Laser-initiated polymerization

Hoyle, C. E. and J. F. Kinstle (eds.), Radiation Curing of Polymeric Materials , ACS, Washington, DC, 1990. Developments in cationic polymerization, laser-initiated polymerization and high energy radiation curing are discussed. 

It is important to note that values are of the same order of magnitude whether conventional UV or laser radiation were used (Table I). This result is in contrast with the recent work of Sadhir et al. (7) on the photopolymerization of maleic anhydride and styrene, induced by the 363.8 nm emission of an argon ion laser. That work showed to be the laser initiation 1000 times more energy efficient than the UV-induced polymerization. This discrepancy may arise from two factors (i) the lower light-intensity used in the laser irradiation that should favor chain propagation and (ii) differences in the way of comparing the... 

Stereolithography is simple in concept and it provides great economies for the design lab as well as for the modeling process. It also provides previously unrecognized challenges for the polymer photochemist, for it is entirely a laser-initiated technology, and the polymerization reactions take place to depths below a finitely thin surface layer. 

The time to measure spectra of this quality under high-pressure conditions has been about I min. The absolute time scale of the experiment depends on the method of initiation. In thermally initiated (spontaneous) polymerizations reaction time can be several hours or even days. In contrast, in excimer laser-initiated free radical polymerizations application of a few laser pulses each of about 20 ns duration can induce changes between subsequent spectra as on this figure. 

The laser-initiated polymerization of styrene with maleic anhydride has been said to occur through either a singlet or triplet excimer (the authors surely mean exciplex ) of the anhydride as shown in Scheme 4. This study provides an interesting comparison between laser- and u.v.-initiated polymerization, and Table 2 shows clearly that the former is a very energy-efficient system. In the laser-initiated polymerization of a thiol-ene system, oxygen inhibition was not a significant problem and similar conclusions were reached on the energy efficiency of lasers. On a related note, the efficiency of the well-known benzo-phenone-triethylamine complex is apparently enhanced if carboxylic acids are added to the system. ... 

Figure 26. Rate of heat evolution during an argon-ion laser-initiated polymerization of 2-ethyl-2-(hydroxymethyl)-l,3-propanediol triacrylate (TMPTA) and l-methyl-2-pyrrolidinone (MP) mixture (9 1) photosensitized by RBAX (10 - m) in the presence of 1) 4-(dimethylamino)benzophenone (0.1 m) and 2) iV,A,3,5-tetramethylaniline (0.1 m).

Figure 35. Rate of polymerization as a function of free energy change (-AGet) for photoinitiated CQ-NPG systems. Polymerizing monomer mixture 2-ethyl-2-(hydroxymethyl)-1,3-propanediol triacrylate (TMPTA) and ethylated bisphenol-A dimethacrylate (bis-EMA) (1 1). Polymerization was initiated by an argon-ion laser beam. Data from Ref. [175].

The pulsed laser-initiated polymerization of multifunctional acrylates has been investigated using a photocalorimeter. In the case where a limited munber of pulses are delivered to the sample, the extent of conversion decreases as the laser repetition rate increases. However, for a large number of pulses, higher repetition rates do not lead to decreases in the overall degree of conversion. 

These results point to two processes, premature radical chain termination and film shrinkage, which compete in determining the ultimate polymerization conversion efficient of multifunctional acrylates. It is obvious that critical attention must be paid to the pulse repetition rate, photoinitiator concentration, and acrylate functionality in developing any photopolymerizable system for laser-initiated polymerization. Future publications on laser-initiated polymerization of multifunctional acrylates will deal with monomer extraction of partially polymerized films, mechanical properties of laser polymerized films, and the Idnetics of single-pulsed systems. 

Propagation rate coefficients for styrene, vinyl acetate, and various acrylates and methacrylates were determined by the so-called PLP-SEC technique, which combines pulsed-laser-initiated polymerization (PLP) with subsequent polymer analysis via size-exclusion chromatography (SEC). The PLP-SEC experiment measures the product of propagation rate coefficient, kp, and monomer concentration, Cm ... 

To illustrate more clearly the nature of free radical polymerization, it is instructive to examine the values of the individual rate constants for the propagation and termination steps. A number of these rate constants have been deduced, generally using nonstationary-state measurements such as rotating sector techniques and emulsion polymerization [26]. Recently, the lUPAC Working Party on Modeling of kinetics and processes of polymerization has recommended the analysis of molecular weight distributions of polymers produced in pulsed-laser-initiated polymerization (PLP) to determine values of... 

Data taken from Refs. [30-32] data in parentheses determined by pulsed-laser-initiated polymerization. Ref [29], Ref [33], Ref [34], Ref [35]. 

Keywords aqueous-phase polymerization free-radical polymerization methacrylic acid PLP-SEC propagation rate coefficients pulsed-laser initiation water-soluble monomers... 

Free-radical polymerization in aqueous solution is of significant industrial importance. To model polymerization processes and product properties, reliable rate coefficients for the individual reaction steps are required. The propagation rate coefficient, kp, may be precisely obtained by the PLP-SEC method, which combines pulsed-laser initiated polymerization with subsequent... 

Summaiy In this short review, selected experimental approaches for probing the mechanism and kinetics of RAFT polymerization are highlighted. Methods for studying RAFT polymerization via varying reaction conditions, such as pressure, temperature, and solution properties, are reviewed. A technique for the measurement of the RAFT specific addition and fragmentation reaction rates via combination of pulsed-laser-initiated RAFT polymerization and j,s-time-resolved electron spin resonance (ESR) spectroscopy is detailed. Mechanistic investigations using mass spectrometry are exemplified on dithiobenzoic-acid-mediated methyl methacrylate polymerization. 

The method of polymerization, being initiated by pulse laser radiation [13] is used for determination of rate constant for chain growth kp lately. It is anticipated that such an estimation method is more correct, than the traditionally used revolving sector method [12], We made om choice on temperature dependence of the rate constant for chain growth that was derived on the ground of method of polymerization, being initiated by pulse laser radiation ... 

The attractiveness and major advantage of using pulsed lasers in kinetic studies is a result of the short duration of the laser pulse (typically 10 to 20 ns). Opposed to more conventional methods such as rotating sector and spatially intermittent polymerization, the initiation period can now be considered to be instantaneous on the time scale of propagation. As a consequence, all radicals formed in one laser pulse are of approximately the same chain length at any moment in time, which greatly simplifies the kinetic scheme. It is this quality of laser-induced experiments that leads to the reliability of the PLP method to determine kp values (in conjunction with an accurate MWD determination). 

Hoyle CE, Hensel RD, Grubb MD. Laser-initiated polymerization of a thiolene systems. J Polym Photochem. 1984 4 69-70. 

---