Vinyl ethers polymerization

A special case of the internal stabilization of a cationic chain end is the intramolecular solvation of the cationic centre. This can proceed with the assistance of suitable substituents at the polymeric backbone which possess donor ability (for instance methoxy groups 109)). This stabilization can lead to an increase in molecular weight and to a decrease in non-uniformity of the products. The two effects named above were obtained during the transition from vinyl ethers U0) to the cis-l,2-dimethoxy ethylene (DME)1U). An intramolecular stabilization is discussed for the case of vinyl ether polymerization by assuming a six-membered cyclic oxonium ion 2) as well as for the case of cationic polymerization of oxygen heterocycles112). Contrary to normal vinyl ethers, DME can form 5- and 7-membe red cyclic intermediates beside 6-membered ringsIl2). 

The second part of the theory, which is a logical consequence of the first, is that monomers that have more than one basic site, e.g., an aromatic ring or an oxygen atom, can form more than one type of complex with the carbenium ion this idea was first proposed by Plesch (1990) in the context of chemically initiated polymerizations. It helps to explain why aryl alkenes and alkyl vinyl ethers polymerize more slowly than isobutene and cyclopentadiene. The reason is that all the complexes formed by the alkyl alkenes are propagators, whereas for the aryl alkenes and vinyl ethers only a fraction of the population of complexes can propagate. 

Cationic photoinitiators are used in coatings, printing inks, adhesives, sealants, and photoresist applications. Most of the applications involve vinyl ether polymerizations or ringopening polymerizations of epoxy monomers (Sec. 7-2b). 

Enantiomer selection is also found in vinyl ether polymerization [226,227], The polymerization of cis- and trans- 1-methylpropyl propenyl ethers using (-)-menthoxyaluminum dichloride [227] and the copolymerization of rac- 1-methylpropyl vinyl ether with optically active monomers [226] are enantiomer selective. 

Recently Vandenberg (189) published additional information on vinyl ether polymerizations with Ziegler type catalysts. Details of the catalyst preparations, polymerization conditions and polymer characterization were presented together with an excellent discussion of mechanism. 

As represented above the initiation of vinyl ether polymerization by trityl salts involves reaction of the olefin with the aliphatic carbon atom. It is worth noting, however, that Magee, Winstein, and Heck (18) have shown previously that trityl cation reacts with the related olefin (CH3)2C=C(OCH3)2 exclusively in the 4-position of one of the aromatic rings ... 

Initiation with Triphenylmethyl Cation. When tetrahydrofuran (THF) is used to dissolve triphenylmethyl hexachlorantimonate at room temperature, there is almost immediate decomposition of the triphenylmethyl cation (6). On the other hand, solutions of the trityl salt in THF can be prepared and stored as deep yellow solutions if maintained at temperatures around — 80°C. At room temperature the initial decoloration of the catalyst is followed rapidly by polymerization of the monomer to poly(tetramethylene oxide), and the actual percentage conversion depends markedly on the temperature. This behavior is typical of systems exhibiting monomer-polymer equilibria (28), and Table III shows values for the equilibrium conversion of monomeric THF to polymeric THF obtained with a variety of catalysts. As for vinyl ether polymerization, it is most convenient to use the trityl hexachlorantimonate salt however, recourse to Table III shows clearly that above room temperature this anion yields less than the expected equilibrium conversion monomer... 

The existence of centres with non-ionic character has already been suspected in studies of polymerizations which are supposed to proceed on carbocat-ions the theory of pseudo-cationic polymerization was proposed [137] (see Chap. 3, Sect. 3.1). The transformation of an ion pair to a covalent compound will evidently be easier for acid centres with heteroatoms, i.e. in heterocycle or vinyl ether polymerizations. Propagation on covalent bonds has actually been observed, first in the studies of oxazoline polymerization [138] and later even with THF [139, 140] and with other monomers (see, for example, refs. 131, 141 and 142). 

The 1 1 adducts of various carboxylic acids and styrene, vinyl ethers, and isobutene have been isolated and used as initiators in the presence of Lewis acid activators. The polymerization rates correlate with the basicity of the leaving groups. However, isobutene polymerizes =103 times slower when initiated by pivalates and isobutyrates in the presence of BC13 than when initiated by acetates, even though they have similar pKa values [106]. Coordination of the covalent adducts with BC13 is evidently hindered when the alkyl substituents are bulkier. More detailed studies on vinyl ether polymerizations using a series of substituted benzoates demonstrate that the pKa values of the parent acid affects both the initiation rate and dynamics of ionization, and therefore the ability to prepare well-defined polymers [107]. 

However, ionization of the adducts should be more pronounced in more polar solvents and at lower temperatures if ionization is exothermic. Most vinyl ethers polymerize under these conditions [114]. Nevertheless, traces of iodine may catalyze polymerization, because Lewis acids act as coiniti ators. Moreover, even styrene oligomerizes in the presence of high concentrations of dry HC1 in polar solvents at -78° C [115]. 

As outlined in Section III. A.3. a, the strength of the Lewis acid with mixed chloride and alkoxy derivatives decreases as the number of chloride ligands are replaced with alkoxy groups. Titanium chloride with one alkoxy group polymerizes styrene and a-methylstyrene Lewis acid with two alkoxy groups is too weak to initiate polymerization of styrene, but will initiate polymerization of a-methylstyrene and vinyl ethers. The Lewis acidity of titanium chloride derivatives with three alkoxy groups are so low that only vinyl ether polymerizations reach reasonable conversions. 

The rates of initiation and propagation are comparable when the covalent initiator and dormant chain ends have similar structures. Therefore, 1-phenylethyl precursors are useful initiators for styrene polymerizations, but are poor initiators for a-methylstyrene and vinyl ether polymerizations. Similarly, cumyl derivatives are good initiators for isobutene and styrene, but are poor initiators for vinyl ethers their initiation of a -methylstyrene is apparently slow [165]. 1-Alkoxyethyl derivatives are successful initiators for vinyl ethers, styrenes, and presumably isobutene polymerizations [165,192]. /-Butyl derivatives initiate polymerization of isobutene slowly [105]. This is mirrored in model studies that show that /-butyl chloride undergoes solvolysis approximately 30 times slower than 2-chloro-2,4,4-trimethylpentane [193]. This may be due to insufficient B-strain in monomeric tertiary precursors [194]. In contrast, monomeric and dimeric or polymeric structures of secondary esters and halides apparently have similar reactivity. 

The energies of activation of vinyl ether polymerizations are much larger isobutyl and isopropyl vinyl ether E = 21 kJmol-1 ethyl vinyl ether Ea 54 kJ mol This indicates that carbenium ions of vinyl ethers are less reactive, probably due to an equilibrium with dormant oxonium ions formed by an intramolecular cyclization [Eq. (67)]. The overall activation energies should also increase to more positive values if formation of the active carbenium ions is endothermic. 

The equilibrium constants with nucleophiles such as tertiary amines are so large, that carbenium ions practically do not exist. Thus, tertiary amines and pyridine apparently react with carbenium ions irreversibly and therefore terminate carbocationic polymerizations. Somewhat weaker nucleophiles such as 2,6-dimethylpyridine (lutidine), sulfides, and tris(p-chlorophenyl)phosphine are good deactivators in vinyl ether polymerizations because they react reversibly with monomer, thus maintaining a low concentration of carbenium ions without causing elimination. However, the equilibrium constants in styrene and isobutene polymerizations with amines, sulfides, and phosphines are too large to generate a sufficient stationary concentration of carbenium ions to complete polymerization in a reasonable amount of time. 

Because the reactivities of ions and ion pairs are similar and only weakly affected by the structure of the counteranions, kp + or kp determined by either stopped-flow studies or y-radiated systems (cf., Section IV. 13) can be used in Eq. (75). The equilibrium constant of ionization can then be estimated from the apparent rate constant of propagation and the rate constant of propagation by carbenium ions [Eq. (77)]. For example, Kf 10-s mol-,L in styrene polymerizations initiated by R-Cl/SnCl4 [148]. Kt for vinyl ether polymerization catalyzed by Lewis acids can also be estimated by using the available rate constant of ionic propagation (kp- = 104 mol Lsec-1 at 0° C) [217], The kinetic data in Ref. 258 yields Kj == 10 3 mol - l L in IBVE polymerizations initiated by HI/I2 in toluene at 0° C and Kf 10-1 mol- -L initiated by HI/ZnI2/acetone can be calculated from Eq. (76). 

As discussed in Section II, hyperconjugation results in as much as 7-12% of the positive charge being located on each /3-hydrogen atom in propagating carbenium ions of isobutene, styrene, and vinyl ether polymerizations. Thus, even weak bases such as monomer, polymer, solvent, counteranion, or an impurity such as water may abstract these /3-protons to produce unsaturated end groups spontaneous loss of proton is highly unlikely [Eq. (88)]. 

Similar processes can occur in polymerizations of styrenes and vinyl ethers. Various colors are observed in vinyl ether polymerizations when protonated polyene sequences are generated by loss of several alcohol molecules [Eq. (122)] [324]. 

However, the danger of this approach is that the traps themselves may cause /3-proton elimination which lead to undetectably low concentration of active (or temporarily deactivated) chain ends. Malonate traps and H NMR analysis of the resulting terminal ester groups have been used successfully to monitor the growing chains in vinyl ether polymerizations [209], This technique measures the sum of dormant species and growing carbenium ions. Another method first used to monitor the active species in ring-opening polymerizations traps the active and dormant chain ends with phosphines [Eq. (135)]. 

Relative to the initiator/activator mechanism shown in Scheme 5, it is interesting to compare vinyl ether polymerizations initiated with the HI/I2 system and with iodine alone. The former system provides living polymers of controlled molecular weights and very narrow MWD [58], whereas the latter has been known for more than a century but fails to give such controlled polymerizations (cf., Sections IV.A) [49,55]. In the iodine-mediated polymerization, iodine serves as both the initiator and activator one molecule of iodine first slowly adds across the vinyl ether double bond to give an adduct. The a-carbon-iodine bond is activated by another molecule of iodine [34,95]. Thus, both systems would in fact form the identical growing chain end [ CH2CH(OR)+.I3 ], and the ob-... 

Vinyl ether polymerizations with the HCl/SnCl4 initiating system in methylene chloride are extremely rapid even at -40° C and cannot provide controlled polymers [71,72,105], The uncontrolled nature of the reaction is attributed to the high polarity of the solvent and to the high Lewis acidity of the tin chloride as the activator, both of which promote the ionization of the growing end. 

A similar effect was observed for trityl derivatives. Initiation of vinyl ether polymerization with trityl salts is very slow and often incomplete [257]. This precludes preparation of well-defined polymers with predetermined molecular weights and narrow MWDs. However, polymerization of vinyl ethers initiated by trityl salts in the presence of tetrahydrothiophene leads to controlled polymers [135]. The equilibrium constant for the formation of sulfonium ions is much smaller for trityl salts than for the growing species (K, < Kp), which increases the ratio of the apparent initiation to the propagation rate constants a thousand times (Scheme 14) ... 

In an extreme case the reactivity difference between the two monomers is positively utilized to obtain block copolymers. For example, Goethals recently polymerized IBVE with CF3S03H in the presence of thietane (a cyclic thioether) [86]. Because the thioether is much less reactive than vinyl ethers, it cannot polymerize and serves as a nucleophilic additive in the first-phase vinyl ether polymerization [64], but once IBVE has been completely polymerized, the cyclic monomer now polymerizes from the living end to form block polymers. 

The terminators for vinyl ether polymerizations include the sodium salt of ethyl malonate [sodiomalonic ester Na CH(C02C2H5)2] [131],... 

---