Ketene acetal monomers: Cationic photopolymerization

A study of the photoinduced cationic polymerization of a novel series of cyclic and acyclic ketene acetal monomers was carried out. It was observed that the cyclic monomers underwent facile cationic polymerization to give high molecular weight polymers while the acyclic ketene acetals did not. Thermally induced polymerization was also observed on standing catalyzed by glass surfaces. Photoinduced cationic polymerizations employing both diaryliodonium and triarylsulfonium salt photoinitiators were studied. It was observed that the position and type of substitutents in the cyclic ketene acetals was the major factor in determining the proportion of vinyl and ring-opening polymerizations that take place. © 1996 John Wiley & Sons, Inc.     

Recent Progress of Photo- and Radiation-Induced Ionic Polymerizations

Photoinduced ionic polymerizations of the monomers α-methylstyrene, cyclohexeneoxide, nitroethylene, and acrylonitrile were carried out in the presence of electron acceptor or donor molecules. These polymerizations are proved to be initiated by ions formed through the dissociation of the photoexcited electron donor-acceptor complex and to proceed by ionic mechanism.

The molecular weight distribution of the polymer and the light intensity dependency on the rate of polymerization indicate that free ionic and ion-pair propagations coexist in the cationic polymerization of α-methylstyrene.

Anionic polymerizations were observed for the nitroethylenetetrahydrofuran and acrylonitrile-dimethylformamide systems.

Radiation-induced cationic polymerizations of styrene and α-methylstyrene were found to proceed by free cationic propagation. The effect of added electron acceptors in these polymerizations was investigated.     

Modulating Polymer Dispersity with Light: Cationic Polymerization of Vinyl Ethers Using Photochromic Initiators

Deterministic methods for tuning polymer dispersity are rare, especially for nonradical polymerizations. Reported here is the first example of photomodulating dispersity in controlled cationic polymerizations of vinyl ethers using carboxy‐functionalized dithienylethene initiators. Reversible photoisomerization of these initiators induces changes in their acidities by up to an order of magnitude. Using the more acidic, ring‐closed isomers as initiators results in polymers with lower dispersities. The degree of light‐induced pK_a change in the initiators correlates with the degree of dispersity change in polymers derived from the isomeric initiators. The polymerizations are controlled, and dynamic photoswitching of dispersity during the polymerization reaction was demonstrated. This work provides a framework for photomodulating dispersity in other controlled polymerizations and developing one‐pot block copolymerization reactions in which the dispersities of component blocks can be controlled using light.     

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.     

Photoinduced cationic ring‐opening frontal polymerizations of oxetanes and oxiranes

The photoinitiated cationic ring‐opening polymerizations of certain epoxides and 3,3‐disubstituted oxetanes display the characteristics of frontal polymerizations. When irradiated with UV light, these monomers display a marked induction period, during which little conversion of the monomer to the polymer takes place. The local application of heat to an irradiated monomer sample results in polymerization that occurs as a front propagating rapidly throughout the entire reaction mass. For the characterization of these frontal polymerizations, the use of a new monitoring technique, employing optical pyrometry, has been instituted. This method provides a simple, rapid means of following these fast polymerizations and quantitatively determining their frontal velocities. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 1630–1646, 2004     

Propenyl ethers. III. Study of the photoinitiated cationic polymerization of propenyl ether monomers

The effects of photoinitiator structure and variations in the experimental parameters on the rate and extent of the photoinitiated cationic polymerization of propenyl ether monomers were studied. It was found that the photoinitiators can be divided into two classes: those which exhibit an induction period and those which do not. It was demonstrated that in those propenyl ether polymerizations using iodonium salts and certain sulfonium salts which do not have an induction period, a free radical chain-induced decomposition of the onium salt takes place. The reactivity of a particular onium salt photoinitiator was shown to be related to its reduction potential. It was also shown that the structure of the monomer plays a major role in the free radical induced decomposition reaction. © 1993 John Wiley & Sons, Inc.     

Design of functional polymers based on organic synthesis - novel polymerizations of allene and propargyl derivatives

This paper describes our recent work dealing with novel polymerization methods of allene and propargyl derivatives. By the radical, cationic, and living coordination polymerizations, polymers bearing exomethylene moieties in the side chain could be obtained. The polymers obtained could undergo the cationic crosslinking reaction. As step-growth polymerizations, spontaneous copolymerizations, polyadditions with dithiols, and coupling polymerizations using palladium catalyst are also described.     

Redox-initiated cationic polymerization: The diaryliodonium salt/benzoin redox couple

A new redox couple based on the copper-catalyzed reduction of diaryliodonium salts with benzoins has been used to initiate cationic polymerizations of cyclic ethers and esters. A proposed mechanism for initiation by this redox couple is based on its stoichiometry and on the nature of the products. It was concluded that initiation of polymerization occurs both by direct arylation of the monomer and by protonation by strong Brønsted acids. The polymerization of several typical cationically polymerizable monomers using this new redox initiator were studied.     

Polymerizations with contribution of covalent and ionic species

The similarities and dissimilarities between the ring-opening and vinyl cationic polymerizations are discussed for systems with simultaneous presence of covalent and ionic species. The common denominator in comparing kinetics of polymerization of these systems is the comparison of the total times of polymerization with the time elapsing when a given macromolecule is active (i.e. ionic) and inactive (e.g. covalent in cationic vinyl and ring-opening polymerization). In the cationic ring-opening polymerization the reactivation of covalent species proceeds mostly by intramolecular nucleophilic dipole-dipole reaction (conformationally assisted) involving the chain unit adjacent to growing ester chain end. This reaction is fast (e.g. in THF polymerization) and is responsible for covalent ion-pair interconversion. In the cationic vinyl polymerization this path does not exist but covalent species (esters) can be faster ionized by using of the nucleophile, increasing thus the rate of exchange between esters and ions. This process provides one of the possible ways for converting the non-living polymerization into the living-dormant process. The cationic polymerizations are then compared with covalent (pseudoanionic) polymerization of cyclic esters, initiated with covalent metal alkoxides like R₂AlOR' or Al(OR)₃.     

Abilities of 1‐(9‐anthrylmethyloxy)‐2‐pyridone and related compounds to initiate radical and cationic photopolymerizations

This study explored the abilities of 1‐(9‐anthrylmethyloxy)‐2‐pyridone and related compounds, which absorb long‐wavelength light (>350 nm), to photochemically initiate radical and cationic polymerizations. It was found that the irradiation of the title compounds initiates the radical polymerization of styrene whereas the cationic polymerization of oxetane proceeds in the presence of these photoinitiators to a negligible extent. The behavior of 9‐anthrylmethyloxyl and amidyl radicals in the photopolymerization process of styrene was discussed based on ¹H NMR, UV, and fluorescence spectral data. In addition, the photoinitiation ability of the anthrylmethyloxyl end group was also investigated by using its model compound. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 2859–2865, 2004     

Synthesis of hyperbranched carbohydrate polymers by ring-opening multibranching polymerization of anhydro sugar

The synthesis of novel hyperbranched carbohydrate polymers, prepared by the ring-opening multibranching polymerizations of anhydro and dianhydro sugars, is described. The hyperbranched carbohydrate polymers were formed by the cationic polymerization of 1,6-anhydro-beta-D-hexopyranose, 1,4-anhydrotetritol, 2,3-anhydrotetritol, and 1,2:5,6-dianhydro-D-mannitol. These polymerizations proceeded without gelation to produce water-soluble hyperbranched carbohydrate polymers with controlled molecular weights and narrow polydispersities. The values for the degree of branching of the polymers were in the range of 0.28-0.50. The polymerization method, which proceeds through a ring-opening reaction by a proton-transfer reaction mechanism, is a facile method leading to a spherical carbohydrate polymer with a high degree of branching.     

Orthogonal Cationic and Radical RAFT Polymerizations to Prepare Bottlebrush Polymers

An orthogonal combination of cationic and radical RAFT polymerizations is used to synthesize bottlebrush polymers using two distinct RAFT agents. Selective consumption of the first RAFT agent is used to control the cationic RAFT polymerization of a vinyl ether monomer bearing a secondary dormant RAFT agent, which subsequently allows side‐chain polymers to be grafted from the pendant RAFT agent by a radical‐mediated RAFT polymerization of a different monomer, thus completing the synthesis of bottlebrush polymers. The high efficiency and selectivity of the cationic and radical RAFT polymerizations allow both polymerizations to be conducted in one‐pot tandem without intermediate purification.     

Structure and reactivity in cationic polymerization of butadiene derivatives. II. 1-phenylbutadiene

The reactivity of 1-phenylbutadiene (1-PBD) in cationic polymerization and the monomer structure were investigated. 1-PBD polymerized at ?78°C in several solvents initiated by cationic catalysts such as stannic chloride and tungsten hexachloride. The polymerizations proceeded predominantly via 3,4-type propagation mode, and gave low molecular weight polymers. More than one double bond of 1-PBD was consumed during the polymerizations, probably due to transfer and cyclization reactions. 1-PBD was several times as reactive as styrene and trans-1,3-pentadiene in copolymerizations. The Hammett plots of reactivities of ring-substituted 1-PBD in cationic polymerization gave the p-value of -1.20, which is 0.6 times that of styrene. The ¹H and ¹³C NMR chemical shifts of ring-substituted 1-PBD were measured and discussed in relation to the reaction mechanism.     

Photoinduced ionic polymerization. V. Coexistence of cationic and anionic polymerizations

Photopolymerization of a mixture of cyclohexene oxide and nitroethylene was carried out with the purpose of carrying out cationic and anionic polymerizations simultaneously in the same system. The excitation of the charge transfer band by light of wavelength longer than 390 nm gives rise to the polymerization of both monomers. No polymer was obtained in the dark. Additives affect the composition of the polymer, the rates of polymerization, and the molecular weight distributions. These data show that cationic polymerization of cyclohexene oxide and anionic polymerization of nitroethylene occurs simultaneously in this system.     

可控自由基聚合与其它活性聚合方法结合制备嵌段共聚物

可控自由基聚合与其它聚合方法结合,可以制备多种类型的嵌段共聚物,因此得到了广泛关注。本文着重介绍可控自由基聚合与离子开环聚合、阴离子聚合、烯类单体的阳离子聚合及其它活性聚合方法结合制备嵌段共聚物的研究现状和进展。     

Redox initiated cationic polymerization

A novel catalytic method for carrying out the cationic polymerizations has been developed based on a redox initiator system in which the reducing component is delivered to the reaction mixture in the vapor state. The redox couple consists of a diaryliodonium salt that is dissolved in the monomer and a noble metal catalyst is added. The silane reducing agent is introduced to the reaction mixture in the vapor state using air as the carrier gas. Reduction of the diaryliodonium salt by the silane results in the liberation of a Brønsted superacid that initiates cationic polymerizations. A study of the effects of variations in the structures of the diaryliodonium salt, the silane, and the type of noble metal catalyst was carried out. In principle, the initiator system is applicable to all types of cationically polymerizable monomers and oligomers including: the ring‐opening polymerizations of such heterocyclic monomers as epoxides and oxetanes and, in addition, the polymerization of vinyl ether monomers such as vinyl ethers. The use of this initiator system for carrying out commercially attractive cross‐linking polymerizations for coatings, composites, and encapsulations is discussed. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 1825–1835, 2009     

Note Copolymerization of Styrene with Maleic Anhydride Initiated by L-Ascorbic Acid

It is well known that maleic anhydride (MAH) behaves as an electron acceptor and forms charge-transfer complexes with donor monomers such as styrene (ST) [1,2], divinyl ether [3], and some of other olefms [4–61. The charge-transfer polymerization of ST with MAH has been extensively studied [1,7–11]. On the other hand, L-ascorbic acid (L-Asc) in combination with a suitable oxidants proved to be an efficient redox initiator for various vinyl polymerizations. Misra et al. [12] showed that the reduction of peroxides by ascorbic acid follows a chain mechanism with ascorbate and other free radicals as intermediates. Thus, we can expect that such an electron donor might initiate the copolymerization of MAH with ST.     

Living Polymers in Ionic and Radical Polymerizations: Recent Developments

The discovery of living polymers, that is, assemblies of polymer molecules formed by anionic polymerization which may grow without chain-breaking reaction and may react subsequently with other monomers and various reagents through their end-groups, has led to great progress in the knowledge of the mechanism of anionic polymerization and to the synthesis of a large variety of well-defined block copolymers, graft co-polymers, and polymers with functionalized end-groups. Since only a limited number of the current monomers are polymerizable by an anionic mechanism, many attempts have been made to obtain similar results by polymerizing other monomers by cationic, radical, and Ziegler polymerization. Systems making it possible to work at temperatures higher than those used for many anionic and most cationic polymerizations would be particularly interesting.     

Effect of alkyl substituents on initiator activity in cationic polymerization of styrene with p-methoxybenzyldialkylsulfonium salts as initiators

The effect of alkyl substituents on cationic polymerization of styrene with p-methoxybenzyldialkysulfonium salts was studied. p-Methoxybenzyl tetramethylene ( 1 ), dimethyl ( 2 ), diethyl ( 3 ), dibuty ( 4 ), and diisopropylsulfonium salts ( 5 ) were synthesized by the reaction of p-methoxybenzyl bromide with the corresponding sulfides, followed by exchange of the counter anion Br^? with SbF^?₆. These sulfonium salts served as potent cationic thermal initiators of which activity was estimated by the bulk and solution polymerizations of styrene. The bulk polymerizations with 1–4 (0.1 mol %) for 30 min gradually proceeded at 30–50°C, but the exothermic polymerization occurred vigorously at 40–60°C. The Polymerization with 5 took place exothermically even at room temperature. Temperature-conversion curves of the polymerizations for 30 and 5 min revealed that the activity of the sulfonium salts was in the following order: 5 > 4 > 3 > 2 ≈ 1 . This order was explained by the order of the bulkiness of the alkyl substituents on the sulfur atom. Number-average molecular weight (M?_n) of polystyrene obtained by the polymerization undergoing no exothermic process was in a range of 6600–16000, which depended on the structure of the alkyl substituents: the more bulky the substituent was, the higher M?_n was.     

Inverse-microsuspension polymerization: Mechanism,kinetics and modelling

A mechanism has been developed for the inverse- microsuspension polymerization of acrylic water soluble monomers. This free-radical reaction scheme includes elementary reactions for polymerization in the aqueous and organic phases, nucleation in the monomer droplets and heterophase oligoradical precipitation. The latter being the dominant initiation process. A unimolecular termination reaction with interfacial species has also been elucidated and has been found to compete with and often dominate over the conventional biomolecular reaction. A kinetic model is developed which includes the influence of ionogenic monomers and polyelectrolytes and is able to predict the rate, molecular weight and composition data well for polymerizations of acrylamide and copolymerizations with quaternary ammonium cationic monomers. A categorization and systematic nomenclature for heterophase water-in-oil and oil-in-water polymerizations is also developed based on physical, chemical and colloidal criteria.