Quasiliving Carbocationic Polymerization. XIII. Polymerization of 2,4,6-Trimethylstyrene

The polymerization of 2,4,6-trimethylstyrene (vinyl mesitylene) has been investigated, and quasiliving polymerizations have been achieved under a comfortably wide experimental condition range. This monomer is particularly suitable for quasiliving polymerizations because the methyl groups in the 2 and 6 positions prevent chain transfer to monomer involving indanyl-skeleton formation. Quasiliving polymerizations readily occurred by the use of cumyl chloride/TiCl₄ or BCl₃ initiating systems in various n-C₆H₁₄/CH₂Cl₂ mixtures at -50°C. Because indanyl-skeleton formation is impossible, the rate of monomer addition can be safely decreased to very low values without risking chain transfer by intramolecular alkylation.     

Quasiliving Carbocationic Polymerization. X. Molecular Weight Averages and Polydispersity

In quasiliving polymerizations with reversible chain transfer (QL_0R 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 QL_0R 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.     

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.     

Quasiliving Carbocationic Polymerization. IV. Polymerization of p-tert-Butylstyrene

The mechanism of polymerization of p-tert-butylstyrene (ptBuSt) initiated by the cumyl chloride/BCl₃ initiating system in CH₂Cl₂ at -50°C has been investigated. At and below ～0.4 M ptBuSt, quasiliving polymerizations proceed, i.e., initiation is instantaneous, termination is absent or reversible, and chain transfer to monomer can be suppressed or eliminated. In the quasiliving range the M _n versus [ptBuSt]₀ plot is linear and passes through the origin, and a M _w/M _n decreases much below 2.0 with decreasing [ptBuSt]. GPC traces change from broad multimodal to narrow monomodal and the color of polymerization charges change from colorless to golden-yellow with decreasing [ptBuSt]. The effect of temperature jump subsequent to monomer addition has been examined; however, it does not explain the peculiar monomer concentration effect on the mechanism. Changes in the ionicity may be responsible for this phenomenon.     

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.     

Open mechanistic problems of quasiliving carbocationic polymerization of olefins mediated by nucleophilic additives

This study is a comprehensive overview of the open problems and the existing views on the mechanism of quasiliving carbocationic polymerizations (QLCP) of olefins mediated by nucleophilic additives. The fundamental and general aspects of ideal living and quasiliving polymerizations involving other mechanisms, such as free radical, group transfer, ring-opening metathesis, ring-opening cationic and anionic processes, have been also analyzed and summarized. Quasiliving carbocationic polymerization of olefins in the presence of nucleophiles, which form complexes with the Lewis acid coinitiators, occur By reversible termination. Four different mechanisms have been discussed in this study: (1) reactivity leveling by nucleophiles (“electron donors”); (2) propagation by species with decreased ionicity (“stretched polarized bonds”) mediated by Lewis acid-nucleophile complexes (LA-Nu); (3) propagation by classical ion pair and free ion species; (4) proton scavenging by nucleophiles and 2,6-di-teri-butylpyridine proton trap. It is shown that mechanisms No. 1, 3 and 4 cannot explain all the existing findings, and although the experimental results can be interpreted with mechanism No. 2, the existence of “stretched polarized bonds” can be questionable. It is also concluded that compared to nonliving carbocationic polymerization, kinetic analysis indicates that the propagating species cannot be the same in quasiliving carbocationic polymerizations and in chain transfer dominated classical carbocationic polymerizations with ion pairs and free ions.     

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. 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. IX. Forced Ideal Copolymerization of Styrene Derivatives

Forced ideal carbocationic copolymerization of α-methylstyrene (αMeSt) with p-tert-butylstyrene (ptBuSt) and (αMeSt) with styrene (St) has been achieved by continuous monomer feed addition to a cumyl chloride/BCl₃ charge at -50°C by keeping the feeding rate of the monomer mixtures equal to the overall rate of copolymerization, The composition of the copolymers was identical to the composition of the monomer feeds over the entire concentration range. A quantitative expression has been derived to show that under forced ideal copolymerization conditions the composition of the copolymer can be controlled by the composition of the feed. Further, conditions have been found for forced ideal quasiliving copolymerizations, i.e., the number-average molecular weight of the copolymers increased almost linearly with the cumulative weight of consumed monomers by the use of suitably slow, continuous feed addition in the presence of relatively nonpolar solvent mixtures (60/40 v/v n-hexane + methylene chloride). In polar solvent (methylene chloride) the molecular weight increase was less pronounced due to chain transfer to monomer involving indane-skeleton formation; however, with charges containing large amounts of ptBuSt the molecular weight increase was surprisingly strong. Interestingly, ptBuSt does not homopolymerize in 60/40 v/v n-hexane/methylene chloride but it readily copolymerizes with αMeSt. This observation was explained by examining the relative rates of terminations of the cationic species involved. Conditions have been found for the pronounced quasiliving polymerization of St. In forced ideal quasiliving copolymerizations neither the molecular weights of αMeSt/ptBuSt or αMeSt/St copolymers nor the initiating efficiencies of the initiating systems used show a depression. The microstructure of representative αMeSt/ptBuSt copolymers obtained under forced ideal quasiliving conditions has been analyzed by ¹³C-NMR spectroscopy. According to these studies, true copolymers have formed and resonance peaks for various triads have been deduced.     

Living carbocationic polymerization. IV. Living polymerization of isobutylene

Truly living polymerization of isobutylene (IB) has been achieved for the first time by the use of new initiating systems comprising organic acetate-BCl³ complexes under conventional laboratory conditions in various solvents from −10 to −50°C. The overall rates of polymerization are very high, which necessitated the development of the incremental monomer addition (IMA) technique to demonstrate living systems. The living nature of the polymerizations was demonstrated by linear versus grams polyisobutylene (PIB) formed plots starting at the origin and horizontal number of polymer molecules formed versus amount of polymer formed plots. obeys [IB]/[CH₃COOR^t · BCl₃]. Molecular weight distributions (MWD) are very narrow in homogeneous systems whereas somewhat broader values are obtained when the polymer precipitates out of solution . The MWDs tend to narrow with increasing molecular weights, i.e., with the accumulation of precipitated polymer in the reactor. Traces of moisture do not affect the outcome of living polymerizations. In the presence of monomer both first and second order chain transfer to monomer are avoided even at −10°C. The diagnosis of first and second order chain transfer has been accomplished, and the first order process seems to dominate. Forced termination can be effected either by thermally decomposing the propagating complexes or by nucleophiles. In either case the end groups will be tertiary chlorides. The living polymerization of isobutylene initiated by ester · BCl₃ complexes most likely proceeds by a two-component group transfer polymerization.     

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.     

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.     

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.     

Living cationic polymerization of olefins. How did the discovery come about?

The tortuous road to living carbocationic polymerizations is chronicled. The impetus for this project was my conviction that, just as living anionic polymerizations have started with a critical insight, a similar breakthrough will also be possible with cations. Upon retrospect, the facts show a three-step progression to the objective: Discovery of 1) controlled initiation, 2) reversible termination (quasiliving systems), and 3) controlled chain transfer. But what good is the discovery of a process without demonstrating its usefulness in terms of desirable products? Thus, a section concerns unique microarchitectures obtainable only by this technique: functional liquids, telechelics, thermoplastic elastomers, etc. The marketing of some of these products has already started, and the fundamental exploration of the promises of this technique is in progress worldwide. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 2285–2293, 1999     

Quasiliving Carbocationic Poiymerization. III. Quasiliving Polymerization of lsobutylene

The polymerization of isobutylene has been investigated by the use of the steady, slow, continuous monomer addition technique in the presence of a variety of initiating systems, i.e., “H₂O”/TiCl₄, “H₂O”/AlCl₃, C₆H₅C(CH₃)₂Cl/TiCl₄, p-ClCH₂ C₆(CH₃)₄* CH₂Cl/AlCl₃ at -50°C. Quasiliving polymerizations have been obtained with the “H₂O” and C₆H₅(CH₃)₂Cl/TiC1₄ systems in 60/40 v/v n-hexane/methylene chloride solvent mixtures with very slow monomer input. After a brief “flash” polymerization, the M _n of PIB increased linearly with the cumulative amount of monomer added (consumed); however, the number of polymer molecules formed also increased, indicating the presence of chain transfer to monomer. With the “H₂O”/TiCl₄ initiating system, M _n,max was 56,000 and M _w /M _n < 2.0. By the use of the C₆H₅C(CH₃)₂CL/TiCl₄ initiating system, quasiliving polymerization has been achieved and chain transfer could virtually be eliminated.     

Degenerative chain‐transfer process: Controlling all chain‐growth polymerizations and enabling novel monomer sequences

The use of dormant species has opened a new era in precision polymerization and has changed the concept of living polymerization. The dormant species can be exchanged into the active species via reversible termination or via reversible chain transfer. Professor Mitsuo Sawamoto has greatly contributed to the establishment of the concepts of living cationic and radical polymerizations based on the reversible activation of dormant species. This highlight, dedicated to Professor Sawamoto on his retirement from Kyoto University, provides an overview of reversible or degenerative chain‐transfer (DT) processes, which are effective in controlling all chain‐growth polymerizations, including radical, cationic, anionic, coordination, ring‐opening metathesis, and ring‐opening polymerizations. In addition, structures with novel sequences accessible only by a combination of different propagating species with a common DT agent are reviewed. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2019 , 57, 243–254     

Living carbocationic polymerization. IV. Living polymerization of isobutylene

Truly living polymerization of isobutylene (IB) has been achieved for the first time by the use of new initiating systems comprising organic acetate-BCl₃ complexes under conventional laboratory conditions in various solvents from ?10 to ?50°C. The overall rates of polymerization are very high, which necessitated the development of the incremental monomer addition (IMA) technique to demonstrate living systems. The living nature of the polymerizations was demonstrated by linear M _n versus grams polyisobutylene (PIB) formed plots starting at the origin and horizontal number of polymer molecules formed versus amount of polymer formed plots. DP _n obeys [IB]/[CH₃COOR^t · BCl₃]. Molecular weight distributions (MWD) are very narrow in homogeneous systems (M _w/M _n = 1.2–1.3) whereas somewhat broader values are obtained when the polymer precipitates out of solution (M _w/M _n = 1.4–3.0). The MWDs tend to narrow with increasing molecular weights, i.e., with the accumulation of precipitated polymer in the reactor. Traces of moisture do not affect the outcome of living polymerizations. In the presence of monomer both first and second order chain transfer to monomer are avoided even at ?10°C. The diagnosis of first and second order chain transfer has been accomplished, and the first order process seems to dominate. Forced termination can be effected either by thermally decomposing the propagating complexes or by nucleophiles. In either case the end groups will be tertiary chlorides. The living polymerization of isobutylene initiated by ester. BCl₃ complexes most likely proceeds by a two-component group transfer polymerization.     

High-conversion polymerization. I. Theory and application to methyl methacrylate

The concept of polymer entanglements has been applied in conjunction with classical free-radical kinetics to describe vinyl polymerizations carried to high conversion. A kinetic model has been developed on the assumption that two populations of radicals exist in a high-conversion polymerization system: those radicals whose chain lengths are long enough to become entangled with neighboring molecules and have, therefore, a restricted mobility; and those shorter radicals whose mobilities are not strongly affected by diffusional effects. It has also been assumed that the kinetic rate constant for the termination step between entangled radicals is inversely proportional to the mean entanglement density. The model contains only two parameters in addition to the kinetic rate constants required to describe low-conversion polymerizations. One of these parameters can be determined, at least in principle, from measurements of solution properties of the polymer-monomer mixtures. The model so developed has been tested against experimental data obtained from the literature on the bulk polymerization of methyl methacrylate. The agreement between predicted and experimental monomer conversions and molecular weight averages is found to be satisfactory.     

Transfer and Termination Reactions in “Living” Carbocationic Polymerizations

Abstract

The available data concerning the polymerization of three classes of monomers deemed to yield living polymers, vinyl ethers, styrenic monomers and isobutylene, are discussed from the point of view of transfer and termination reaction. In the case of vinylethers, linearity of [Mbar]_n with a yield up to 30,000 has been obtained, but when higher [Mbar]_n are planned, there is evidence for the occurrence of transfer reactions. In the case of isobutylene, indene, and p-Me-styrene, the linearity (up to [Mbar]_n ～ 10⁵) of [Mbar]_n with the amount of monomer polymerized which has been observed (but only at low temperature) is compatible with values of transfer constants to monomer measured in “conventional” systems. In these living systems, irreversible terminations are often not very important but may become significant toward the end of monomer consumption. The main termination process is reversible termination which may lead to narrow molecular weight distributions. The linearity of [Mbar]_n with yield is not conclusive evidence for the absence of transfer and termination and for the presence of particular active centers. The control of the polymerizations achieved up to now can be accounted for by the mechanisms of conventional cationic polymerizations, transfer reactions included.     

Living ring opening metathesis polymerization of mesogenic norbornene derivatives

A new class of SCLCPs was prepared by polymerizing mesogenic norbornene derivatives using Mo(CHCMe₂R)(N-2,6-C₆H₃-i-Pr₂)(O-t-Bu)₂ (R = CH₃ or Ph). Monomers based on norbornene ring systems were chosen because the rings are highly strained and therefore yield irreversible polymerizations. The Mo-alkylidene initiators were chosen because they initiate norbornene derivatives relatively fast and quantitatively, and provide stable chain ends which have low reactivity to both the internal double bonds of the polymer backbone and the functional groups present in the monomers. The apparent absence of termination and transfer reactions in ROMP results in polymerizations which appear to be living, and the fast initiation and irreversible chain growth leads to polymers with narrow molecular weight distributions in which the degree of polymerization is controlled by the ratio of monomer to initiator used. The resulting well-defined polymers were used to determine the most basic structure-property relationships of this new class of SCLCPs. The thermotropic behavior of both terminally attached and laterally attached SCLCPs based on polynorbornene backbones becomes independent of molecular weight at approximately 25 repeat units. In addition, polydispersity was found to have no effect on the breadth of nematic phase transitions in the terminally attached polymers, with the transition temperature determined simply by the number average degree of polymerization.