Awad Albalwi

Hindered amine stabilizes

By

Awad Nasser Albalwi

School of Chemistry

University of Wollongong

(June,2010)

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Abstract

A B3LYP, HF, AM1 and PM2 computational studies of the reaction of hindered amine (HALS)

has been perfumed. Four different theories were used to calculate the bond dissociation energy

(BDE). In two molecules studied the nitrogen were protonated and not protonated. BDE were

calculated when aromatic rings were substituted with NO2 and OCH3. B3LYP was the best

theoretical calculation level, The BDE was grater when nitrogen in HALS was protonated. There

was no big significant difference in BDE when aromatic ring of hindered amine was substituted

with NO2 and OCH3.

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Introduction:

Hindered amine light stabilizers (HALS) are among the most efficient polymer stabilizers known.

Bis (2,2,6,6-tetramethyl-4-piperidinyl) sebacate, is a typical (HALS). Since the early 1970s,

HALS have become a highly important class of light stabilisers for polymers. They stabilise wide

range of commercial polymers and are particularly effective for stabilization of polyolefins when

use where resistance to deterioration by light and weathering are important. Hinder amines

have also been used as stabiliser against light induced degradation of polymers such as

polyolefin and polyurethane1&4. Polypropylene is an example of a major commercial polymer

which would never have achieved any practical use without the development of a good

stabiliser system. Polyolefin needs protection in all the stages of its life cycle. In order for an

antioxidant to improve the long-term weathering performance of an automotive

clearcoat/basecoat paint which is a polymer system, it must inhibit clearcoat photo-oxidation at

the onset of exposure and sustain the inhibition for many years. While there is ample evidence

that hindered amine light stabilizer (HALS) additives can inhibit the photo-oxidation of

automotive clearcoats polymer,

It is generally known that the photo oxidation of hydrocarbon polymer under natural weathering

is initiated by ketones or hydroperoxide impurities. HALS acts as a scavenger for free radicals

that would otherwise degrade or discolour HALS are efficient inhibitors of the photooxidation of

polyolifins HALS act as scavengers for free radicals that would otherwise degrade or discolour

the polymer coating. Hindered amine has been employed in the automotive and wood coating

sectors of the surface coatings industry for many years. In both applications, HALS are

incorporated into a non-pigmented (or ‘clear’) topcoat by addition to the unstabilised wet paint or

lacquer during formulation. The presence of HALS unequivocally improves gloss retention and

long-term durability of these formulations 2 .

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Mechanisms of action of hindered amine stabilizers.

Hindered amines have multiple mechanisms of functioning as photostabilsers of polymers. The

chemical mechanism of HALS action remains an active area of research interest, but there is

general agreement that the critical active species in the stabilisation process is the oxidised

form of the HALS in question, namely the corresponding nitroxyl free radical.

Although the sequence of reactions involved is often generically described by the so-called

‘Denisov cycle’ .The mechanism of HALS protective mechanisms is actively been pursued by

many researchers and agreed that the active radical nitroxyl radical is the stabilising spices.

Savaging of radicals is generally considered to be the mechanism by which HALS inhibits

polymer auto oxidation. The stabilising spice in HALS is the oxidised form nitroxyl free radical.

The mechanism of nitroxyl radical trapping studies with various hindered amines is an active

area of research.

HALS is able to break several radical chains. HALS amino ether’s ability to convert back to

nitroxyls by reacting with alkyperoxy radicals in known as Denisov cycle(scheme.1). This

process is over simplified caused there are other reactions and diffusion taking place.

Scheme.1: Denisov cycle.

Some researches. focused on the possible deactivation mechanisim for HALS molecules i.e

finding the ideal initial nitrogen substitute and the size of R on piperidine ring.

Their primary mode of functioning appears to be the trapping of carbon –centred radicals by

nitroxyls and the regeneration of nitroxyls from N-alkyloxy product The nitrogen oxidation

mechanism where the alkylperoxy radical attack the nitrogen of the aminoether, forming an

oxidised tetra coordinated nitrogen intermediate. The intermidate undergo α-hydrogen

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abstraction and a N-O bond cleavage to produce the nitroxyl radical a carbonyl compound

Hindered amine stabilizers are able to participate in inhibiting both photo-oxidation reactions--

they trap acylperoxy radicals, converting them to carboxylic acids and are converted to nitrosyl

radicals in the process; the nitrosyl radicals trap alkyl radicals and the hindered amines trap

alkylperoxy radicals to inhibit the other oxidation pathway.

1- Denisov Mechanism. Nitrosyl are regenerated from N-alkyloxy hindered amines in a fast,

efficient reaction with acylperoxy radicals and in a slow, inefficient reaction 1 The Denisov

mechanism shown in eqn (1) and Scheme 1 was postulated on the basis of findings that nitroxyl

radicals retarded the photo-oxidation of polypropylene powder containing dicumyl peroxide as

initiator 3.

1

,

When the photolysis of the polymer containing hindered amine nitrosyl was carried out in an

argon atmosphere, the nitrosyl radical concentration was found to decrease to zero and

subsequently, when air was introduced into the reaction vessel, nitroxyl radicals were again

found to be formed during continued photolysis. These observations led to the proposal that the

nitroxyls were being regenerated from the N-alkyloxy compounds by reaction with peroxy

radicals, and dialkyl peroxides were also formed.

2- Sedlar and his co-workers have proposed that complexation between hindered amine and

hydroperoxides locates the hindered amine in the region where the radical products from

photolysis of the polymer hydroperoxide can interact easily with the hindered amine and form

products which do not have any oxidation propagating qualities:

3 -Carlsson and his co-workers have also proposed complexation between hindered amines

and polymer hydroperoxides. In addition, they found that when a prephotooxidized film of

polypropylene was contacted with a solution of hindered amine, or when they had imbibed

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hindered amine into the polymer film from Stabilization mechanisms of hindered amines

solution, dark reactions took place in which nitroxyls were generated probably through

intermediate hydroxylamines., Carlson et al postulated that the prephoto-oxidized polypropylene

film contained activated hydroperoxide functionalities on alternating carbon atoms based on the

accepted back-biting mechanism of peroxy radicals in polyolefins 3.

They pointed out that activated hydroperoxides have been reported in the literature, most

particularly in a paper by Ball and Bruice. In their work, Ball and Bruice found that-

hydroperoxyflavin reacted readily with amines. With secondary amines the activated

hydroperoxide formed hydroxylamines; with tertiary amines, amine oxides; and hydroxylamines

were found to be further oxidized to nitrones by this reagent. In that paper, Ball and Bruice

reported the rate constant for the reaction of -hydroperoxyflavin with N,N-dimethylaniline to be

about 400000 times greater than the rate constant for the reaction of di-t-butyl hydroperoxide

with that same tertiary amine 3.

4- Carlsson et al., investigators reported that the reaction between triacetoneamine and

cyclohexylperoxy radicals had a rate constant of 3 mol- 1 s- 1 and yielded cyclohexylperoxide and

aminyl radicals.The aminyl radicals were subsequently oxidized to nitroxyls.

5- Toda and his co-workers of the Sankyo Corporation of Japan reported that hindered amines

react rapidly with peracids as indicated in equation(2). The reaction is stoichiometric and results

in the formation of nitroxyl radicals:

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6 Felder and his co-workers of the Ciba-Geigy Corporation in Switzerland reported that

hindered amines are effective scavengers of acylperoxy radicals 3.

3

The Felder group also reported that the photo-oxidation of isooctane, a model for

polypropylene, initiated by t-butoxy radicals from the photolysis of di-t-butyl peroxide, resulted in

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the formation of peracids, hydroperoxides, and peroxides. About 40% of the active oxygen in

the products was in the form of peracids. Furthermore, on analysis of the products formed, the

Felder group established that the attack of isooctane by alkylperoxy radicals was nearly

statistical and that primary hydrogens were as readily abstracted as secondary or tertiary

hydrogens. This result is in contrast to the generally accepted concept of hydrogen atom

abstraction in polymer molecules, with the tertiary carbon atoms in polypropylene, for instance,

generally being accepted as the preferred site of attack 4.

Felder's results of random attack by t-butoxy radicals in the photooxidation work contrast with

results obtained by Niki and Kamiya in the thermal oxidation of hydrocarbons also with t-butoxy

radicals, generated from the thermolysis of di-t-butyl peroxyoxalate. In the latter work with a

number of hydrocarbons the attack at the primary, secondary and tertiary positions was in the

ratio 1:7:20, respectively 4.

• Felder and his co-workers postulated further that since primary attack occurred frequently in

the photolysis of isooctane it was likely that significant amounts of aldehydes were being formed

as a result of termination of primary peroxy radicals and that subsequently the oxidation of

these aldehydes resulted in peracids. That was one of the first publications in which the

suggestion was made that aldehyde oxidation may be a key, but neglected, pathway in the

photo-oxidation of hydrocarbon polymers. In fact, a number of articles in the literature indicate

that among the photooxidation products of polyolefins are acids, esters and even peracids.

A computational study of the possible regeneration mechanism should provide useful insight in

to most of the molecular structures of various HALS1.

The purpose of this paper to examine the following hypothesis:

1- There is correlation / relationship between an increase the size of group (R) and an

increase the Bond dissociation energy of Hindered amine (HALS).

2- There is no effect of substitution on the aromatic ring of HALS with OCH3&NO2 in various

positions.

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3- There are differences in BDE between none & protanated nitrogen of Hindered amine

(MO1).

4- There is no relationship between change of such group (OCH3, NO2) on the same

position on the aromatic ring (HALS) & BDE changes.

In this project the molecular MO1 is refer to this structure:

The Molecular MO2 is refer to these structure:

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R R

R R

MO2(+)Protonated

MO2Non-protonated

MO1Non-protonated

MO1(+)protonated

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Procedure

Firstly, the calculations were performed with the GAUSSIAN09 (G09) programme in order to

select the best level of theory to calculate the Bond Dissociation energy (BDE). Four level of

theories (B3LYP, AM1, HF & MP2) at the basis set 3-21(G) were used to calculate the BDE of

these reactions (Scheme3&4):

Scheme3: breaking reaction of the bond O-R of Hindered amine MO1 .

Scheme4: breaking reaction of the bond O-R of Hindered amine MO1(+) .

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In order to calculate the BDE, the reactants and products structures were built by drawing each

of them on the built molecule page in the job manager (GAUSSIAN09 (G09)). After constructing

the molecule, comprehensive cleanup using idealized Geometry & Mechanics was used to get

the best molecular structures. .In addition the theory level was basis set, optimize + Vibe freq

calculation, charge and multiplicity were selected from Configure Gaussian Job Options page.

After the calculation was done successfully, the electronic energy for every molecule was

determined from the final block of output of (G09). The BDE was calculated by using the

formula:

BDE = ∑ reactants energy -∑ products energy

Comparison between the 4 levels of theories was done.

Comparison of the four level of theories depends on how long every theory takes and how

accurate they are.by using results from research papers and experimental data

After selecting the suitable theory, the comparison between different basis sets of the selected

theory, in calculation time and BDE results were done.

By using the B3LYP 3-21G data sets (Optimize + Vib Freq - Gaussian )., BDE of breaking

reaction of the bond O-R, when the substituting the aromatic ring with different groups such as

OCH3 & NO2 in various position (meta, Ortho & Para) were calculated. The calculation was

applied when the nitrogen is protonated & non protonated (scheme 5&6).

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Scheme 5: Substituting the aromatic ring with group NO2 in various position (meta, Ortho &

Para).

Scheme 6: Substituting the aromatic ring with group OCH3 in various position (meta, Ortho

& Para).

The results of this project were compared with experimental data and different level theories

from other research papers.

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Results:

Table.1 Comparison between experimental and calculated BDE (O_R) for HALS Molecular 1 (MO1)

[kJmol_1] BDE 3LYP

BDE AM1 BDE HF BDE MP2 BDE exp (from

research

BDEPM3

paper)

BDEDFT

M1-OH= M1-O.+H. 271.65 162.34 199.10 185.15 291 296 279M1-OCH3= M1-O. + CH3 140.46 120.76 111.62 96.04 197 178 185MO1-OC(CH3)3 = MO1-O. +.C(CH3)3

186.53 172.98 100.71 74.72 n/a 94 n/a

Graph.1.: Comparison between experimental and calculated BDE (O-R) for HALS Moleculer1.

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Graph.2: Comparison of BDE when an increase of R from H to CH3 between experimental, PM3, DFT from research paper & calculated with HF, B3LYP, MP2 and AM1.

. Graph.3: The comparison of calculation time of energy with different levels of theories

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Graph.4: The Comparison of calculation time of energy with different basis sets of B3LYP for molecular 1.(MO1).

Graph. 5: Comparison of BDE changes with an increase the basis sets of B3LYP for HALS Molecular No.1 with different R group ( R=H, CH3 & C(CH3)3).

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Graph.6: Comparison of BDE between none & protanated nitrogen of Hindered amine (MO1) with different group of R ( H, CH3 & C(CH3)3.

Graph.7: Comparison between BDE change with HALS molecular No1 with different group of R ( H, CH3, CH2CH3, CH(CH3)2 , C(CH3)3 ).

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Graph.8: comparison between change the group (OCH3) on the aromatic ring of HALS molecular No2 (MO2) – none protonated Nitrogen -and BDE changes at B3LYP/3-21(G).

Graph.9: comparison between change the group (OCH3) on the aromatic ring of HALS molecular No.2 (MO2(+)) - protonated Nitrogen -and BDE changes at B3LYP/3-21(G).

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KJ/mol

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Graph.10: comparison between change the group (NO2) on the aromatic ring of HALS molecular No2 (MO2) – none protonated Nitrogen -and BDE changes at B3LYP/3-21(G).

Graph.11: comparison between change the group (NO2) on the aromatic ring of HALS molecular No2 (MO2) – proton ted Nitrogen -and BDE changes at B3LYP/3-21(G).

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Graph.12: comparison of BDE between different groups (NO2 & OCH3) on the aromatic ring of HALS molecular No2 (MO2) – none- protonated Nitrogen -at B3LYP/3-21(G).

Discussion:

It is observed that very few experimental BDEs of HALS have been reported in literature.

BDE of O-R in HALS compound have been investigated for various group ( R= H, CH3 &

C (CH3)3 using different level of theories and different data sets, Calculated BDE from research

papers were compared with calculated BDE in this paper (Table .1 and figure. 1) . It was found

that the BDEs of O-R (HALS) were decreasing from H> CH3> C (CH3)3 using HF & MP2

theories. However , the BDE was random from H> C(CH3)3 > CH3 using B3LYP & AM. It was

found that in most cases B3LYP/3-21(G) calculations were slightly closer to BDE experimental

value when R= H& CH3 .It is also observed that the results coming from HF & MP2 were more

reasonable, thus the stability of these groups were increasing from C (CH3)3 > CH3>H. The

stability of those group lead to decrease the BDE of O-R in HALS. It is interesting to note that

the calculated BDE using B3LYP /3-21(G) of this paper was in agreement with the

experimental values . Graph.2 has shown that The B3LYP /3-21(G) was closer R2=0.86 to the

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experimental value & DFT (R2 = 0.99) level theory from journal article than other theoretical

calculation (HF, AM1 & MP2) 6. Thus , It has chosen the B3lYP/3-21(G) to calculate the BDE for

various structures in this project . In addition, The B3LYP/3-21(G) takes short calculation time

Graph 3&4.

Figures 8 and 9 show the effect of substituting the aromatic ring with OCH3 in Meta, .or tho and

Para positions, there was no significant change in BDE when OCH3 was substituted on all three

positions of the aromatic ring. Figures 8 and 9 have shown that there was difference in BDE

when the nitrogen is proton ted and not proton ted. In protonated Nitrogen the BDE is greater

than that of non protonated by about 7% .

figures 10and 11 show the substitution of NO2 on the aromatic ring, In figure .10 there was no

change in the BDE when NO2 was substituted in ortho and para positions. However BDE

decreased significantly in meta position and this is not normal compare to other positions.

Figure 12 shows comparison between 2 different substitutions ie NO2 and OCH3.on aromatic

rings. There were no different in BDE in ortho and para positions when NO2 and OCH3 were

substituted on the aromatic rings. However the BDE of OCH3 was three times greater than that

of NO2 in meta position figure.12. When aromatic ring was substituted by various groups,

there was no effect on BDE , if we assume that the BDE on meta position of NO2 is wrong.

Figure. 6 has indicated that protonated Nitrogen of HALS gives an increase in BDE than non

protonated . Thus the HALS (MO1) with protonated Nitrogen might be more stable than non

protonated Nitrogen. In addition , protonated Nitrogen of HALS might lead to increase the

lifetime of the paint that contains the HALS Molecule. From the result , it can be said , the

increase the size back rings of HALS is not significant in an increase the stability of HALS in

comparison between non protonated nitrogen of MO1 & MO2. How ever, in protonated Nitrogen

of MO1(+) &MO2(+) cases , the MO1(+) was greater in BDE than MO2(+) (graph 6,9&11). Thus

, the MO1(+) is more stable than MO2(+).

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Conclusion

Computational analysis now show that there is a relationship between the size of R and BDE of

HALS. cause When R increases, BDE decreases. It is also observed that there was no

significant change in BDE when OCH3 was substituted on all the three positions of the aromatic

rings of HALS. Computational calculations also show that there was difference in BDE between

protonated and non protonatednitrogen of HALS.

Acknowledgments:

I would like thank School of Chemistry - University of Wollongong for computational facilities on

GAUSSIAN09 and Drawing program and huge sources. Many thanks to Dr: Yoke Berry for help

in writing this paper.

References:

1-Possi, Aventurini and A Zedda J. AM Chem .SCI (1999)121,,7914-7917

2-F,.Gugumus Polymer Degradation and Stability (1995) 50, 101-116

3- P.P. Klemchuk , M.E Gande Polymer Degradation and Stability (1988),22,241-274

4- T.A. Lowe, M.R.L Paine,D.L.Marshall.L.A.Hick,J.A.Boge,P.J.Barker , S.J.Blanksby J Mass Spec (2010) 45(5) 486-496

5- G.Geuskens ,M.N.Kanda Polymer Degradation and Stability (1996),51, 227-232.6- A Gaudel,S., D. Siri, P.Tordo ,ChemPhysChem,(2006),7,430-438

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