Polymerization quantum yield

For an accurate evaluation of the efficiency of UV photons in initiating the polymerization of acrylate photoresists, it is necessary to determine the polymerization quantum yield, , that corresponds to the number of acrylate functions which have polymerized per photon absorbed. can be expressed as the ratio of the rate of polymerization to the absorbed light-intensity and calculated from the following expression, after introducing Equations 1 and 2 ... 

Figure 6. Dependence of the polymerization quantum yield ( ) on the light-intensity (Iq) in the laser-induced polymerization of epoxy-acrylate photoresists (— expected variation of on I0 for a half-order kinetic law).

Fig. 3. a) Polymerization quantum yield of the thioxanthone — diphenyliodonium-chloride — methyl methacrylate system vs. diphenyliodoniumchloride concentration (X = 365 nm, I bs = 5xl0 7 ein-stein 1"1 s in acetonitrile/water 90 vol%, [MMA] = 1 moll-1) b) Plot corresponding to Eq. (12b)... 

Table 7 give some data of the polymerization quantum yields (p) as a measure for the efficiency of various donor-diphenyliodonium salt combinations in the cationic polymerization of PGE. 

Kinetics and Mechanism. Practical applications of onium salt photo-initiated polymerization generally require use of spectral sensitizers. Under conditions of electron transfer mediated sensitization, the initiating cation, R+, thus derives from the sensitizer, not the onium salt (see above), that is, a Lewis, rather than a Bronsted, acid functions as the polymerization catalyst. As a result overall polymerization quantum yields are very dependent on sensitizer [93,166]. 

In systems undergoing cross-linking polymerization, it is usually not possible to evaluate the kinetic chain length because of the formation of a polymer of infinite molecular weight One of the distinct advantages of photoinitiation is to make this quantity accessible through quantum yield measurements. The kinetic data reported in Figures 6 and 7 have been used to evaluate the polymerization quantum yield. Op, i.e. the number of epoxy or vinyl ether functions polymerized per photon absorbed by the irradiated sample. Op was calculated from the equation ... 

A similar study has been performed on EPI blends in which the vinyl ether was replaced by an acrylate monomer (HDDA) to produce, by different mechanisms, two interpenetrating polymer networks. With the onium salt as sole photoinitiator, the cationic polymerization of the EPI epoxy groups occurred as fast in the formulation containing 20% of HDDA by weight as in the EPI/DVE-3 blend, to reach nearly 100% conversion within 0.6 s (Fig. 11). The polymerization quantum yield was found to be similar to that measured in the EPI/vinyl ether blend Op 650 mol E. By contrast, the acrylate double bonds were found to polymerize at a much slower pace, most probably because of the low reactivity of the free radicals generated by the cationic-type photoinitiator. 

From the slope of the RT-FTIR kinetic curves it is easy to evaluate the rate of polymerization (Rp) which can be calculated at any moment of the reaction. It is therefore possible to plot the Rp values as a function of the conversion rate. The overall polymerization quantum yield, ( )p, can be calculated from the ratio of polymerization, Rp, over the absorbed light intensity. 

The quantum yield of the initiation process (<, ) is quite low 8 x 10, indicating the great stability of the chelate ring toward photolysis. However, the quantum yield of photodecomposition 4>d) under similar condition is 2 X 10, which is higher than It is clear, therefore, that not every molecule of Mn(acac)3 that is decomposed initiates polymerization apparently, ex-... 

The ion-pair complex formed by the interaction of hydroxobis(8-quinolyloxo) vanadium (V) [VOQ2OH] and /i-butyl amine is also effective in photoinitiation of polymerization of MMA in bulk and in solution [40]. The quantum yield of initiation and polymerization determined are equal to 0.166 and 35.0, respectively. Hydroxyl radical ( OH) is reported to be the initiating radical and the following photoreaction is suggested ... 

The quantum yield of polymerization is 6.72 and for photoinitiation < / = 2.85 x 10 . The polystyrene produced with this initiator shows photosensitivity when irradiated with UV light (A = 280 nm). This polymer, which carries two photosensitive end groups of - SC(S) N(CH3)2, behaves as a telechelic polymer and it is useful for production of ABA block copolymer. 

Some of the derivatives 96 show high photoluminescence quantum yields of up to 73%, and can be applied as blue and green polymeric emitters in organic materials-based LEDs [122]. 

The quantum yields, the number of olefinic double bonds consumed to form cyclobutane per absorbed quantum, of the oligomerization and polymerization of 2,5-DSP, P2VB and 3 OMe have been measured by using monochromatic light. The quantum yield ( ) is defined by the equation... 

Table 3 Quantum yields () of oligomerization and subsequent polymerization of 2,5-DSP, P2VB, and 3 OMe. ...

DSP crystal, a detailed picture of the lattice motion and related displacements was constructed and related to the topochemical postulate and the mechanism of phonon assistance. Holm and Zienty (1972) have measured the quantum yield for the overall polymerization process of a,a -bis(4-acetoxy-3-methoxybenzylidene)-p-benzenediacetonitrile (AMBBA) crystals in slurries and reported it to be 0.7 on the basis of the disappearance of two double bonds ( = 1.4 if assigned on the basis of the number of double bonds consumed). 

The processes by which unsaturated monomers are converted to polymers of high molecular weight exhibit the characteristics of typical chain reactions. They are readily susceptible to catalysis, photoactivation, and inhibition. The quantum yield in a photoactivated polymerization in the liquid phase may be of the order of 10 or more, expressed as the number of monomer molecules polymerized per quantum absorbed. The efficiency of certain inhibitors is of a similar magnitude, thousands of monomer molecules being prevented from polymerizing by a single molecule of the inhibitor. ... 

If the reactive species in the chemical activation step initiates a radical chain with a chain length CL, then the overall quantum yield based on the ultimate product is X CL, and can be greater than 1. Photons are rather expensive reagents, and are only used when the product is of substantial value or when the overall quantum yield is large. Examples are the use of photoinitiators for the curing of coatings (a radical-polymerization process (Section 7.3.1)), and the transformation of complex molecules as medications. 

At the beginning of the polymerization reaction, the viscosity of the medium is low, and efficient rotation (about the ethylenic double bond and the single bond linking ethylene to the phenyl ring) accounts for the low fluorescence quantum yield. After a lag period, when approaching the polymer glassy state, the fluorescence intensity increases rapidly as a result of the sharp increase in... 

In the literature, fluorescence spectroscopy in OFD has been limited to the use of ultraviolet (UV) or visible dyes as molecular probes.(1) The most common fluorescent dye used in OFD is fluorescein and its derivatives/21 23) Fluorescein possesses a good fluorescence quantum yield and is commercially available with an isothiocyanate functionality for linking to the polymeric support/24-26 Additionally, selective laser excitation can be performed because the absorbance maximum of fluorescein coincides with the 499-nm laser line emitted from an argon laser. Unfortunately, argon lasers are costly and bulky, thus limiting the practicality of their use. Similar difficulties exist with other popular commercial dyes. 

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