Photoinitiators allow the production of polymers and coatings with high control and variety in the process parameters. [1] A broad range of technical applications, unthinkable a few years ago, have now been firmly established. [2] These developments were also driven by the syntheses of new types of photoinitiators. [3] Starting from simple peroxides or a-hydroxy ketones, functionalized oxime esters, [4] organo phosphorus, [1] or, more recently, germanium compounds [5,6] have been developed as highly sophisticated photoinitiators with specific properties. [7,8] Among these, intensively studied mono-acylphosphane oxides (MAPOs, such as Ph 2 PO(COMes), Mes = mesityl; trade name Lucirin TPO) and especially bis-(acyl)phosphane oxides [BAPOs, such as PhPO(COMes) 2 ; trade name IRGACURE 819] stand out owing to their excellent efficiency and activity. [9][10][11][12][13][14][15][16] Irradiation with even a weak light source in the visible range leads to the formation of a phosphinoyl and an acyl radical through a Norrish type I cleavage reaction [Eq. (1)]:BAPOs offer several unique advantages: 1) Photolysis yields a total of up to four radicals, with the phosphinoyl radical about 1000 times more reactive than the acyl radicals. 2) Light in the visible region is absorbed by BAPOs (l = 360-440 nm), but the cleavage products are transparent. This property leads to a high curing depth and allows BAPOs to be applied as initiators, even for relatively thick clear coatings. 3) BAPOs show a relatively high thermal stability (> 100 8C) and can be easily stored. [17,18] Despite their widespread industrial use, very few BAPO derivatives have been synthesized to date. [19] The reported synthetic routes requiring a primary phosphane (RPH 2 ) or a metallated derivative (RPH 2Àx M x ) are incompatible with many functional groups and only aryl or alkyl substituents are bound to the phosphorus atom. Herein, we report a simple method for the synthesis of P-functionalized BAPO derivatives (Scheme 1), which offers interesting possibilities for surface modification.NaPH 2 is easily obtained from elemental phosphorus, sodium, and tert-butanol in the form of a sodium tert-butylate aggregate, which is a versatile starting material for functional phosphorus compounds. [20] Without prior isolation, the aggregate compound NaPH 2 (NaOtBu) x 1 was reacted with mesitoyl chloride to give sodium bis(mesitoyl)phosphide 2 in > 80 % yield as bright yellow crystals (Scheme 1). The structure of 2 (Figure 1) was obtained from a X-ray diffraction study with crystals obtained from a hot toluene/THF mixture (10:1) that contained residual dimethoxyethane (DME). [21] This compound Na 2 [P(COMes) 2 ] 2 ·2 THF·DME, contains two almost planar six-membered NaO 2 C 2 P rings formed by the P(COMes) 2 À anion, which is structurally similar to an acetylacetonate ion, chelating one sodium cation. These rings aggregate to form a strongly folded central Na 2 O 2 ring. Scheme 1. Synthesis of P-functionalized bis(acyl)phosphane oxides (BAPOs). FG = functional group, Mes = mes...
Got a light? An efficient method for the synthesis of phosphorus‐functionalized bis(acyl)phosphaneoxides (BAPOs) was developed, which allows the preparation of photoactive polymers or grafting of these photoinitiators to various surfaces. Irradiation in the presence of polymerizable monomers leads to coatings that can be deposited imagewise.
Styrene is the classical monomer obeying zero-one kinetics in radical emulsion polymerization. Accordingly, particles that are less than 100 nm in diameter contain either one or no growing radical(s). We describe a unique photoinitiated polymerization reaction accelerated by snowballing radical generation in a continuous flow reactor. Even in comparison to classical emulsion polymerization, these unprecedented snowballing reactions are rapid and high-yielding, with each particle simultaneously containing more than one growing radical. This is a consequence of photoinitiator incorporation into the nascent polymer backbone and repeated radical generation upon photo-irradiation.
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