Laser-initiated polymerization

J. Photo&em. Photobiol. A: Chem., 77 (1994) 1-7

1

Review

Laser-initiated

polymerization

Thomas P. Davis LkpattmentofPolymer Science, School of Chemical Engineeting and Inducrrial Chemistry, Uniwrsily of New South Wales, P.O. Bar 1, Kensington, Sydney, N.S. W. 2033 (Australia)

(Received June 30, 1993; accepted July 8, 1993)

Abstract A review of laser-initiated polymerization is presented. Particular attention is given to the derivation of kinetic rate parameters from pulsed-laser-initiated polymerizations.

1. Introduction The application of lasers to polymerization chemistry has been increasing in recent years. The unique features offered by lasers, namely the production of monochromatic light of high intensity and the ability to flash light in short pulses (on a nanosecond time scale), have been utilized to synthesize polymers and to measure polymerization kinetic rate constants. UV and near-UV radiation is widely used to initiate polymerization reactions; however, the limited penetration of visible and UV light into organic materials can restrict the application of this method. The use of lasers overcomes this to some extent and provides the means to achieve very fast cure rates for thermosetting coatings; for example, Castle [l] has shown that the rapid curing of thick polymer specimens (2 in) can be achieved by irradiation with visible laser light. Practical applications for lasers include usage in the in situ curing of dental resins [2] and the production of contact lenses [3]. Jn these instances the absence of significant heating in the initiation process is essential. There is also the impetus to apply this polymerization technique to microlithography, by utilizing a highly focused light beam to initiate highly localized polymerization, thereby allowing the direct writing of structures on the micrometre scale [4].

lOlO-6030/94/$07.00 0 1994 Elswier Sequoia. All

rights reserved

The applications of lasers to other macromolecular syntheses include the polymerization of the following: solid formaldehyde [5], monomers containing metal ions [6], liquid crystalline monomers [7] and epoxy resins [8]. Experimental results for polymerizations performed using conventional UV sources and UV laser light have been compared by Fouassier et al. [9]. The application of laser light to polymerization kinetic experiments can be subdivided into four areas. (i) Studies on the initiation and calculation of triplet lifetimes. This work has led to the identification of multiphoton absorption processes and novel synthetic routes. (ii) Studies on free radical polymerization kinetics using time-resolved pulsed-laser polymerization. (iii) Studies on free radical polymerization kinetics using a pulsed UV laser to initiate the polymerization with the subsequent derivation of kinetic rate constants from the molecular weight distribution of the polymer formed. (iv) Studies on crosslinking reactions utilizing a pulsed UV laser and following conversion using either IR spectroscopy or differential scanning calorimetry (DSC}. The purpose of this paper is to review the developing area of laser-initiated polymerization.

2

2. Initiation and multiphoton processes

T.P. David I Laser-hilimed

potymetimion

absorption

It has been recognized that the adoption of pulsed-laser UV light sources instead of conventional UV irradiation sources can lead to different chemical reactions even when the total energy and wavelength used are the same. These differences originate from the mdtiphoton absorption processes that occur on laser irradiation. McGimpsey and Scaiano [lo] have reported the irradiation of benzil with pulses of 308 nm light from an excimer laser. This leads to the production of a triplet state (A,, = 480 nm). When the intensity of the transient absorption signals was monitored as a function of the laser dose, a non-linear dependence of the detectable triplet yield on the laser dose was observed. They postulate that this is caused by photon absorption by transient intermediates produced during the early part of the laser pulse. In this case laser excitation of benzil leads to symmetric Norrish type 1 cleavage, which does not occur on UV lamp irradiation. This phenomenon of multiphoton absorption is encouraged by high transient concentrations of intermediates (capable of absorbing at the wavelength A) and a high photon flux. These photoprocesses are “laser specific” and can involve species such as free radicals [ll], carbenes [12], ylids [13] and biradicals 1141.Another example of this photochemistry has been given by Johnston and Scaiano [15] for the photodecomposition of 2,2,6,6-tetraphenylcyclohexanone. Conventional UV lamp photolysis gives the following. UV laser photolysis yields. These multiphoton processes are significant in laser-initiated polymerization as they may be exploited to produce laser-specific free radical sources. In addition, they can interfere with the predicted photochemistry to yield unexpected adducts. This is discussed later.

Additional studies on initiation using laser light sources have been reported. These include work on thioxanthones [16], metal carbonyl complexes [17] and hydroxy alkylphenones [18].

3. Time-resolved pulsed-laser experiments This work has been pioneered by Buback et al. [19] and involves the photoinitiation of vinyl polymerization using a UV pulsed laser (pulse width, 10 ns). The polymerization is then monitored on a time scale of microseconds and milliseconds using IR spectroscopy. The initial work in this area was on ethylene polymerization. Analysis of the experimental results is based on the propagation rate equation d[M]/dr = - (G,[M][R-]

(1)

Assuming a second-order radical termination rate, integration yields [M]/[M,] = (2k,[g]t + 1) -05kp’kc

(2)

where [M,] and I&,] refer to the initial monomer concentration and the (maximum) radical concentration immediately after laser pulse absorption respectively_ Introduction of the Lambert-Beer law for the polyethylene band at 2857 cm-l gives an expression for the increase in polyethylene concentration A[P] A]P] = (EP&)-’ log&/l)

(3)

Utilizing the relationship [W) gives

= Pfol

- WW

(4)

3

T.P. David I Laser-initiated polymerization

l(t)/& = ed

- 2.303[M,J~~~1

x [l - (ZX,[R& + 1) - *.=p’k’]}

(5)

where l(t)PO is the time-dependent relative IR detector intensity. A least-squares fit of the experimental data to eqn. (5) with known values for the optical path length I and the initial monomer concentration [M,] leads to the derivation of values for k,/k, and k&,1. Buback et al. [19] then proceeded to use this experimental technique to investigate the chain length dependence of propagation and termination rate coefficients in ethylene polymerization. In this work, they subdivided the conversion time range into several intervals and subjected each period to a local analysis by fitting the data using a leastsquares procedure to eqn. (5) above. This yielded a series of k,/k, and k,[&] data for polymer radicals of increasing size. Assuming that [G] is constant throughout the data set, the variation of k, and k, with chain length becomes accessible. These results yielded a strong chain length dependence for k, and, surprisingly, a significant chain length effect on kp. However, subsequent work by Buback [20] and Schnoll-Bitai et al. [21] indicated an error in this analysis originating in the assumption that [&] remains constant. In fact, this can only be regarded as true for chain-length-independent rate constants. As pointed out by Schnoll-Bitai et al. [21], “it is impossible to obtain two independent time profiles, k,(t) and k,(t), from one single experimental time profile, [M](t)“. In addition, the fitting procedure needed to derive the rate constant data required a curve fitting procedure to five parameters; however, the accuracy of the experimental data was not sufficient for this multiparameter fitting. If [&] can be measured, this will give direct access to both k, and k,. A method for this has recently been proposed and is described later. Subsequently, Buback and Schweer [22] analysed the chain length dependence of k, for ethylene polymerization assuming chain-length-independent values for the propagation rate constant. In this case the chain length dependence of k, can be expressed by a power law k,(t) =kt, o[@)l”

(6)

where k,%, is the termination rate coefficient of the primary radicals. The chain length of growing radicals (at low conversions) can be expressed by J+) = k,[M&

(7)

[M,] is considered to be constant during single pulse experiments. An expression for the timedependent radical concentration can be obtained assuming a second-order termination reaction [R’](t)/[~]=[KIRO](a+l)-V+‘+l]-’

(8)

with K= 2k,(k,W,])”

(9)

From eqn. (1) the rate of polymerization d[M]/&=

is obtained

-k,[M][Rb][q&](cy+l)-‘r”+‘+l]-’

(IO) Utilizing this rate expression together with data fitting to a conversion-time plot for a single pulse experiment yielded values of Q: for ethylene of around - 0.20. Further work by Buback and Schweer [23] has investigated the effect of conversion on the rate parameters. For ethylene polymerization, k, was found to be constant up to 50% conversion where the viscosity of the reactants exceeded the viscosity of pure ethylene by five orders of magnitude. This study on the effects of conversion was extended to butyl acrylate, and concentrated on the reactive diffusion control of k,. In the conversion regime where termination is solely by reactive diffusion k,,, = constant

x k,[M]

(11)

or (kp/ktjRD = constantW1[M]-’

(12)

Buback et al. [24] studied the dependence of k,/k, for butyl acrylate up to 60% conversion, and showed that the termination step in butyl acrylate appears to be controlled by reactive diffusion over a wide conversion range (7%-57%, not just at the final stages of conversion as suggested previously). This fascinating result has several ramifications, as in this conversion region k, should not be chain length dependent (as it is controlled by kp). The generality of reactive diffusion control over termination needs to be evaluated for other monomer systems. More recently, Buback et id. [25] have studied the conversion dependence of rate coefficients for styrene using a modified piLocedure yielding [&I, k, and k, from a single polymerization experiment. The procedure combines IR and/or nearIR measurement of monomer conversion as a function of laser repetition rate with an IR spectroscopic estimation of [Rb] via the quantitative determination of end groups introduced by the photoinitiator.

4

T.P. David khzser-initiated poiymerimtion

4. Kinetics from pulsed-laser polymerization (PLP) and molecular weight analysis The initial work in this area was by Aleksandrov et al. [26] who derived equations describing the molecular weight distribution of a polymer formed by PLP. One of the significant advantages in utilizing a pulsed laser as an initiation source is that the initiation process is very rapid with respect to the time taken for primary radical addition to the monomer. This allows simplification of the kinetics to provide treatment of initiation as an instantaneous process. Aleksandrov et al. [26] compared experimental data for the PLP of methylmethacrylate (MMA) with their calculated predictions and achieved satisfactory results despite the fact that they were limited to viscometry for their molecular weight analysis. This work was followed 8 years later by Olaj at the University of Vienna, who went on to develop the PLP method into a promising new polymerization kinetics technique for deriving absolute propagation rate constants. Olaj et ~2. [27] derived the following polymerization rate expression for PLP

Y’R,*_k [Ml f-

2

Thus k,k, can be obtained from a plot of y 8s. In tr or a plot of y us. ln[I] where [I] is the concentration of photoinitiator which, in turn, is considered to be proportional to the radical concentration p

kP k In(&) (

y = k, k, In(k&)

Y= kJM& This process is repeated many times molecular weight distribution of the formed will show a peak characteristic

(17) and the polymer of PLP,

h[l+y [l+(l+&)lE]]

(13) where zf is the dark time between laser pulses and p is the pseudostationary radical concentration. The left-hand side of eqn, (13) gives the fraction of monomer polymerized per pulse. As with the established rotating sector method, the ratio k,l k, appears in this expression. k,,lk, is extracted by selecting experimental conditions such that pk,t,s 1. In this way a series expansion of the expression under the square root is possible. The result is

Y=

Olaj et al. [27] tested their theory with experiments with styrene and obtained a value for k,/k, of 1.0X 10m6. The separation of the two individual rate parameters kp and k, required a knowledge of p by this method. Unfortunately p is not readily accessible. Subsequently Olaj et al. [28] developed a technique whereby the propagation rate constant was obtainable directly from the molecular weight distribution of the polymer formed by PLP. The PLP process is illustrated in Fig. 1. The laser pulse instantaneously generates a population of radicals; the radical concentration decays according to a second-order rate law, until the next pulse, when a large concentration of new radicals is formed. At this point, those surviving radicals from the first pulse are subject to a vastly increased probability of termination. The chain length of the polymer formed between two consecutive pulses is given by the simple expression

-t 2 In tf t + 2 ln[I] t

where p =qI]

(161

5

u

Fig. 1. A schematic representation of the pulsed-laser polymerization process. A population of radicals is generated “instantaneously” by a laser pulse. These radicals decrease in concentration during the dark period, their decay is governed by a second-order rate law. At the next laser pulse, a new population of radicals is generated. Any radicals that survive the preceding duration of the dark period are subject to an increased probability of termination at this point. This sequence of events is repeated many times, resulting in the formation of polymer chains with a characteristic chain length distribution.

T.P. Davis I Laser-initiated poiymetization

which can be related directly to k,. Olaj et al. [28] also showed that Poisson broadening could be accounted for by taking the inAexion point on the low-molecular-weight side of the molecular weight distribution as the best estimate of Lo, the chain length generated between laser flashes, as illustrated in Fig. 2. Olaj et al. [28] reported a k, value for styrene at 25 “C of 107 1 mol-’ s-’ measured at a chain length of 2740. They subsequently amended this value to 80 1 mol-’ s-’ on reevaluating their GPC (gas permeation chromatography) data. A similar problem with accurate GPC data was alluded to by Davis et al. [29] and is pertinent since the technique is totally reliant on accurate and precise GPC data. This experimental procedure was adopted by Davis and coworkers [29-331 in a series of papers investigating both homopolymerization and copolymerization kinetics. All the homopropagation k, Arrhenius parameters reported in the literature obtained using PLP are listed in Table 1.

tf = i 4iL J L (b)

(cl tf-

set

0.33 set

Fig. 2. GPC chromatograms for poly(methylmethacrylate) polymerized by a pulsed-laser technique. The chain length generated between consecutive laser pulses is obtained from the inflexion point. The second peak in chromatogram (b) has the same retention time as the primary peak in chromatogram (a) and corresponds to the polymer chains that survive for the time period 2,.

5

An extension of this procedure was suggested by Holdcroft and Guillet [36] who used a pulsedlaser technique in a microemulsion system. In the microemulsion system the low-molecular-weight tail typical of PLP samples polymerized in solution is not present. In solution this arises from natural bimolecular termination between pulses; however, this does not occur in emulsion systems. This work was extended to a dual laser technique [37]. The initiation and termination processes were controlled independently using two laser pulses of different wavelength. Photoinitiation using AIBN (azobisisobutyronitrile) was followed by termination by flash photolysis of 2-naphthylmethyl-lnaphthylacetate (NMNA).

The idea behind this work was to eradicate the low-molecular-weight tail (as described above) and the high-molecular-weight species that originate from radicals which survive for periods greater than I~,thereby generating monodisperse polymer. In fact this was complicated by a two-photon absorption process that resulted in a mechanism for the photoaddition of naphthylmethyl groups to the polymer chain. Olaj and Schnoll-Bitai [38] proceeded to demonstrate that k, values are- also obtainable from the molecular weight distribution by deriving the equation v,,Pw= (3 - S)k,2[M]z/k,

(18) where 6 is the relative contribution of disproportionation to total termination. This means that the ratio kp2/k, is readily accessible from the product v,PW utilizing the same molecular weight distribution from which k,, is derived. They reported data for styrene in bulk and in toluene solution and for MMA in bulk all at 25 “C [39]. Very recently O’Driscoll and Kuindersma [40] have called into question the suitability of PLP. for deriving k, from these experiments. They

T.P. David / Laser-initiatedpoiymerirntion

6 TABLE

1. Arrhenius

parameters

for propagation

rate constanfs

derived

from pulsed-laser

Monomer

Comments

FrequeIKy factor (x10-6)

Styrene

Bulk

19.9

p-Methoxystyrene

Bulk

Methylmethacrylate

Bulk

Ethylmethacrylate

Bulk

polymerization

experiments

E.4 (kJ mol-‘)

Reference

30.78

31

0.59

23.W

33

0.492

18.10

31

1so

20.46

31

3.44

23.3

31

0.293

16.19

31

n-Butylmethacrylate

Bulk

Lamylmethacrylate

Solution toiuene

Acrylamide

pH 1, 0.5 mol drr-’ aqueous solution

Not given

-20

pH 4, 0.5 mol dm-’ aqueous solution

Not given

-20

Methacrylamide

pH 1, 1 mol dm-’ aqueous solution

Not given

2Q

34

t-Butylmethacrylate

Bulk

25.1

27.7

35

50% (v/v)

modelled the PLP process using a Monte Carlo procedure and allowed for the fact that k, is not a constant, but chain length dependent. In this non-stationary process radicals of different lengths are continuously being generated and any value of k, obtained will be an average value and will be dependent on the molecular weight distribution (and hence the pulse frequency). It now appears unlikely that a single PLP experiment will yield reliable data on both k, and k,. An experimental analysis of the effect of laser pulse repetition rate, laser light intensity and photoinitiator concentration on the molecular weight distribution was carried out by Hoyle et al. [4143] on a number of monomer systems. This work was essentially qualitative in nature and demonstrated the effects of pulse repetition rate and initiator concentration on the molecularweight distribution. They also simulated the molecular weight distributions using a sequence of successive Poisson distribution functions [43, 441.

5. Studies on crosslinking polymerizations Decker and Moussa [45] have utilized a krypton ion laser operated in the continuous wave mode as an initiating source to study a number of crosslinking reactions. Real time IR (RTIR) spectroscopy was used to study the conversion profiles of the monomers under investigation.

34

Decker and Moussa [45] utilized the classical rate equation to analyse their results Rp=kp/(2kJo~5[~O(l -e-2.“)Oi]0~5[M]

(19)

where A is the absorbance of the sample, 1, is the photon flux and 0, is the initiation quantum yield. However, the use of this equation for studying high conversion crosslinking polymerization is questionable. This equation can only apply to Iow conversion kinetics, and it does not apply to higher conversion regimes where termination is reduced by either the Trommsdorf effect or by crosslinking reactions that will induce a similar kinetic response from the polymerization system. It is well known that the relationship R, a k,lkt2 is derived assuming a steady state hypothesis. Typical conversion-rate profiles for photopolymerizations involving either acrylates or crosslinking monomers show clearly that only an instantaneous R, is accessible, as the steady state assumption is not applicable. Despite the shortcomings of their kinetic analyses, the work of Decker and Moussa [45] illustrates the applicability of laser-induced polymerization and shows the very high rates of cure that can be achieved. Hoyle and Trapp [46] utilized a pulsed-laser initiating source coupled with a DSC set-up for monitoring conversion. Studies on 1,6-hexanediol and trimethylolpropane triacrylate diacrylate showed the versatility of the equipment for studying crosslinking reactions.

T.P. Davis f Laser-initiated

and Jenkins [47] and Hoyle et al. [48] developed methods for quantifying oxygen inhibition in laser-initiated polymerization. Both Decker

6. Conclusions The work described in this review clearly demonstrates the recent advances made in the measurement of polymerization rate Constants. These advances are largely due to the utilization of laser light sources. These new experimental techniques have provided data on both termination and propagation kinetics which have given considerable insight into the failure of the terminal model in copolymerization and the significance of reactive diffusion in termination reactions. As a result an IUPAC working party [49] has recommended these

procedures

for future kinetic analysis.

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