have demonstrated the versatility of nickel in enabling catalytic 1,2-dicarbofunctionalization using a wide range of carbon-based electrophiles and nucleophiles and in a fully intermolecular fashion. Though this area has emerged only recently, the last few years have witnessed a proliferation of publications on this topic, underscoring the potential of this strategy to develop into a general platform that offers high regio- and stereoselectivity. This minireview highlights the recent progress in the area of intermolecular 1,2-dicarbofunctionalization of alkenes via nickel catalysis and discusses lingering challenges within this reactivity paradigm.

Nickel-catalyzed three-component alkene difunctionalization has rapidly emerged as a powerful tool for forging multiple C–C bonds in a single step.     

Recent developments in miniemulsion polymerization

Some developments in miniemulsion polymerization aiming at taking advantage of its unique mechanisms minimizing the drawbacks of this technique are discussed. The discussion includes preparation of highly concentrated latexes, miniemulsion polymerization in continuous stirred tank reactors (CSTRs), and elimination of the low-molecular-weight hydrophobe.     

Exchange reactions in the living cationic polymerization of alkenes

Living cationic polymerization of alkenes is discussed in terms of rapid dynamic equilibration between carbenium ions and either covalent species or onium ions. Rates of exchange between these species have strong effect on polydispersities of the synthesized polymers. In the discussed living systems, rates of exchange are faster than rates of propagation. Propagation proceeds via classic carbocationic mechanism.     

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.     

Addition reactions of carbocations with alkenes: Studies on the mechanism of carbocationic polymerization

In certain cases, Lewis acid catalyzed addition reactions of alkyl halides with alkenes terminate at the 1:1 product stage. This telomerization reaction has been employed to study the efficiency of initiating systems of carbocationic polymerization and the relative reactivities of alkenes towards carbenium ions. A direct determination of addition rate constants is possible in some cases.     

Noncoordinating anions in carbocationic polymerization

The initiation and catalysis of isobutylene polymerization from several new metallocene and nonmetallocene initiator-catalysts that contain the noncoordinating anions (NCA), B(C₆F₅)₄⁻ and RB(C₆F₆)₃⁻, is reported. Application of these initiator-catalysts is extended to styrenics and vinyl ethers. The NCA does not contribute to termination and can be used in low concentrations compared with conventional Lewis acids. These qualities provide for isobutylene polymerizations that yield low M_n oligomers or high M_n polymer, dependent upon the initiator and polymerization conditions. Mechanistic aspects of initiation, transfer and termination as well as the participation of adventitious water are considered for each class of initiator-catalyst. The influence of the NCA on the stereoregularity of cationic styrene polymerization is also considered. NCAs do not cause the stereospecific carbocationic polymerization of styrene. We suggest that under conditions not conducive to carbocationic polymerization, NCA/metallocenes mediate the coordination polymerization of styrene. © 1997 John Wiley & Sons, Inc.     

Metal-catalyzed living radical polymerization: discovery and developments

Control of radical polymerization has been one of the most challenging frontiers in polymerization chemistry. This review presents the discovery of metal-catalyzed living radical polymerization and recent developments in the evolution of catalysts in terms of versatility and activity, scope of monomers, controlled polymerization in water, catalyst removal, and precision synthesis of well-controlled polymers such as random, block, end-functionalized, and star polymers.     

Kinetic simulation of living carbocationic polymerizations. II. Simulation of living isobutylene polymerization using a mechanistic model

This paper discusses the kinetic simulation of TiCl₄--coinitiated living carbocationic isobutylene (IB) polymerizations governed by dormant-active equilibria, using a mechanistic model. Two kinetic models were constructed from the same underlying mechanism: one using a commercial simulation software package (Predici^®), and the other using the method of moments. Parameter estimation from experimental batch reactor data with Predici yielded a rate constant of propagation k_p = 4.64 × 10⁸ ± 2.75 × 10⁸ L/mol s, with no constraints imposed. This agrees with k_p data measured with diffusion clock and competition methods, but disagrees with kinetically obtained k_p values. Estimation of rate constants with Predici^® and the GREG parameter estimation software packages revealed that it was difficult to estimate the complete set of kinetic parameters, due to correlated effects of the parameters on model predictions. Estimability analysis confirmed that some of the strongly correlating parameters could not be estimated simultaneously using the available experimental data. Using k_p = 6 × 10⁸ ± 2.75  × 10⁸ L/mol s measured by Mayr, and using starting estimates of other rate constants defined by experimentally observed correlations, yielded the set of rate constants required for the simulations. Both kinetic models yielded good agreement with experimental data, with the exception of M_w values that slightly diverged from the theoretically predicted ‘M_w−M_n = constant’ relationship. This may indicate the occurrence of a minor side reaction. However, the k_p/k₋₁ = 17.5 L/mol average run length calculated from measured and simulated MWD data agrees well with earlier literature values.     

Criteria for living systems with a special emphasis on living cationic polymerization of alkenes

A number of new living systems have been reported in recent years. Classic anionic polymerization of nonpolar monomers allows the synthesis of well-defined high molecular weight polymers (DP > 1000), block copolymers, chains with perfect terminal functionalities and behaves as a true living system. Some new systems abuse the term “living polymerization.” A relatively modest criterion for living systems is proposed “3 X 10,000,” i.e., k_p/k_t > 10⁴ mol^-1 L, k_p/k_tr > 10⁴, 1/k_t/tr > 10⁴ s (translated to < 10% of chains deactivated at t ≈ 1000 s), which is related to a typical limit of the polymeric chain dimensions (DP ≈ 100) and standard synthetic manipulations (≈ 15 min). New living cationic systems are discussed in detail with special emphasis on exchange phenomena. © 1993 John Wiley & Sons, Inc.     

Tailored macromolecules by living carbocationic techniques

This presentation concerns our latest results for the controlled synthesis of various homopolymers, blocks, random copolymers, networks, by living carbocationic polymerization (LC^⊕ Pzn). We start with an analysis of the Winstein ionicity spectrum, a roadsign toward LC^⊕ Pzn systems. Living isobutylene (IB) homopolymerization can be effected by the addition of common anion salts to conventional polymerization charges, e.g., by adding nBu₄NCl to a 2-chloro-2,4,4-trimethylpentane (TMPCl)/TiCl₄/IB system. Various AB diblocks have been prepared by sequential monomer addition. The first clean svnthesis of an AB poly(olefin-b-alkyl vinyl ether) diblock, ie., poly(isobutylene-b-methyl vinyl ether), by sequential monomer addition was accomplished. These novel amphiphilic diblocks promise to be excellent nonionic detergents. The field of linear A-B-A triblocks and three-arm star blocks where A = glassy and B = rubbery (polyisobutylene) segment, is vigorously expanding. Several new thermoplastic elastomers (TPEs) have been prepared, characterized and tested, e.g., poly-(indene-b-isobutylene-b-indene). The synthesis of random copolymers, e.g., isobutylene/2,4,4-dimethyl-1,3-pentadiene, by living reactivity leveling has been demonstrated. Transformation of the polyisobutylene cations PIB^⊕ and ^⊕PIB^⊕ yielded the corresponding anions PIB^⊖ and ^⊖PIB^⊖, which upon block polymerization of butadiene (Bd) and methyl methacrylate (MMA), respectively, gave rise to the new diblock PIB-PBd and triblock PMMA-PIB-PMMA. In the latter, the largely syndiotactic sPMMA endblocks were stereocomplexed with isotactic iPMMA and thus the upper use-temperature of these novel TPEs was enhanced. Novel amphiphilic networks comprising hydrophilic polymethacrylates (e.g., dimethylaminoethyl methacrylate (DMAEMA) or hydroxyethyl methacrylate (HEMA)) crosslinked by methacrylate-telechelic PIBs were prepared and their potential usefulness for biomedical applications has been studied.     

Recent developments in polymerization by an alternating intra-intermolecular mechanism

Recent developments in polymerization by an alternating intra-intermolecular mechanism are summarized and discussed briefly. The methods used to prove the structure of typical polymers, which in turn established the mechanism of polymerization, are reviewed. A study of larger ring formation by Marvel and Garrison is also summarized. Ring of sizes from 7 to 21 members were formed from α-diolefins in yields of from 4 to 25%. A study of certain unsymmetrical dienes has shown that cyclization occurs in yields up to 60%, but that gelation eventually occurs because of the difference in relative reactivities of the two olefinic double bonds. Copolymerization of certain 1,4-dienes with certain olefins to produce linear copolymers containing six-membered rings is discussed. In an effort to explain the strong driving force to close a six-membered ring during polymerization of 1,6-diolefins, a pronounced interaction between the double bonds, which can be represented by several resonance forms of the molecule, is proposed.     

"活性"/可控自由基聚合新进展

概述了当前“活性”／可控自由基聚合(CPR)的三种主要方法，硝基氧调介聚合(NMP)、原子转移自由基聚合(ATRP)、可逆加成-断裂链转移聚合(RAFT)，特别是近年来的进展情况。     

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.     

Living and apparently living carbocationic polymerizations

Some years ago, the occurrence of living carbocationic polymerization had not generally been expected to be possible, since it is well known that most carbocationic species are quite unstable and have very short lifetimes, and since transfer to monomer had been shown to be important, particularly near room temperature. However, during the last years, many reports of living carbocationic polymerizations have been made. They were based on the observation of various features usually linked to living polymerizations, such as a linear increase of mol. wts. with conversion, sometimes even after several monomer additions, which was attributed to the absence of termination and transfer. In some cases, narrow mol. wt. distributions were also obtained. There seems to be now a general agreement that in these polymerizations a reversible termination occurs, making eventually further growth possible on all macromolecules. Another general feature of those apparently living systems is that the ratio of propagation rate and initiation rate is not too high, so that the concentration of macromolecules is approximately equal to that of the initiator. But the experimental data do not necessarily imply, as this has been generally assumed, that transfer is absent and that the nature of active sites is completely different from those in more classical systems. It is shown that the values of transfer constants already measured in these last ones are compatible with the results obtained in the apparently living systems. A perfectly linear relationship between number-average degree of polymerization (D̄P̄_n) and polymer yield may be observed even up to mol. wts. of about 2.10⁴ with transfer constants k_trM/k_p as high as 5.10⁻⁴ in apparently living systems. Termination and transfer might be, however, reduced in some cases by various means that are examined, such as the presence of polar additives, a lowering of temperature and the presence of excess monomer. The distinction between systems obeying all the main criteria for living polymerization and those which are only apparently living is discussed.