Functional Organo-Nano Particles by RAFT Copolymerisation

A significant impact of this work on the use of polymers is expected because the developed organo-nano particles (ONP) mixed into standard polymers will make them unique and traceable. The doping of polymers with non migrating ONP was demonstrated and applications for the recycling of plastics were discussed. Thus, perylene derivatives were linked to polymerisable vinyl groups and copolymerized under RAFT conditions (Reversible Addition Fragmentation chain Transfer) with styrene and methylmethacrylate, respectively, to obtain fluorescent ONP with sizes of 40 nm or even less and narrow distributions of molecular weight in most cases with polydispersities PD of 1.1 and lower.


Introduction
Nano technology is very promising because of many novel possibilities and is now concentrated to inorganic materials such as titanium dioxide, zinc oxide, alumina and silica.However, the persistency of the majority of such materials is the subject of controversy discussion concerning hazards to human health and environment [1] [2] [3] [4], respectively.A sufficient broad experimental basis for a realistic estimation is still lacking.On the other hand, organic materials are generally long-term degradable where organic nano particles (ONP) would be an attractive alternative for applications in mass products [5].Moreover, a comparably low lifetime in the environment can be expected because of their large sur-face for chemical and biological attack and degradation; thus, ONP can be estimated as "green materials".The possibility of the application of ONP found only little attention although the introduction of functionalities such as fluorescent chromophores in organic materials is well-established by the methods of preparative organic chemistry.We prepared fluorescent organo-nano particles in preceding work by polymer analogous reaction [6] with reactive chromophores.
A free radical-induced copolymerisation of polymerisable chromophores with various monomers was successful.Nano dimensions were obtained by the application of high concentrations of initiators in rapid reactions [7] where high stationary concentrations of growing chains cause efficient terminations of radicals resulting in short chains and nano dimensions of the polymers.Basically, fluorescent organo-nano particles (ONP) could be prepared by this method, however, with comparably broad distribution of molecular weight and size, respectively.Moreover, the controlling of the reaction was difficult and scaling-up problematic because of the Trommsdorff [8] [9] [10] effect.An easier processing radical reaction leading to a more uniform distribution of size would bring about an appreciable progress.
The reversible addition of radicals to the trithiocarbonate structure of 1 causes a low stationary concentration of free radicals both with uniform conditions for   the chain propagation and a suppression of the bimolecular termination by combination and disproportionation reactions [19] [20] [21].The consequence is a narrow distribution of the molecular weight and a targeted nearly uniform size of the thus formed polymeric particles, respectively.As an alternative, we polymerised MMA (methyl methacrylate) under RAFT conditions where we applied the reagent [14] 2 because of more similarity with MMA and the polymeric PMMA than 1.The monomeric styrene was copolymerized with vinylphenyl groups attached to chromophores for the introduction of fluorescence into the ONP.Alternatively, methacylic esters of chromophores were copolymerised with MMA.

Synthesis of Fluorescent Labels
Perylenes [15] [16] were applied as basic structures of fluorescent chromophores because of their chemical and photochemical stability and high fluorescence quantum yields.Their inherently low solubility was overcome by the attachment of long-chain secondary alkyl groups (swallow-tail substituents) such as the 1-hexylheptyl group.
Vinylphenyl-modified perylenes were targeted for co-polymerisation with styrene.
Thus, we condensed the corresponding anhydride function with 4-aminostyrene to obtain 3 [17] (see Scheme 1).For the more bathochromic spectral region in the UV/Vis the aromatic core of 3 was laterally extended by a phenylimidazolo group [11].Thus, the corresponding N,N'-bis-1-hexylheptylbiscarboximide was partially hydrolysed under rough alkaline conditions to end-up in a difficult separable mixture of regio isomeric anhydrides-carboximides that was directly condensed with 4-aminostyrene in the same manner as described for 3 to obtain Scheme 1. Synthesis of fluorescent labels with vinyl groups.
the mixture 4a/b.This mixture could not be separated on a preparative scale; however, the UV/Vis spectral properties of 4a and 4b are so similar that a separation is not necessary for practical applications (TLC separation, nearly uniform UV/Vis spectra).For covering the even more bathochromic spectral region terrylenebiscarboximides were applied meaning a naphthalene-core-prolonged perylenebiscarboximide.Synthesis started similarly to 4a/b with a terrylene biscarboximide [18] with two even more effectively solubilising 1-nonyldecyl substituents, hydrolysing to give the corresponding anhydridecarboximide and its condensation with 4-aminostyrene to obtain 5.For the more hypsochromic spectral region perylenebiscarboximide with two solubilising 1-hexylheptyl substituents was core-modified by means of a Diels-Alder-Clar reaction with maleic anhydride leading in a five-membered ring anhydride [25] that was condensed with 4-aminostyrene to obtain the benzoperylene-derived label 6.Furthermore, benzoperyleneanhydride-carboximide [22] was allowed to condense with 4-aminostyrene in the same manner to obtain the benzoperylenedicarboximide 7.
We prepared methacrylic esters of chromophores for more similarity in the co-polymerisation with MMA.Thus, the well-accessible perylenetetracarboxylic-3,4-anhydride-9-carboxylicacid-10-potassium salt [26] [27] was condensed with 3-hydroxypropylamine, then with 1-hexylheptylamine and allowed to react with methacroylchloride to obtain 8 (see Scheme 2).Two methacroylester groups were attached in 9 for cross linking.Thus, the perylene anhydride-carboximide with the solubilising 1-hexylheptyl substituent attached to the nitrogen atom was condensed [23] with aminodihydroxypropane and allowed to react with methacroylchloride for the preparation of 9 where the chromophore remains attached to the side chain of the polymer.For a cross-linking across the chromophore perylenetetracarboxylic bisanhydride was condensed [24] with 2-aminomethyl-2pentylheptyl-1-ol where the solubilising effect was brought-about by the geminal alkyl groups.Further reaction with methacroyl chloride gave 10.The solubilising effect of the geminal alkyl groups could be further increased by means of a prolongation of the alkyl groups to obtain 11 in the same manner as described Scheme 2. Synthesis of fluorescent labels with methacrylic ester groups.above for 10.

Fluorescent Organo-Nano Particles (ONP)
Radical RAFT polymerisation (Reversible Addition-Fragmentation chain Transfer) mediated and over-all controlled by 1 was applied to a mixture of styrene and 3 until 7 for the preparation of ONP 12 until 16 as co-polymers (see Scheme 3).The reactions proceeded smoothly without problems concerning the Trommsdoff effect.A comparably narrow distribution in molecular weight of 12 was obtained with polydispersities PD as low as about 1.1 (1.04 until 1.19); see 12a until 12g in Table 1 and Table 3.
The molecular weights M n of 12 decrease with increasing concentrations of 1 from 23,300 to 3300 (12a until 12g) and the size decreases from 66 nm to 7 nm where the smaller nano particles seem to be more compact presumably because of the local influence of the chromophore.An increase of the concentration of labelling agent 3 (12h until 12l) deceases also the molecular weight, however, not as pronounced as with increasing concentrations of 1.An aggregation of 3 at higher concentrations is indicated by a colour deepening from orange to red and Scheme 3. Fluorescent organonanoparticles (ONP). is made responsible for the lowering of the size by impeding the polymerization.
Finally, the size of the nano particles can be controlled with 1 in the same manner as with the monomers of co-polymerisation of styrene such as for 4a/b (13a until 13d) and 7 (16a until 16d).
A further type of ONP was prepared on the basis of PMMA (polymethyl methacrylate) where methyl methacrylate was co-polymerised under RAFT condition.Markers 8 until 11 were applied and the reaction was controlled by means of 2; see Table 2  reaction time of 24 h (17a until 17e).The shortening of the reaction time to 3 h (17f until 17h) and even to 1 h (17i) decreases the size of the ONP appreciably until 11 nm.The lowering of the concentration of the RAFT reagent 2 (17g, 17h and 17j) causes an increase of the molecular weight and the size of particles, respectively.The same influence was found for 1 and polystyrene even for short reaction times (17i).
The bis-ester 9 can expected to act as a cross-linker where the chromophore is situated at the side chain (18a until 18d).The concentration of the RAFT re- solubilising groups causes the formation of larger ONP (18b and 20).The properties of cross-linked and linear ONP seem to be similar, however, the cross-linking causes of the particles to dissolve more slowly in organic solvents.
A comparably low molecular weight was found for the nano particles 12g by means of GPC and could be verified with MALDI as is shown in Figure 1.The pattern of peaks corresponds to the mass differences of units of styrene.A uniform size can be seen in Figure 2.
The ONP exhibit a comparably narrow distribution in size determined by means of dynamic light scattering (DLS); this corresponds to their low values of the polydispersity PD (see Table 1 and Table 2).The very small particles 17i exhibit a broader distribution in size; this may be caused by the influence of the larger share of the chromophore.The distribution in size of the typical samples 18a until 18d is shown in Figure 3.
The thermal stability of ONP was tested by means of thermogravimetry (TGA) and reported for the typical samples 12a and 17a in Figure 4.The particles were completely stable until 200˚C.A loss of mass between 10 and 13% proceeds slightly above 200˚C and is attributed to a loss [28] of the terminal trithiocarbonate group.On the other hand, this does not affect the function of the fluorescent nano particles being thermally stable until more than 300˚C.
Thus, the ONP can be applied under conditions of the processing of technical polymers.
The perylene-derived chromophores remain nearly unaffected by the incorporation into ONP as is shown in Figure 5 where both the structured absorption           preferentially for larger ONPs (slope < 1 in Figure 8) and an increase of the polydispersity PD to about 1.4; see Table 5 and Figure 8.We conclude that the stability of the ONPs is high enough for processing, whereas a slow degradation can be expected in the environment attributed to the very high surface of the particles.

Conclusion
Organo-nanoparticles (ONP) with narrow distribution of size can be prepared by RAFT polymerisation where the co-polymerisation with vinyl-substituted chromophores introduces fluorescence as a functionality of such materials.The size of the ONP is controlled both by the concentration of the applied RAFT reagent and the amount of added polymerisable chromophore for co-polymerisation.
Small ONP are more compact than larger ones indicated by the relatively smaller size compared with their molecular weight.Adapted perylene-derived chromophores allow the preparation of strongly fluorescent ONP with emission covering the whole visible region.Application of ONP as non-migrating makers of polymers is of interest such as for recycling applications where a binary coding [30] of applied n fluorescent marker allows a characteristic labelling of 2 n-1 materials.Moreover, such marking may be applied for efficient and easily detectable tamper- [31] and forgery-proof [32] optical elements.

ONP-doped polymers
by means of polymerisation: 50 … 100 ppm ONP and AIBN (1.5 mg, 0.009 mmol) were stirred with the monomer (9 g) styrene and methylmethacrylate, respectively, until homogeneous, treated with further monomer (1 g), stirred for 15 min, treated with AIBN (1.5 mg, 0.009 mmol), polymerised at 70˚C for 1.5 h and hardened at 47˚C for 3 d.The fluorescence of the ONP could be detected with optical excitation at 490 nm and corresponds to the fluorescence of the isolated ONP.ONP-doped polymers by means of incorporation: 50 … 100 ppm ONP (until 300 ppm ONP for 15 and 16) and technical Delrin (polyoxomethylene, 3 g) were treated with chloroform (1 mL), homogenized by stirring, allowed to evaporate in air, melt by means of a heat gun at about 300˚C with stirring and kneading for 3 min and shock cooled in liquid nitrogen.The fluorescence of ONP could be detected with optical excitation at 490 nm.Degradation of ONP-doped Delrin: Doped Delrin was refluxed with concentrated hydrochloric acid (bath 120˚C) until dissolution (15 min until 1 h depending on technical processing), allowed to cool, extracted with chloroform and characterized by UV/Vis spectroscopy.Absorption and fluorescence spectra of the applied chromophores were obtained.

agent 2
influences the size not as pronounced as for 8 (18b and 18c); the reaction time and the concentration of the cross linker 9 (18a and 18c) are more important (18b and 18d).An aggregation of the chromophore constraining the growths of the chains is made therefore responsible; the latter is indicated by a colour deepening of the ONP from orange to red with increasing concentration of the marker.Surprisingly, it seems of minor importance whether the chromophore is placed in the cross linking position or not(19a until 19d).A lowering of the concentration of the cross linker increases the size of the particles (19a and 19b) where the reaction time is more important (19b and 19c) than the concentration of 2 (19c and 19d).Finally, an increase of the chain length of the DOI: 10.4236/gsc.2018.83017H. Langhals et al.

Figure 1 .
Figure 1.Segments of the MALDI spectra of the styrene-based ONP 12g in reflection mode.Left: Positive ionisation.Right: Negative ionisation (matrix IAA + AgTFA in THF).

Figure 3 .
Figure 3. Size distribution d in nm of the ONP 17a until 17j by means of DLS.

Figure 8 .
Figure 8. Aging of ONP: The number average of the molecular weights M n , of ONPs (see Table5) and the influence of the expo- Figure 8. Aging of ONP: The number average of the molecular weights M n , of ONPs (see Table 5) and the influence of the exposition to air at room temperature after a period of three years [M n (3 years)]; the slope < 1 indicates stronger alterations of larger particles.Inset upper left: Linearity between M n and M w for ONPs; the slope slightly larger than 1 indicates a higher uniformity of smaller particles.Inset lower left: Linearity between M n (3 years) and M w (3 years) for ONPs after an exposition to air after a period of three years (3 years); the slope slightly higher than the slope of the upper left diagram indicates a stronger alteration of the larger ONPs by ageing.Table 5. Aging of ONP: The number average of the molecular weights M n , the weight average of the molecular weight M w of ONP and changes after the exposure to air at room temperature for a period of three years [M n (3 years), M w (3 years) and PD (3 years)].

Table 2 .
Synthesis of methylmethacrylate-based ONP (MMA) according to the general

Table 3 .
ONP by the copolymerisation of 3 until 7 and styrene under RAFT condition mediated by 1; M n and M w by GPC (UV detector, acetonitrile, calibration with polystyrene).Size by DLS.

Table 4 .
and Table4.The scope of reproducibility of the synthesis is in-ONP by the copolymerisation of 8 until 11 and methyl methacrylate under RAFT condition controlled by 2; M n and M w by GPC.Size by DLS.

Table
1 and Table2, and the light emission of ONP 12 until 16 covers the most of the visible region as is shown in Figure6.
The ONP can be incorporated into polymers for applications such as fluorescent labelling.The more styrene-similar ONP 12 until 16 were spread in monomeric styrene and the more methacrylate-like ONP 17 until 20 preferently in DOI: 10.4236/gsc.2018.83017268 Green and Sustainable Chemistry H. Langhals et al.