Photoinitiation of Polymerization by Chloro-oxobis(2,4- pentanedionato)vanadiurn(v) in the Presence of Electron Donors BY S . M. ALIWI AND CLEMENT H. BAMFORD* Department of Inorganic, Physical and Industrial Chemistry, Donnan Laboratories, University of Liverpool L69 3BX Received 17th April, 1974 When a strong electron donor D, such as dimethyl sulphoxide (DMSO) or one of a wide range of amino-compounds, is added to VO(a~ac)~Cl a colour change from deep blue to pale green-yellow occurs. The change appears to be complete if [D] is slightly in excess of [VO(a~ac)~Cl] and all strong donors examined give effectively similar final absorption spectra. At the same time the solution acquires marked electrical conductivity. It is proposed that ion-pair complexes of the type (I) / o\+ (0 are formed between donor and chelate The complexes are effective photoinitiators of frwradical polymerization ; the reactions occurring at A = 365 nm with D = DMSO and 3) = pyridine (Py) have been studied with methyl methacrylate as rnoa~mer.The polymerization is not complicated by retardation and the rate of initiation at a volume fraction of additive ua = 0.1 is independent of monomer concentration (with benzene and ethyl acetate as diluents). The quantum yields of initiation are relatively high, 0.59 and 0.125 for D = DMSO and Py, respectively ; these values are 29 and 6 times those obtained with VO(acac),Cl in the absence of D. When D = DMSO both chlorine and DMSO residues are found in the poly- mers, but with D = Py only chlorine has be& detected.Negligible quantities of acetylacetone fragments are incorporated in either case. The products are VIV derivatives, VO(acac)zD with D = Py and VO(acac),D and VOC12D3 with D = DMSO. In the latter case acetylacetone is also formed ; both this product and VOC12DJ are considered to arise from interaction of the chelate with HCl produced in another secondary reaction. Spectrophotometric measurements show that the rate of decomposition of (I) for a given incident intensity is independent of 0,; on the other hand, the rate of initiation decreases markedly with increasing ua for Va > 0.1, approximately. Several reaction mechanisms are discussed and it is concluded that the observations are consistent with a primary act consisting of electron transfer from D to vanadium.Secondary processes in- volving the resulting radical cation D+ and C1- occur leading to C1 atoms and also, when D = DMSO, to cH2SOCH3 and HCI; both el and CH2SOCHJ initiate polymerization. With increasing ua, solvent separation of Df- and C1- retards reaction between these species and D t is consumed in a competitive processes ; the decrease in the rate of initiation with increasing Ua is thus understandable. The relevant kinetic parameters for this mechanism are evaluated. During the course of this work it was found that pyridine has a greater influence on the propaga- tion coefficient of methyl methacrylate than any other solvent so far examined. The presence of electron donors often has a marked influence on the thermal initiation of free-radical polymerization by transition-metal chelates, evidenced by a greatly enhanced rate of initiation and the appearance of monomer-selectivity.v 2 Information about the photoinitiation by chelates is relatively scanty, but unexpected 52S, M. ALlWI AND C. H. BAMFORD 53 solvent effects have been found in polymerizations photoinitiated by MnIII(fa~ac)~ (facac = CF3COCHCOCH3). In this paper we report an extension of our work on photoinitiation by VO(a~ac)~Cl [(acac) = CH3COCHCOCH3] to systems containing the electron-donors dimethyl sulphoxide (DMSO) or pyridine (Py). These donors, and many others, convert VO(acac),Cl into ion-pair complexes [D-+VO(aca~)~l+Cl- (D = electron donor), so that the nature of the photosensitizing species is fundamentally changed by the additive.The systems with which we are concerned may therefore be expected to show analogies with the ion-pair complexes Fe"'X- (X- = OH- or C1-) first studied by Evans, Santappa and UriS These workers demonstrated that on irradiation of FelILX- (A = 310, 365, 405 nm) free radicals or atoms are generated which can initiate the free-radical polymerization of vinyl monomers such as acrylonitrile, methyl methacrylate or methacrylic acid in aqueous solution at 25"C, e.g. Fe1ILCl- + hv+ Fe" + c l I i I I e l + CH2=C+C1-CH2-C. etc. (1) Initiation occurs with low quantum yields ( 5 x lo-' and 0.13 for X- = OH- and Cl-, respectively) and the rate of initiation was shown to be independent of monomer concentration and equal to the rate of consumption of ferric ion. This type of photo-chemical activity is quite general and many examples of it are now known.6 We shall see that photoinitiation by [D+VO(acac),]+Cl- is more complex, exhibiting features which are not implicit in (1).EXPERIMENTAL The techniques were as described in a previous paper with the addition of the following. Gas-liquid chromatography was carried out with a Pye-Unicam gas-liquid chromato- graph (model 24) employing dual flame-ionization detectors, a column (2m) containing 5 % silicone on Embacel (60-100 mesh) at 82°C was used. The participation of acetylacetonate radicals in initiation was examined by comparing the radioactivities of polymers prepared from VO(gcac)2C1 having ligands labelled with 14C with those of the labelled chelate itself. Radioactivities were measured by internal sample liquid scintillation counting with a Packard 3003 Tri Carb liquid scintillation spectro- meter. A standard toluene-based organic scintillator was employed ; in all cases the chemical compositions of the solutions, and hence the counting efficiencies were identicaL2 MATERIALS VO(aca~)~Cl was synthesized as described by Funk, Weiss and Zeising ; dichlorotri- (dimethylsulphoxide)oxovanadium(w) (VO(DMS0)3C12) was prepared as reported by Selbin and Holmes.* Azobisisobutyronitrile (Koch-Light) was recrystallized twice from AnalaR grade methanol and once from chloroform. It was then stored in the dark in vacuum.Dimethyl sulphoxide (B.D.H.) was dried for one week by molecular sieves, then frac- tionated under nitrogen (14mmHg) as recommended by Martin, Weise and Niclas.lo Scavenging of DMSO was effected by the technique of Atkinson, Bamford and Eastmond.2*11 Pyridine (B.D.H., AnalaR) was dried over fresh barium oxide for 24 h before being frac- tionated under an atmosphere of nitrogen.12 Benzene (B.D.H., AnalaR) was dried with sodium-wire for 24h, then distilled in vacuum and stored in the dark under nitrogen.Ethyl acetate (Hopkin and Williams, AnalaR) was dried for one week over anhydrous potassium carbonate and then fractionally distilled (b.p. 77°C at 76 mmHg). Methanol (AnalaR) was used without further purification. VO(acac)zC1 with labelled ligands was prepared from [l ,3-14C2]acetylacetone, synthesized by Claisen condensation l3 from [1,3-14Cz]acetone (Radiochemical Centre, Amersham) and ethyl acetate.Methyl methacrylate was purified by the method of Bamford and Lind.954 PHOTOINITIATION OF POLYMERIZATION RESULTS AND DISCUSSION KINETICS OF PHOTOINITIATED POLYMERIZATION The rate o of photoinitiated polymerization (A = 365 nm) of methyl methacrylate in the presence of dimethyl sulphoxide or pyridine is shown in fig. 1 to be proportional to ~ O ( a ~ a c ) ~ C l ] + over the range studied for constant incident intensity I. and reactant composition (volume fraction of additive v, = 0.1 in each case). Fig. 2 indicates that, for constant [VO(a~ac)~Cl], (I) is proportional to I$ for the system containing DMSO. Polymerization under these conditions is therefore an uncomplicated free-radical process. The overall rate observed cu, contains small contributions from thermal and uncatalyzed reactions (rates cot, (I),, respectively). The latter were 102[VO(acac)2CI]~/mol~ dm-4 FIG.1 .-Dependence of initial rate of photosensitized polymerization of methyl methacrylate at 25°C on [VO(acac),Cl]*. 0, Dimethyl sulphoxide as additive, Va = 0.1 ; 0, pyridine as additive, va = 0.1 ; h = 365 nm ; I. = 1.72 x einstein dm-3 s-l. 1041t/einstein* dm-3 s-* FIG. 2.-Dependence of initial rate of photosensitized polymerization of methyl methacrylate at 25°C on 18. [VO(a~ac)~Cl] = 1 . 0 ~ mol dm-3 ; Va(DMS0) = 0.1 ; A = 365 nm.S. M. A L I W I AND C. H. BAMFORD 55 measured directly under the appropriate conditions, cot without irradiation but with all components present and co, with irradiation but with monomer alone in the dilato- meter.Both cot and co, were less than 3 % of coo. Values of co in fig. 1 and 2 have been derived from coo by means of the relation 0 = (co:-w:-co:)+. Values of the kinetic parameter k,k, (kp, k, being the rate coefficients of propaga- tion and second-order termination, respectively) were determined as a function of u, from observations of rates and degrees of polymerization, with the aid of relation (1) of the previous The transfer constants to monomer and DMSO have values of 2 x and 7.1 x (60°C), r e ~ p e c t i v e l y . ~ ~ * ~ ~ Under our conditions, transfer has a negligible effect on molecular weights and calculations of k,k;+. In these experiments the conversion of monomer was kept below 3%. The results, presented in fig. 3 and 4, show that in both systems k,k;+ increases markedly with u,.- - - - - 1.8 1.6 1.4 o a 1.2 5. 2 1.0 -0.8 -0.6 0.4 0.41 I I I ' ' ' I ' I .o 0 0.2 0.4 0.6 0.8 v,(DMSO) FIG. 3.-Dependence of kpkt-* and kp/k; for methyl methacrylate at 25°C on volume fraction of dimethyl sulphoxide. 0, kpkF* ; photoinitiation by VO(acac)zC1 + DMSO ; [VO(acac),CI] = 1 . 0 ~ mol dm-3 ; h = 365 nm. A, k,kt-* ; photoinitiation by azobisisobutyronitrile (5 x mol dm-3). 0, kp/kg. 0.4 2 2 .o 1.6 1.2 0.8 0.4 0.0 A? 0 0 . 2 0.4 0.6 0.0 1 .o v,(p yridine) FIG. 4.-Dependence of kpkt-* and kp/kg for methyl methacrylate at 25°C on volume fraction of pyridine. mol dm-3 ; 0, kPkt-* ; photoinitiation by VO(acac)2C1 + Py ; [VO(acac),Cl] = 1 .O x h = 365 nm. 0, k,/k;.56 PHOTOINITIATION OF POLYMERIZATION If it is assumed that k, is affected only by the viscosity of the reaction medium, being inversely proportional to the latter as demonstrated by North and Reed,16 relative values of kp may be calculated from these observations and are shown in fig.3 and 4. Clearly both dimethyl sulphoxide and pyridine belong to the class of additives which significantly increase the observed propagation coefficient of methyl methacrylate. Our findings with DMSO agree within experimental error with those of Bamford and Ferrar;2 the behaviour of pyridine in this context does not appear to have been previously recorded, but comparison of fig. 3 and 4 reveals that its “activity” exceeds that of DMSO and is therefore greater than that reported for any other diluent. A recent discussion of phenomena of this type has been given by Bamford.17 Some values of k,k;* obtained with photoinitiation by azobisisobutyronitrile are shown in fig.3 and fall into line with those already mentioned. Table 1 shows that the apparent values of k,k,& for reactions photo-initiated by VO(acac)2C1 + DMSO (0, = 0.166) are not significantly dependent on [VO(acac),Cl] ; these data are therefore consistent with the effective absence of chain-transfer, retardation and primary termination. TABLE l.--k,k;* AS A FUNCTION OF [VO(a~ac)~Cl] WITH DMSO AS ADDITIVE; Ua = 0.166, A = 365 nm, 25°C 104[VO(acac)~Cl]/mol dm-3 0.666 1.00 2.00 4.00 5.00 8.00 10.00 20.00 30.00 k P t k-*/mol-* dm3 s-3 0.0649 0.0695 0.0713 0.0675 0.0639 0.0642 0.0637 0.0669 0.0641 mean: kPkt-* = 0.0662 mol-* drn’s-’ The order of the overall reaction in monomer for 21, (DMSO) = 0.1 was determined with benzene and ethyl acetate as diluents.A small correction was applied in the latter case to allow for changes in viscosity of the medium,16 but no correction was necessary when benzene was the diluent. Fig. 5 presents log w against log [MI plots and shows that under the conditions studied the order in [MI is close to unity; the slopes in fig. 5 are 0.99 and 1.08 for benzene and ethyl acetate, respectively. The rate of initiation is therefore effectively independent of the monomer concentration. 0 0.2 0.4 0.6 0.8 1.0 log[MMA]/mol dm-3 FIG. 5.-Initial rate of photosensitized polymerization of methyl methacrylate at 25°C as function of monomer concentration. [vO(a~ac)~Cl] = L O X mol dm-3 ; A = 365 nm; lo = 1.8 x einstein dm-3 s-l.0, diluent ethyl acetate ; 0, diluent benzene.S. M. ALIWI AND C . H. BAMFORD 57 o, 0.1 0.2 0.3 0.4 va(DMSO) FIG. 6.-Dependence of initial rate of photosensitized polymerization of methyl methacrylate at 25°C on volume fraction of dimethyl sulphoxide. [VO(aca~)~Cl] = 1.4 x mol dm-3 ; h = 365 nm ; I , = 1.8 x 10-6einstein dm-3 s-l. ..I kn 40.8 Ua(DMS0) FIG. 7.-Dependence of rate and quantum yield of initiation at 25°C on volume fraction of dimethyl sulphoxide. [VO(aca~)~Cl] = 1.4 x mol dm-3 ; h = 365 nm ; lo = 1.8 x 10-6einstein dm-3 s-l. -, $calculated from rates of polymerization(fig. 6) and values of kPkt-+ (fig. 3) with the aidof eqn(2) ; ---- , -d[I]/dt. va(pyridine) FIG. 8.-Dependence of rate and quantum yield of initiation of 25°C on volume fraction of pyridine.[VO(a~ac)~Cl] = 1 .O x 10-6einstein dm-3 s-l. -, 9 calculated from rates of polymerization and of kpkc8 (fig. 4) with the aid of eqn (2) ; - - -, - d[I]/dt.58 PHOTOINITIATION OF POLYMERIZATION The results so far described show that the rate of polymerization conforms to the conventional relation (2), in which [MIo is the bulk monomer concentration and 9 the rate of initiation. U) = kpkc3[M]f~ = kpk,'[M]o(l -~,)9'. (2) This equation has been used in deducing rates of initiation from observations of w over a range of Va with the aid of the results in fig. 3 and 4. The variation of cr) with va (DMSO) is illustrated in fig. 6 and values of Y calculated from w against va curves for the two additives are presented in fig.7 and 8. The dependence of 9 on v, is similar for DMSO and Py ; a maximum in 9 occurs near o, = 0.1, It follows from the results in fig. 1-3 that, for oa = 0.1, the rates of initiation are given by DMSO : 9 = 136.9 Io[VO(acac)2Cl]mol dm-3 s-l ( 3 4 Py : 9 = 23.8 Io[VO(acac)2Cl]mol dm-% s-l (3b) QUANTUM YIELDS These were determined by the techniques described in the previous paper and the results are displayed in tables 2 and 3. Quantum yields for initiation and the overall reaction are denoted by 4i and 40, respectively. In bulk methyl metha- crylate 4i = 2.06 x ; thus it is clear that addition of DMSO or Py (u, = 0.1) produces a large increase in the quantum yield of initiation (by factors of 29 and 6, respectively). TABLE 2.-QUANTUM YIELDS IN METHYL METHACRYLATE+ DIMETHYL SULPHOXIDE (Ua = 0.I). [VO(aca~)~Cl] = 10-3mol dm-3 ; 3, = 365 nm ; 25°C ; kpkL3 = 0.062 mol-3 dms s-*. lO6I0/ lo71abs/ 1 0 4 4 10~91 40 4i einstein dm-3 s-* einstein dm-3 s-1 mol dm-3 s-1 mol dm-3 s-1 1.91 3.97 2.55 2.35 642 0.59 1.91 3.97 2.50 2.26 630 0.57 1.70 3.53 2.26 1.84 640 0.52 1.70 3.53 2.58 2.40 730 0.68 mean : 4i = 0.59 TABLE 3.-QUANTUM YIELDS IN METHYL METHACRYLATE+ PYRIDINE (Va = 0.1). [VO(aca~)~Cl] = mol dm-3 ; 3, = 365 nm ; 25°C ; k,k,* = 0.063 mol-3 dm3 s-3. 1061,1 1071~bs 1 0 4 4 1 0 8 9 1 40 4i einstein dm-3 s-1 einstein dm-3 s-1 mol dm-3 s-1 mol dm-3 s-1 2.37 4.64 1.31 5.88 282 0.127 2.37 4.64 1.28 5.63 276 0.122 mean : $i = 0.125 IDENTIFICATION OF TERMINAL GROUPS I N POLYMERS Experiments with VO(a~ac*)~Cl prepared from acetylacetone containing [1 ,3-l4CZ] acetylacetone were carried out to examine the participation of acac radicals in initia- tion.Poly(methy1 methacrylate) specimens prepared by photoinitiation with VO(a~ac*)~Cl in the presence of dimethyl sulphoxide (u, = 0.166) were purified by several reprecipitations into methanol and submitted to scintillation counting. In a typical experiment it was found that the number of counts per mole of polymer was 9.3 x 106/100 s, the number per mole of initiator (chelate) being 2.6 x 108/100 s.S . M. ALIWI AND C . H . BAMFORD 59 The proportion of growing chains with acetylacetone terminations was therefore less than 3 %. Thus acat radicals are not significantly involved in initiation. Chlorine contents were determined by neutron-activation analysis, with results given in table 4.Initial determinations by this technique of the sulphur contents of polymers prepared in the presence of DMSO gave high values, corresponding to 4.77 S-atoms per polymer chain. Related findings with DMSO labelled with 35S have been recorded by other workers '9' using different initiators (including azobis- isobutyronitrile) and were attributed to the presence in DMSO of radioactive im- purities capable of copolymerizing with methyl methacrylate. Attempts were there- fore made to purify the DMSO by scavenging the impurities by radicals obtained from azobisisobutyronitrile, following the technique of Atkinson, Bamford and Eastmond." Two such treatments led to a considerable reduction in the sulphur content of the polymers, although it was clear that a significant amount of impurity remained in the DMSO.It was therefore decided to estimate the extent of incorpora- tion of sulphur in the polymer during photoinitiation by VO(a~ac)~Cl+ DMSO by an indirect method. Polymers were prepared at 25°C from methyl methacrylate + (scavenged) dimethyl sulphoxide (u, = 0.05) with photosensitization (A = 365 nm) either by the VO(acac),Cl + DMSO complex or by azobisisobutyronitrile (AZO) ; conditions were arranged so as to obtain effectively equal rates of polymerization in these experiments. After suitable purification by reprecipitation, the polymers were submitted to neutron-activation analysis and the difference in sulphur contents was attributed to incorporation of sulphur-containing fragments on photoinitiation by VO(acac),Cl + DMSO.This technique has evident disadvantages, but a satisfactory alternative could not be devised without the availability of pure DMSO. Results are summarized in table 4. TABLE 4.-NEUTRON-ACTIVATION ANALYSIS OF POLYMERS. initiating system 10-5Fn no. of C1 atoms S atom per polymer molecule VO(acac),Cl+DMSO 0.962 0.40rfr0.03 1.12kO.13 AZO+DMSO 1.12 0.0 0.41 0.08 VO(a~ac)~Cl+ Py 1.39 0.92rfr0.11 0.0 (Ua = 0.05) (u, = 0.05) (Va = 0.1) [VO(acac),Cl] = mol dm-3 It appears that photoinitiation by VO(acac),Cl + DMSO leads to polymer molecules containing on the average 0.40 Cl and 0.71 S atoms. Since the ratio of combination to disproportionation in the termination reaction at 25°C is 0.52,19 the total endgroup content arising from initiation should be 1.21 ; hence initiation by e l and by DMSO fragments accounts for nearly all the observed photoinitiation by VO(acac),Cl + DMSO.In the case of VO(a~ac)~Cl+Py, the bulk of the initiation arises from C1 atoms. We have previously shown that photoinitiation by VO(a~ac)~Cl in the absence of strong electron donors proceeds through formation of chlorine atoms. SPECTRAL A N D CONDUCTIVITY OBSERVATIONS ON COMPLEX FORMATION Addition of a strong electron donor such as dimethyl sulphoxide, pyridine or one of a wide range of amino compounds to a solution of VO(acac),Cl in methyl metha- crylate produces (in inactive light) a colour change from deep blue to pale greenish- yellow as illustrated by the absorption spectra in fig. 9. The change appears to be60 PHOTOINITIATION OF POLYMERIZATION complete for concentrations of electron donor slightly in excess of [VO(acac),Cl] and further addition produces no effect on the spectrum.All strong donors give final spectra of the same type, which is independent of the solvent initially present (methyl met hacry late, benzene, cyclo hexane , acetone, e t h y 1 acetate) . 325 350 400 450 500 550 600 650 700 750 800 wavelength /MI FIG. 9.-Absorption spectra. Solvent methyl methacrylate ; 25°C. (1) VO(a~ac)~Cl(l.O x rnol dm-3); (2) VO(acac)2Cl ( 1 . 0 ~ mol dm-3)+DMS0 (Va = 0.1); (3) VO(acac)2C1 ( 1 . 0 ~ loA3 mol dm-3)+DMSO(~, = 0.1) after irradiation ( A = 365 nm) for 20 min ; (4) as (3), but after irradiation for 45 min. Pathlength 10 mm. We believe these results indicate the formation of a new complex between VO(acac),Cl and the donor additive D and propose that it is an ionic complex of the type shown in (I).(I) [ (acac), ij' V C1- D The band in the absorption spectrum of VO(acac),Cl in methyl methacrylate solu- tion with a peak near 600 nm corresponds to charge transfer from p-orbitals of C1 to d-orbitals of V, so that its disappearance on complex formation is under- standable. The extinction coefficients of the complexes at A = 365 nm are 101.2 and 96.0 mo1-ldm3 cm-' for D = DMSO and Py, respectively. TABLE 5 .-ELECTRICAL CONDUCTIVITIES OF VO(aCaC)2 c1 SOLUTIONS AT 25°C solution tVO(acac)~C11/ ua 1 0 6 ~ specific mcl dm-3 conductancelf2-1 cm-1 MMA 0.0 MMA+ DMSO 0.0 MMA+VO(acac)zC1 2.36 x MMA+ VO(acac)zC1 + DMSO 2 .0 ~ 10-3 2 . 0 ~ 10-3 2.0 x 10-3 2.0 x 10-3 2.0 x 10-3 M MA + VO(acac)2 C1 + PY 2.37 x 2.37 x 2.37 x 1W2 2.37 x 0.0 0.1 0.0 0.1 0.2 0.4 0.6 0.8 0.1 0.4 0.6 0.8 0.1 0.1 0.12 8.2 13.0 17.3 20.0 24.0 0.63 1.5 4.2 9.8S. M. ALIWI AND C . H . BAMFORD 61 In agreement with the views expressed above, we find that addition of the donor is accompanied by a large increase in electrical conductivity. Some typical results are presented in table 5. On addition of pyridine to a concentrated solution of VO(acac),Cl in ether light green crystals separated. After washing with ether and drying these gave the following elemental analysis ( %) : C 48.4, H 4.8, N 4.01. Calculated values for (I) (D = Py) are C 47.15, H 5.04, N 3.71. Attempts to isolate the complex formed between VO(acac)2C1 and DMSO have not been successful.CHEMICAL CHANGES PRODUCED BY IRRADIATION Irradiation at 25°C of a solution of VO(acac),Cl in methyl methacrylate containing DMSO by light with A = 365 or 436 nm produces a colour change from greenish- yellow to pure green; the corresponding spectra are shown in fig. 9. Essentially similar results are obtained when DMSO is replaced by Py (fig. 10). The low- intensity peaks at 770, 650 and 400 nm for the DMSO complex which develop on irradiation are near those typical of d-d transitions in V" (e.g. vanadyl ion) deriva- tives,20p21 indicating that reduction of Vv (do system) to VIV (dl system) has occurred. 1.0 - 0.9 = 0.8 - ?? 0.7 - 8 0 . 6 - rn - 0.5 - *= 0.4 - a 0.3 - 0.2 0.1 .-. 8 - - 350 400 450 500 550 660 650 o* ' wavelength /nm FIG. 10.-Absorption spectra.Solvent methyl methacrylate ; 25°C. (1) [VO(aca~)~Cl] = 1.0 x mol dm-3 +Py (ua = 0.1) ; (2) as (l), after irradiation for 45 min ; (3) as (l), after irradia- tion for 90 min ; (4) as (l), after irradiation for 150 min. - - -, spectrum of VO(acac),(l.O x lo-' mol dm-3)+Py (ua = 0.1). Wavelength of irradiation 365 nm ; pathlength 10 mm. The precise location of the bands depends on the nature of the E.s.r. observations confirm this photo-reduction. Both VO(acac),Cl and the complex (I) are diamagnetic and do not give an e.s.r. spectrum. On irradiation of the DMSO complex an e.s.r. spectrum develops steadily as shown in fig. 11 ; this spectrum is typical of those of V02+ derivatives e.g. vanadyl porphyrin 23 and vanadyl acetyl- acet onate.24 After prolonged irradiation at 25°C of VO(acac)2C1+DMS0 in ethyl acetate or benzene solution (A = 365 nm) pale green-blue crystals separated out with a composi- tion corresponding to VO(DMS0)3C12 : found C 19.41, H 4.97, Cl 19.09, S 27.26% ; calculated C 19.35, H 4.88, C1 19.20, S 26.00%. The crystals had U.V. and i.r. spectra identical with those of a specimen of VO(DMS0)3C12 prepared from VOClz and DMSO.* Examination of the irradiated benzene solution by gas-liquid chroma- tography established the presence of acetylacetone, this being the only detectable volatile product.62 PHOTOINITIATION OF POLYMERIZATION Different results were obtained with VO(acac),Cl+ Py in benzene solution. Prolonged irradiation in this case does not yield any precipitate and the final absorp- tion spectrum is indistinguishable from that of the VO(acac),Py adduct (fig.10, dotted line). Further, no acetylacetone could be detected by gas-liquid chromato- graphy after irradiation. The significance of these observations is discussed below. -+ H FIG. 11.-E.s.r. observation at 77 K ; methyl methacrylate solution. mo 1 dm-3. (1) VO(acac),Cl ; (2) VO(acac)2C1+DMS0 (0, = 0.1) before irradiation ; (3) as (2) after [VO(a~ac)~Cl] = 1.0 x irradiation for 3 min ; (4) as (2) after irradiation for 8 min. h = 365 nm. RATES OF COMPLEX DECOMPOSITION A N D RADICAL YIELDS The spectral changes accompanying irradiation described in the previous section and illustrated in fig. 9 and 10 may be used to estimate rates of decomposition of the ion-pair complex.These, together with the rates of initiation of polymerization, (fig. 7 and 8) enable the radical yield n (i.e. the number of initiating radicals produced by photolysis of one molecule of complex) to be deduced. TABLE 6.-VALUES OF k d AND F2 AT 25°C. OPTICAL DENSITY MEASUREMENTS AT 775 nm. [VO(a~ac)~Cl] = 10-3mol dm-3 ; u,(DMSO) = 0.1 ; DILUENT BENZENE ; A(irradia- tion) = 365 nm; I,, = 10-6einstein dm-3 s-' - 107 d[o1 volume 104ki/s-' d t I 10~91 n fraction cf mol dm-3 s-1 mol dm-3 s-1 monomer 0.90 1.72 6.19 4.97 0.80 0.55 1.51 5.48 4.97 0.91 0.18 1.71 6.21 4.97 0.80 Optical densities at wavelengths of 775,400 and 340 nm were measured as functions of the time of irradiation. In all cases it was found that A , - A , decreases exponen- tially with time (A, being the optical density after irradiation for time t ) , indicating first-order decomposition of the complex.From the slopes of the log (A, --At) against t lines the first-order decomposition coefficient k, was calculated. Table 6 presents values of kd for v,(DMSO) = 0. I, for a range of monomer concentrationsS . M. ALIWl AND C. H . BAMFORD 63 with benzene as diluent, together with the corresponding values of the rates of complex decomposition, and n. It is clear that under these conditions kd is not much affected by changes in [MI and that n has a value approaching unity. Table 7 shows kd for a range of values of v, for DMSO and Py as additives; no diluent was present in these experiments. Apparently kd is sensibly independent of Va in the range examined.Although n is approximately unity for small v,, it decreases steadily with increasing v,. The differences in kd values in tables 6 and 7 are probably within experimental error. Rates of decomposition of (I) calculated from the values of kd in table 7 are indicated in fig. 7 and 8 to facilitate comparison with #. Since the extinction coefficients are independent of v, the data presented in tables 2, 3 and 7 allow estima- tion of 4i as a function of u, (fig. 7 and 8). REACTION MECHANISMS We believe the experimental observations are best interpreted in terms of a primary photolytic act of electron transfer to vanadium. Two processes may be distinguished, depending on whether transfer occurs from (i) Cl- or (ii) D. Transfer from a chelate ligand appears to be excluded since there is no evidence for the participation of acetylacetonate radicals.In both cases a VIV derivative is formed ; the other primary products are (i) a C1 atom and (ii) a cation radical D'. We now consider the nature of the secondary reactions which must be invoked to explain the experimental data. (i) When D = Py, direct initiation of polymerization by el accounts for the ob- served chlorine content of the polymer. The spectral indications (fig. 10) that the final vanadium-containing product is VO(a~ac)~Py are consistent with this mechanism. If D = DMSO the situation is more complicated since the polymer contains not only C1 but also fragments of DMSO. This suggests that only a fraction of the C1 atoms initiate polymerization, the remainder entering into hydrogen-abstraction reactions with DMSO [eqn (4)] to give CH3SOCH2 radicals which subsequently initiate.(4) Reaction (4) is energetically unfavourable compared to direct addition of C1 to mono- mer, but this may not be important if (4) involves the complexed DMSO molecule in the original ion-pair or takes place before the excitation energy of the complex has been lost. The vanadium derivative resulting from the primary electron transfer is VO(acac),(DMSO), but we shall see that this may react further. (ii) If Dr- is the primary product it is necessary to assume that reaction with Cl- takes place. When D = Py this is simply electron transfer (5) ; the latter is followed by initiation by el : When D = DMSO two processes must be postulated to account for the end-groups in the polymer : (CH3)ZSO + Cl+CH,SOCH, + HC1 Pyt+Cl--+Py+Cl. ( 5 ) (CH3)2SOtCl-+(CH3)2S0 + C1 (6) (CH3)2SOf+ Cl-+CH3SOCH2 +HCl.(7) For reasons given later, we do not believe that direct initiation by Dt is important. Eqn (5), (6) and (7) are no doubt an oversimplification of the mechanism and all the reactions probably involve formation of (DMSO - Cl) as an intermediate within the solvent cages (11) [eqn (S)] : D ~ C I - ~ D - - ci.64 PHOTOINITIATiON OF POLYMERIZATION Complexes of this type have been suggested by Cooper et a125 On the basis of pulse radiolysis studies of hexamethylphosphoric triamide (HMPT) in the presence of NaBr, Koulkes-Pujo and co-workers 26 postulated the formation of a bromine atom charge-transfer complex (HMPT * * Br) which was considered to react with Br- to form Br;.Related processes are likely with DMSO (Koulkes-Pujo et aL2’) ; in our systems formation of (D - Cl) and reaction with monomer would be equivalent to (5) or (6) followed by initiation by el. Decomposition of (D Cl) into the pro- ducts on the right of (7) appears to be feasible on energetic grounds. Koulkes-Pujo et aL2’ did not report observation of CB3SOCH2, but its formation from (D * * Br) may be less likely. These considerations do not materially affect the kinetic treatment, which we base on reactions (6) and (7). The reactions involved in initiation according to mechanism (ii) are summarized in (8). D (1) k - 1 I ( a ) 0 I Ainactive products (4 0 (11) (D = DMS0)- k3 II --+ C1+ (acac),~ :el t DMSO 0 I I LCH3SOCH2 + HC1+ (acac),VIV :f) DMSO i II (acac) VIv t k s DMSO -+inactive products.( 9 ) 1 I I I C1+ CH2=C+Cl-CH2-C* I I I 1 CH3SOCH2 + CH2=C-+CH,SOCH2-CH2-C- The deactivation step (8a) is included to accommodate the observation that the quantum yield for initiation is less than unity (tables 2 and 3). Reaction (8b) is the competing electron transfer leading to the solvent-caged species (11) and reactionsS . M. ALIWI AND C. H. BAMFORD 65 (8c), (8e) and (8f) result from (5), (6) and (7), respectively. Processes (84 g), repre- senting deactivation of (11), will be discussed later. (8h) is merely the formation of the VO(acac),+DMSO adduct by coordination of DMSO. Reactions (9a, b) are directly responsible for initiation and will be supposed to consume all the radicals with 100% efficiency.TABLE 7.-vALUES OF kd AND IE AT 25°C. OPTICAL DENSITY MEASUREMENTS AT 340 AND 350 nm FOR DMSO AND Py, RESPECTIVELY. @-radiation) = 365 nm v, 104kd/s-1 [(I)ll - o7 d[o1 , 1 0791 n d t mol dm-3 mol dm-3 s-1 mol din-3 s-1 DMSO. I. = 1.0 x 10Weinstein dm-3 s-l 0.1 1.40 10-3 1.40 1.37 0.98 0.2 1.43 7 . 5 ~ 10-4 1.07 0.70 0.65 0.4 1.33 7 . 5 ~ 1 .oo 0.13 0.13 0.8 1.34 7 . 5 ~ 1.01 - - mean 1.38 Py. lo = 1.76 x 10-6einstein dm-3 s-l 0.1 0.436 10-3 0.436 0.43 0.99 0.2 0.415 10-3 0.41 5 0.395 0.95 0.4 0.460 10-3 0.460 0.27 0.59 0.6 0.426 10-3 0.426 0.17 0.40 0.8 0.442 10-3 0.442 0.073 0.165 mean 0.436 Mechanisms (i) and (ii) are stoichiometrically similar; however, they may be distinguished kinetically and we shall see that (ii) is preferable from this point of view. The nature of the final products indicates the occurrence of further secondary reactions which we now describe.The products of photolysis in systems containing DMSO, viz. VO(DMS0)&12 and acetylacetone, probably originate from interaction of VO(acac),(DMSO), formed in (8e, h) and HC1, arising from (Sf), e.g. by the sequence DMSO V0(a~ac)~(DMS0) + HCl+VO(acac)(DMSO),Cl+ acacH (10a) DMSO VO(acac)(DMSO)2C1 + HCl+VO(DMSO),Cl, + acacH. (lob) Disproportionation of VO(acac)(DMSO),Cl formed in (1Oa) into VO(DMS0)3C12 and VO(acac),(DMSO) is also possible. In either case, the overall reaction is ZDMSO VO(acac)2(DMSO) + 2HC1+VO(DMS0)3C12 + 2acacH. (1 1) We have obtained direct experimental evidence for this process. A solution of VO(acac), in DMSO, on treatment with anhydrous HCl, was found to produce VO(DMS0),C12, apparently instantaneously.These reactions elucidate satisfactorily two aspects of the experimental data : (a) the appearance of acetylacetone without intermediate formation of acetylacetonate radicals (the latter being excluded by the absence of acetylacetonate residues from the polymer) and (b) the fact that no acetylacetone is formed in the presence of pyridine as additive (no HCl is generated in this system). 1-366 PHOTOINITIATION OF POLYMERIZATION By assuming stationary concentrations of [(V0(a~ac)~D)+Cl-]*, (II), Cl and CH3SOcH2 we obtain from (8), (9) the following relations. k 2 k3+k4 3 = kl~o~{VO(acac)2D]+C~-]- k-,+k2 - k3+k,+k,’ (124 k3 +k4 k3 + k4 + kg’ n = In (12c), y is the ratio of combination to disproportionation in the termination reaction and fc, fD are the average numbers of C1 atoms and D residues per polymer molecule.Eqn (12a) is consistent with the experimental observations on rates of initiation summarized by (3a, b) (cf. fig. 1,2,5). According to tables 6 and 7, n for both systems is close to unity for v, = 0.1, approximately, hence we believe that under these condi- tions k5 is negligible [eqn (12e)l. Comparison of equations (12a, b, c) with the data in equations (3a, b) and tables 2, 3, 4 then yields the parameters in table 8. TABLE 8.-REACTION PARAMETERS, Ua = 0.1, APPROXIMATELY D DMSO PY k1/mol-l dm3 232 190 kZ lk-1 1.4 0.14 kdk3 1.8 0 For very weak absorption the theoretical values of k,/mol-l dm3 are 233 (DMSO) and 221 (Py).The main kinetic differences between the DMSO and Py systems reside in the relatively high value of k2 compared to the deactivation coefficient k-l in the former and in the absence of a reaction corresponding to (8f) in the latter. Fig. 7 and 8 show that the rates and quantum yields of initiation are functions of v, and we now consider the implications of these results. The observed increases in 9 and 6i with v, for v, <0.1 are probably not mechanistically significant ; in this region complex formation may be incomplete and the complex is not completely soluble in the reaction mixture. However, neither of these features is present when v, > 0.1. We have already seen (cf. fig. 3 and 4) that there is no evidence for retarda- tion with increasing v, so that there are no reasons for questioning the validity of the values of 3.Inclusion of the deactivation step (13) k D [(V0(a~ac)~D)+Cl-]* + Djinactive products (1 3) could account for decreasing 9 and +i with increasing v, It would require replace- ment of the factor k2/(k1 +k2) in (12a, b) by k2/(kd1 +k2 +kD[D]) and the resulting mechanism (which would be of the Stern-Volmer type) would predict a linear relation between 3-l or 4;’ and kD[D]. This is not obeyed for either system since such plots are strongly convex to the [D] axis.S . M. ALIWI A N D C. H. BAMFORD 67 It seems to us more likely that the effects we are considering arise from increasing separation of the constituent ions of the complex (VO(acac)2D)+C1- with increasing [D] which is reflected in the conductivity changes presented in table 5.Solvent- separation would reduce the probability of the D'f-Cl- interactions leading to (8c, e,f), since these depend on reactions of species within the solvent cage (11) as already described. At large separation, Cl- may be effectively outside the cage. On this view, the coefficients k3 and k4 [scheme (S)] are functions of Ua and decrease relatively rapidly as v, increases. Reduction in the rates of reactions (8c, e,f) naturally increases the relative importance of (8d, g), so that while k5 is insignificant at v, = 0.1 it becomes increasingly important as v, increases. The nature of (8d, g) is not established by the present results. These reactions are written as first-order processes, but they may be more complex without invalidating the general argument, although, of course, the simple kinetics in (12) may then not apply for v, > 0.1.Thus interaction of species derived from (11) could give rise to inactive (non-radical) products. This might proceed through monomer insertion between V and Df- in (11), forming an adduct which then reacts with a second molecule of the same kind : 0 0 I1 II D+ ! I1 2D II 1 t M-D+ D (acac), V C1- +M + (acac), V C1- I I M-D+ 0 0 2 (acac), V C1- -+ 2 (acac), v+cI-D+M-MD+C~-. I I These reactions resemble those suggested in the previous paper [eqn (12)-(14)] to account for a similar phenomenon, viz. the existence in the photolysis of VO(acac),Cl and Mn(acac)3 of reactions involving monomer which lead to chelate decomposition by non-radical route^.^'^ Alternatively, (11) may first react with the additive D to give a relatively unreactive adduct.It is important to note that k, does not depend on 21, (table 7); this is consistent with (12d) since the latter does not contain k3 or k,. On the other hand, n, which involves both these coefficients (12e), decreases markedly with increasing va (table 7). Mechanism (i), discussed earlier, is based on electron transfer from C1-, and is not easily reconciled with these observations. Decrease in the probability of electron transfer accompanying increased ionic separation would be expected to produce an increase in the rate of the competing deactivation reaction (8a) and hence a decrease in k,. This could only be avoided if the excited complex were to enter into some other decomposition process at precisely the rate required to compensate for the reduction in the rate of electron transfer.Such a mechanism, holding over a range of v, and for both systems, seems highly improbable. This difficulty does not arise in mechanism (ii) since the electron transfer in (8b) is not sensitive to changes in ionic separation. According to mechanism (ii), which we are advocating, direct electron transfer from CI- to vanadium does not occur. This is probably a consequence of the stereochemistry of the complexes (I), in which close approach of C1- and V may be difficult. The mechanism and experimental findings imply, however, that electron68 PHOTOINITIATION OF POLYMERIZATION transfer from dimethyl sulphoxide and pyridine is more facile than that from acetyl- acetone ligands.A variant of this mechanism which we have not so far considered is that initiation occurs exclusively by Dt, leading to the incorporation in a single polymer chain of both D and C1, the latter being bound ionically in terminal groups Cl-D+-. The analytical results in table 4 do not support such a view since (when D = DMSO) the C1 and D contents of the polymer are not equivalent ; further, a large proportion of the chains would have end-groups containing neither species and this, in the absence of chain-transfer, would be difficult to understand. An additional shortcoming of the mechanism is that it does not explain the dependence of 3 on q, for either additive. D. J. Lind, Ph.D. Thesis (University of Liverpool, 1967). C. H. Bamford and A. N. Ferrar, Proc. Roy. Soc. A, 1971,321, 425 ; for discussion and re- ferences see C. H. Bamford in Reactivity, Mechanism and Structure in Polymer Chemistry, ed. A. D. Jenkins and A. Ledwith (Wiley, New York, 1974), chap. 3. C. H. Bamford and A. N. Ferrar, J.C.S. Faraday I, 1972, 68, 1243. S. M. Aliwi and C. H. Bamford, J.C.S. Faraday I, 1974,70,2092. M. G. Evans, M. Santappa and N. Uri, J. Polymer Sci., 1951, 7,243. see V. Balzani and V. Carassiti, Photochemistry of Coordination Compounds (Academic Press, New York, 1970). ’ H. Funk, W. Weiss and M. Zeising, Z. Anorg. Allgem. Chem., 1958, 296, 36. * J. Selbin and L. H. Holmes, J. Inorg. Nuclear Chem., 1962,24, 1111. C. H. Bamford and D. J. Lind, Proc. Roy. SOC. A, 1968,302,145. lo D. Martin, A. Weiss and H. J. Niclas, Angew. Chem. Int. Edn., 1967, 6, 318. l1 W. H. Atkinson, C. H. Bamford and G, C. Eastmond, Trans. Furaduy Soc., 1970,66, 1446. l2 N. Rabjohn, Org. Synth., Vol. IV, 480. l3 A. I. Vogel, Practical Organic Chemistry (Longman, Green & Co., London, 1959), 863. l4 C. H. Bamford, R. W. Dyson, G. C. Eastmond and D. Whittle, Polymer, 1969, 10,759. l5 S. N. Gupta and U. S. Nandi, J. Polymer Sci., 1970,8, 1493. l6 A. M. North and G. A. Reed, Trans. Faraday SOC., 1961,57, 859. l7 C. H. Bamford Molecular Behaviour and the Development of Polymeric Materials ed. A. Led- l8 M. U. Mahmud, Ph.D. Thesis (University of Liverpool, 1972). l9 C. H. Bamford, R. W. Dyson and G. C. Eastmond, Polymer, 1969, 10, 885. 2o T. R. Ortolano, J. Selbin and S. P. McGlynn, J. Chem. Phys., 1964, 41, 262. 21 J. Selbin, T. R. Ortolano and F. J. Smith, Inorg. Chem., 1963,2, 1315. 22 J. Selbin, Chem. Rev., 1965, 65,153. 23 D. Kivelson and S. K. Lee, J. Chem. Phys., 1964,41, 1896. 24 I. Bernal and P. H. Rieger, Inorg. Chem., 1963, 2,265. 25 T. K. Cooper, D. C. Walker, H. A. Gilles and N. V. Klassew, Canaci’. J. Chem., 1973,51,2195. 26 A. M. Koulkes-Pujo, L. Gilles, B. Lesigne, J. Sutton and J. Y. Gal, Chem. Comm., 1974,71. 27 A. M. Koulkes-Pujo, L. Gilles, B. Lesigne and J. Sutton, Chem. Comm., 1971, 749. with and A. M. North (Chapman and Hall, London, 1974), chap. 2.