Of maleimides and vinyl ethers

Photoinitiator Free Polymerization of Maleimides and Vinyl Ethers... 

The photopolymerization of mixtures of maleimides and vinyl ethers is shown to be an efficient, rapid process in the absence of external photo initiators. Polymerization proceeds both in the presence and absence of oxygen. Films produced by the photopolymerization of maleimide/vinyl ether systems exhibit little absorbance at wavelengths greater than 300 nm. The thermal stability of these films are also excellent. 

The electron-transfer reaction as an initiation mechanism of the copolymerization of maleimide and vinyl ethers was investigated by time-resolved optical spectroscopy and FT EPR. Maleimide vinyl ether resins polymerize upon UV irradiation without the addition of a photoinitiator. The hrst step of initiation is an electron transfer from the ground-state vinyl ether molecule to the triplet maleimide. The maleimide radical-anion was detected by FT EPR in systems with hydroxybutyl vinyl ether and thiocyanate as electron donors whereas the maleimide hydrogen-adduct radical was found with 2-propanol as triplet quencher. 

According to the 1 1 stoichiometry between maleimide and vinyl ether, the polymers were AD (acceptor-donor) alternating copolymers. These new copolymers, being hemihydrolyzed, allowed the design and growth of robust... 

DPC has in recent years been applied to studies on a range of photopolymers including DF 2000 photopolymer [5], cinnamoylphenyl methacrylate-glycidyl methacrylate copolymer [6], multiethylene glycol dimethacrylate [7], Ebecryl 270 (aliphatic urethane diacrylate) 1,6-hexanediol diacrylate (Darocur 1173 2-hydroxy-2-methyl-phenylpropan-l-one) [8], epoxy acrylates [9], epoxy vinyl ether formulations [10], polyacrylates, maleimides, and vinyl ethers [11], hydroxylated poly(imides) [12], and polystyrene-poly(n-butylacrylate) copolymers [13]. 

A variety of alternating copolymers based on H-allyl- and N-(3-ethynylphenyl)maleimides, with substituted styrenes and vinyl ethers, have been prepared and their response to x-ray irradiation studied. Broadband and monochromatic x-ray exposures were conducted at the Stanford Synchrotron Radiation Laboratory. Sensitivities were observed to correlate with mass absorption coefficients of the copolymers and were found to be as high as 5-10 mJ/cm2. Preliminary fine line lithographic studies indicate 0.5 ion resolution capabilities. 

Employing an HIPE technique, poly(aryl ether sulfone) monoliths were obtained by the copolymerization of maleimide-terminated aryl ether sulfone macromonomer with styrene, DVB, or bis-vinyl ether in a solution in which petroleum ether (80% by volume) was dispersed [373]. The resulting product possessed an open-cell structure with porous cell walls and enlarged thermostability compared with poly(styrene-co-DVB) mono-hths. Unfortunately the utilization of the material as a possible medium for chromatographic separation has not been reported. 

There are definite attractions for monomers that can be used without the aid of initiators. Such monomers are maleimides. The monomers based on vinyl acrylate are also capable of self initiation. The vinyl ester itself, however, is too volatile for practical use and its initiation of polymerization is slower than obtained with the traditional photoinitiators. When the acrylate group is replaced by crotonate, cinnamate, fumarate, or maleate chromophores, these monomers copolymerize readily with thiol and vinyl ether monomers and initiate free-radical polymerization upon direct excitation in the absence of any added photoinitiator. 

Decker, C., Morel F., Jonsson, S., Clark, S. and Hoyle, C.E., Light-induced polymerization of photoinitiator-liee vinyl-ether-maleimide systems, Macrotnol. Chem. Phys., 1999, 200, 1005-1013. 

Clearcoats based on combinations of various maleimides (MI) and vinyl ethers (VE) or VE blends were tested with respect to their UV-photocuring in air [175]. It was found that MI/VE binders and reactive thinners could be cured without additional photoinitiators. Moreover, the equimolar MI/VE blends could be used as photoinitiators for acrylate systems, although their reactivity compared with commercial Norrish type I initiators proved to be poor. The observed limitations, e.g. changes in the properties of cured UPRs, limited reactivity in the air and irritation effects, could be ehminated using aliphatic MI types with further increased reactivity, higher functionality and/or molecular weight and lower irritation. 

A value for the polymerization enthalpy of 21.5 kcal/mole can be used to estimate percent conversion and rates for N-substituted maleimide/vinyl ether and maleic anhydride/vinyl ether copolymerizations. A value of 18.6 kcal/mole can be used for the enthalpy of polymerization of acrylate monomers to convert heat evolution data to percent conversion. Since the molar heats of polymerization for N-substituted maleimide vinyl ether copolymerization and acrylates vary by less than 20 percent, the exotherm data in the text are compared directly. 

Finally, we should indicate that we have not ruled out the possibility that there is a contribution to initiating photopolymerization in maleimide/vinyl ether systems from an exciplex type complex between an excited state maleimide and ground state vinyl ether. A biradical formed from such a complex might initiate free radical polymerization in lieu of cyclization to form a 2 + 2 adduct. However, we note that at present we have no evidence for such a reactive exciplex. 

Difunctional vinvl ether/difunctional N-maleimide. Up until this point, our results have centered on the reactivity of monofunctional maleimide divinyl ether mixtures. From Kloosterboer s26 work for acrylate polymerization, it is known that the rate of polymerization of a free-radical process is increased dramatically as the functionality of the acrylate is increased. In order to enhance the polymerization rates of maleimide divinyl ether systems, it was decided to synthesize difimctional maleimides for copolymerization with difunctional vinyl ethers. The results in Table V indicate that the photoinitiated TTDBM [bismaleimide made from maleic anhydride and 4,7,10-... 

These copolymers are prepared by the solution free radical polymerization of the electron-poor monomer (substituted maleimide) and the electron-rich monomer (substituted styrene or vinyl ether). Predominantly alternating copolymers result from such polymerizations (IQ). We will report on this unique copolymerization that permits the copolymerization of two double bonds in the presence of a third reactive double bond elsewhere. 

When using a suitable type I photoinitiator, e.g., acylphosphine oxides and non-amine-containing acetophenones in this system, the polymerization process is very efficient and the resulting product is a 1 1 alternating copolymer. This system is susceptible to oxygen inhibition, but to a much lesser extent than acrylate polymerization. An important advantage of this system is its low toxicity. Typical components include a variety of vinyl ethers and unsaturated esters, such as maleate, fumarate, citra-conate, imides (maleimides), or N-vinylformamides. The system is also... 

Among all remaining monomers, those containing (per)fluorinated side chains such as fluorinated acrylates, vinyl ethers or esters, maleimides and styrenic monomers are also very interesting and have been studied in (co)telo-merisation. Most of them have been previously reviewed [15]. However, they are not mentioned in this chapter. 

The opposite type of reaction has also been reported, viz. one in which the heterocyclic molecule reacts via an electron-rich double bond with electron-poor olefins, in particular with tetracyanoethylene. Tanny and Fowler43 found that 2-azabicyclo[3.1.0]hex-3-enes reacted with tetracyanoethylene via a (2 + 2)-cycloaddition of the enamine double bond to give 13. Other electron-deficient reactants, such as JV-phenyl-maleimide, reacted differently, yielding an 8-azabicyclo[3.2.1]oct-2-ene (16). This type of reaction possibly occurs via a concerted [ 2 +ff2 +n2]-cycloaddition.43 At room temperature tetracyanoethylene also readily formed (2 + 2)-cycloadducts with heterocycles that contained a vinyl ether group for instance, 3,4-dihydro-2ff-pyran, 2,3-dihydrofuran, and 2,2-dimethyi-l,3-dioxole afforded the adducts 17-19 in yields of... 

The subjects of reports on radiation-induced polymerization have included the following monomers ethylene,70 tetrafluoroethylene,71 acrylonitrile,72 acrylic acid,78 and methacrylonitrile,74 alkyl acrylates and methacrylates,76 styrene,78 other vinyl monomers,77 78 acrylamide,79 vinylcarbazole,80 maleimide,81 pentenes,82 aminoalkyl monomers,83 isobutyl vinyl ether,84 and buta-1,3-diene.85... 

Jonsson et al., studied a difunctional maleimide with two different electron donors, a vinyl ether and phenyl dioxolane. They observed that structural modifications increase the electron density in the vinylic C=C bond of the donor monomer and promote higher rates of co-polymerization with maleimides. They also compared a vinyl ether with exomethylenic dioxolane and showed increased "reactivity" for the dioxolane monomer. As a result, the mechanism of reaction of maleimide with a donor monomer, a vinyl ether was... 

High-molecular-weight A-substituted maleimides have been prepared and used as polymeric food antioxidants which can achieve the desired gastrointestinal nonabsorption. A-(3,5-Di-t-Bu -hydroxyphenyl)maleimide was prepared in two steps (a) formation of 2,6-di-t-Bu-4-aminophenol either from 2,6-di-(-Bu-phenol by nitration followed by reduction, or from 4-aminophenol by alkylation, (b) amida-tion of maleic anhydride with the 2,6-di-t-Bu-4-aminophenol followed by dehydration. The nonabsorbable poly(A-(3,5-di-t-Bu-4-hydroxyphenyl)maleimide)s were prepared from the monomeric maleimides by free radical homo- and copolymerization with comonomers of alkyl vinyl ethers (Scheme 5.5) [43]. 

In some cases, a monomer may function as a photoinitiator and become incorporated into a copolymer chain. This has been shown for styrene and other conjugated monomers when exposed to deep-UV light however, the initiation efficiency in these systems is substantially less than when a photoinitiator is present (p. 223 of Ref 33, and Ref 34). A better polymerizable initiator scheme is illustrated by the acceptor-donor chemistry of maleimide-donor systems (35-37). Maleimide acts as both photoinitiator and comonomer in the presence of hydrogen donors such as vinyl ethers or vinyl esters (38,39) an example of this copolymerization is shown in equation 5. It shows the molecular structure of the acceptor er -butylmaleimide (left), the donor 4-hydroxy-butyl vinyl ether (right), and their corresponding copolymer repeat unit. 

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