Initiation of Polymerization - American Chemical Society

atom, but only 1/9 the mass. At the end of its ... available the kinetic isotope effect can also be calculated. .... indicating that there is little m...
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M u o n i u m as a H y d r o g e n - l i k e P r o b e to S t u d y M o n o m e r Initiation K i n e t i c s 1

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J. M. STADLBAUER, Β. W. NG, Y. C. JEAN, Y. ITO, and D. C. WALKER

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University of British Columbia, Department of Chemistry and TRIUMF, Vancouver, B.C. V6T 1Y6 Canada The radioactive muonium atom(Mu) has the same ioni­ zation potential and Bohr radius as the hydrogen atom, but only 1/9 the mass. At the end of its 2.2µsec intrinsic life-time the positive muon (which acts as the Mu nucleus) decays into an ener­ getic positron. This decay can be observed using fast single particle counting methods. Because of this ease of detection and its hydrogen-like pro­ perties, Mu makes an excellent probe to directly study hydrogen atom reactions: for example, hydro­ gen atom initiation of monomer polymerization. We have examined the addition reaction kinetics of Mu with acrylamide, acrylic acid, acrylonitrile, methylmethacrylate and styrene, a l l in aqueous solution. Their second order rate constants were found to be, respectively, 1.9, 1.6, 1.1, 1.0 and 0.11 x 10 M S . As proof that Mu does add across the vinyl bond we have observed the muonium containing free radicals in the pure liquid monomers and obtained their hyperfine coupling constants. 10

-1

-1

First of a l l what is muonium, what is its source, how do we observe i t , and why is it useful? Muonium (Mu) is an atom comprised of a positive muon nucleus, (u ), and a bound elec­ tron. This bound electron can have its spin parallel or antiparallel to the muon nuclear spin resulting in 'triplet' and 'singlet' muonium atoms, respectively. The atom has a mass 1/9 that of the hydrogen atom, H; but because the reduced masses are +

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Current address: University of Missouri-Kansas City, Department of Physics, Kan­ sas City, MO 64110. 2

Current address: University of Tokyo, Research Center for Nuclear Science and Technology, Tokyo, Japan. 0097-6156/83/0212-0035$06.00/0 © 1983 American Chemical Society Bailey et al.; Initiation of Polymerization ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

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n e a r l y the same, Mu has e s s e n t i a l l y the same Bohr radius and i o n i z a t i o n p o t e n t i a l as hydrogen. Muonium i s formed when a p o s i t i v e muon thermalizes i n a target and p i c k s up an e l e c t r o n from the stopping medium i n t o a bound s t a t e . Muons, both high energy (28 MeV) and low energy (4.1 MeV), are the product of p o s i t i v e pion decay, i n f l i g h t o r at r e s t , r e s p e c t i v e l y . π+

»μ+ + ν . μ The pion i t s e l f i s g e n e r a l l y produced

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9

Be

+ lp

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+ ir+.

(1 through the r e a c t i o n (2

Intense high energy proton beams r e q u i r e d f o r these s t u d i e s are c u r r e n t l y a v a i l a b l e a t TRIUMF (Vancouver, Canada), LAMPF. (Los Alamos, U.S.A.), SIN ( V i l l i g e n , S w i t z e r l a n d ) , and KEK (Tsukuba, Japan). Under a transverse magnetic f i e l d the f r e e muon and other diamagnetic muonic species precess with a Larmor frequency of 0.0136 MHz/G, while t r i p l e t muonium precesses a t 1.39 MHz/G. The f a c t o r of 102 between these p r e c e s s i o n frequencies makes i t easy to d i s t i n g u i s h between the species present. The i n t r i n s i c l i f e ­ time of the muon, 2.2 χ 10~^s, allows enough time to study the chemical r e a c t i o n s of muonium i n many chemical environments (Γ-4). When the muon decays i t e j e c t s a p o s i t r o n and two n e u t r i n o s . The p o s i t r o n s , with a maximum energy of 52 MeV, are e a s i l y detectable using nuclear physics s i n g l e - p a r t i c l e f a s t counting systems. The experimental data are tabulated as time histograms that are then computer analyzed. A d e t a i l e d d i s c r i p t i o n of the MSR (Muonium Spin Rotation) method i s a v a i l a b l e elsewhere ( 3 ) . Muonium has been observed i n pure hydrocarbons ( 5 ) , a l c o h o l s (6,7), and water (4). Because Mu r e a c t s slowly with these pure l i q u i d s , g i v i n g observable r e a c t i o n l i f e t i m e s of Mu up to 4us, they can be used as solvents to study various s o l u t e s of i n t e r ­ e s t . As the f r e e t r i p l e t Mu atom r e a c t s with the s o l u t e i t s observed p r e c e s s i o n frequency i s damped and a decay constant, λ , can be obtained. The concentration dependence of the decay con­ stant provides second order chemical r a t e constants f o r Mu a d d i ­ t i o n , a b s t r a c t i o n , s p i n conversion, and o x i d a t i o n - r e d u c t i o n r e a c t i o n s . When analogous hydrogen atom r a t e constants are a v a i l a b l e the k i n e t i c isotope e f f e c t can a l s o be c a l c u l a t e d . Muonium i s u s e f u l as a hydrogen-like probe to study the atomic and r a d i c a l r e a c t i o n s of hydrogen because Mu's r e a c t i o n decay constant i s d i r e c t l y observable, while most hydrogen atom data come from measured r e l a t i v e r a t e constants. The more we know of the r e a c t i o n s of hydrogen, the simplest and most abundant element i n the universe, the sooner we w i l l be b e t t e r able t o understand more complex atoms and t h e i r r e a c t i o n s .

Bailey et al.; Initiation of Polymerization ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

4.

STADLBAUER

ET

37

Muonium in Monomer Initiation Kinetics

AL.

Experimental Chemicals. A c r y l o n i t r i l e (AN), methylmethacrylate (MMA), and styrène were s u p p l i e d by A l d r i c h and p u r i f i e d before use. A c r y l i c a c i d (AA) and acrylamide (AM) were purchased from Eastman. The a c r y l i c a c i d contained 200 ppm p-methoxyphenol as i n h i b i t o r . However, a t the s o l u t e concentrations of 5.0 χ 10~ M and 7.4 χ 10~*M the i n h i b i t o r ' s c o n c e n t r a t i o n would be ^ 10"^M and, t h e r e f o r e , could not c o n t r i b u t e s i g n i f i c a n t l y to the observed muonium decay. The water s o l v e n t was t r i p l y d i s t i l l e d , i n i t i a l l y from permanganate s o l u t i o n .

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Apparatus. The schematic diagram i n Figure 1 shows the general experimental set-up f o r a low energy 'surface' muon beam. Right and l e f t p o s i t r o n d e t e c t o r s are made of p l a s t i c s c i n t i l l a t ­ or 'paddles' connected to RCA 8575 p h o t o m u l t i p l i e r tubes by lengths of l i g h t pipe; the whole of which are wrapped to exclude outside l i g h t . Detectors can be s e t up f o r double or t r i p l e coincidence counting to help minimize background. Graphite degrader between the d e t e c t o r s helps maximize s i g n a l amplitudes. The Helmholtz c o i l s , which provide the e x t e r n a l magnetic f i e l d , are mounted t r a n s v e r s e l y to the beam. A s i g n a l from the TM ( t h i n s c i n t i l l a t o r - m u o n ) counter s t a r t s the Ins r e s o l u t i o n c l o c k . A s i g n a l from e i t h e r r i g h t or l e f t d e t e c t o r s i n coincidence mode stops the c l o c k i f t h i s s i g n a l appears between a set "gate" of 0 to 4μβ. I f the p o s i t r o n s i g n a l i s not c o i n c i d e n t or appears a f t e r 4ys i t i s not counted and the system s t a r t s over. U s u a l l y about 50,000 good events are counted per minute. MSR: Muonium Spin R o t a t i o n Measurements. For the k i n e t i c p o r t i o n of t h i s study a low energy (4.1 MeV) beam of muons from the M20 channel at TRIUMF was focused on a shallow T e f l o n c e l l with 0.007 cm Mylar as muon windows. These target c e l l s contained approximately 80 ml of sample s o l u t i o n which was bubbled w i t h high p u r i t y He to remove d i s s o l v e d oxygen. In the case of these v o l a t i l e a c r y l i c s o l u t e s the bubbling gas was f i r s t passed through a prebubbler c o n t a i n i n g a s o l u t i o n of the same concentration as the sample. Muonium p r e c e s s i o n s i g n a l s were observed at a f i e l d of 8G provided by Helmholtz c o i l s transverse to the beam and centered on the t a r g e t . Figure 2 shows some time-histograms of both raw and f i t t e d data. The data i s computer f i t t e d using MINUIT, a χ minimization program, to a nine parameter f u n c t i o n . Equation (3) shows that f u n c t i o n with two parameters, background and muon decay, subtracted out. 2

A ( t ) - Αμ cos(o) t + φ ) + A y

M

exp(-xt) cos(u) t - φ ) M

Μ

(3

where A^ and A J J are, r e s p e c t i v e l y , the muon and muonium s i g n a l amplitudes, and ω are t h e i r f r e q u e n c i e s , φ and φ^ t h e i r Μ

μ

Bailey et al.; Initiation of Polymerization ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

Bailey et al.; Initiation of Polymerization ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

Downloaded by CORNELL UNIV on August 17, 2016 | http://pubs.acs.org Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch004

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Bailey et al.; Initiation of Polymerization ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

Downloaded by CORNELL UNIV on August 17, 2016 | http://pubs.acs.org Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch004

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INITIATION

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i n i t i a l phases, while λ i s the muonium decay constant. Using equation (4), the obseved λ and the known s o l u t e c o n c e n t r a t i o n [S], the bimolecular r a t e constant, k , can be c a l c u l a t e d f o r the r e a c t i o n between Mu and the s o l u t e . M

λ = λ

0

+ k [S]

(4

M

The decay constant f o r the pure water s o l v e n t , λ , has a value o f ( 2 . 4 i 0 . 6 ) x l 0 s " 05-8). Two Xs, r i g h t and l e f t , a r e obtained per experiment and are p l o t t e d against s o l u t e concentra­ t i o n . The slope o f the best l i n e i s taken as k . 0

5

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M

MRSR; Muonium R a d i c a l Spin R o t a t i o n . In order t o prove that Mu was indeed adding across the v i n y l double bond o f styrene and the l i q u i d a c r y l i c s , high energy muons (24 MeV) were made to s t r i k e g l a s s , round-bottomed f l a s k s c o n t a i n i n g neat samples. Two methods o f degassing were u t i l i z e d : i ) a freeze-pump-thaw c y c l e followed by vacuum s e a l i n g , and i i ) by bubbling the samples on l i n e using a s p e c i a l probe and ground g l a s s f i t t i n g s . Data were analyzed using a Fast F o u r i e r Transform (FFT) program t o o b t a i n the r a d i c a l frequencies and the hyperfine coupling constant, a , as described elsewhere (9,10). μ

Results Table I l i s t s the decay constants, λ, obtained f o r the d i f ­ f e r e n t concentrations of the monomers s t u d i e d . These λs are the average of the l e f t and r i g h t values obtained f o r each concentra­ t i o n (11). Though s t a t i s t i c a l e r r o r s range from 5% t o 17%, ex­ perimental i r r e p r o d u c i b i l i t i e s i n t a r g e t geometry, f i e l d homoge­ n e i t y , detector t h r e s h o l d s , muon beam asymmetry and background r e s u l t i n a more probable e r r o r of ±25% ( 8 ) . T h i s l e v e l o f r e p r o d u c i b i l i t y i s q u i t e reasonable when compared to r a t e con­ stants obtained by competitive r a t e techniques and d i r e c t p h y s i c a l methods. F i g u r e 3 shows the FFT s p e c t r a o f styrene a t 1500G and 2500G. The l a r g e low frequency peak i s due to the muon while the two higher frequency peaks are due t o the r a d i c a l . A d d i t i o n o f these two r a d i c a l frequencies y i e l d s the muon hyperfine c o u p l i n g constant α . [Peaks a t 23 MHz are due t o the r a d i o frequency (RF) of the c y c l o t r o n . Peaks a t m u l t i p l e s of 125 MHz are due t o the counting system's c l o c k frequency]. These s p e c t r a show that the coupling constant f o r styrene i s