1364
Znd. Eng. Chem. Res. 1995,34, 1364-1370
Development and Optimization of Fast Quantitative Carbon-13 NMR Characterization Methods of Novolac Resins Ben T. Ottenbourgs, Peter J. Adriaensens, Bert J. Reekmans3 Robert A. Carleer, Dirk J. Vanderzande, and Jan M. Gelan* Institute for Materials Research, Department of Organic Chemistry, Limburg University, B-3590 Diepenbeek, Belgium
A n improved 13C NMR quantification technique of random and high-ortho novolacs in THFCDCl3 solutions is reported. Full quantitative spectra of phenol formaldehyde resins with high signal-to-noise ratios are obtained by using chromium acetylacetonate under optimized spectral conditions within a few hours of spectrometer time. Attached proton test (APT) spectra were acquired to enable proper peak assignments in the region with significant overlap. For several novolacs, prepared under different catalytic conditions, the degree of polymerization, degree of branching, number average molecular weight, isomeric distribution, and number of unreacted ortho and para phenol ring positions could accurately be determined. The improved quantification method and the use of the aromatic instead of the aliphatic region permitted to measure for the first time small differences in network parameters for comparable novolac samples.
Introduction Phenol formaldehyde resins are useful polymers in a wide range of applications, as they are used in thermal insulation materials, molding compounds, foundry, and the wood products industry (Knop and Pilato, 1985). Their molecular structure depends on the reaction conditions and the catalyst levels. Under acidic conditions, reaction of an excess of phenol with formaldehyde leads to novolacs. Alternatively the use of basic conditions and an excess of formaldehyde will lead to the generation of resoles (Knop and Pilato, 1985). Random novolacs (Figure 1)are produced when oxalic acid, acetic acid, sulfuric acid, phosphoric acid, or p-phenolsulfonic acid is used as catalyst (Knop and Pilato, 1985; Hollingdale and Megson, 1955). High-ortho novolacs are formed in the pH range 4-6 with bivalent metal acetates as catalysts (Knop and Pilato, 1985). The ability to analyze the structure of these resins is important because of its relationship to resin functions. The kinetics of the curing reactions strongly depend on the nature and amount of reactive positions on the phenol ring. 13C NMR has been the most successful analytical tool to characterize phenol formaldehyde resins, since contributions of the different reactive positions can be resolved and quantified. However, it has been difficult to obtain fully quantitative 13C NMR data due to various poorly defined NMR factors as NOE effects and long 7'1 and short TZrelaxation times. They are caused by molecular associations, which influence the viscosity and lead t o line broadening (Kim et al., 1990,1992,1993; Kim and Amos, 1991). Moreover, due to the rather long 2'1 relaxation times, 13C " M R spectra of phenol formaldehyde (PF) resins often'have a rather low signal-to-noise ratio (unless extremely long total acquisition times are used) making accurate peak area measurements very difficult. These quantification drawbacks, combined with severe chemical shift overlap, have hampered a detailed structure interpretations of phenol formaldehyde novolac resins.
* To whom all correspondence should be addressed.
+ Present address: Materials Processing Laboratory, National Institute for Materials and Chemical Research, Tsukuba, Ibaraki 305, Japan.
This paper discusses a 13C NMR study of PF resins with emphasis on quantitative applications. Important factors in describing the structure of novolacs, as there are the degree of polymerization, the phenollformaldehyde ratio after reaction, and the number average molecular weight, have been determined. Furthermore, the extent of branching and the content of unreacted ortho and para positions, which influence the kinetics of the curing reactions, have been derived from the 13C NMR spectra.
Experimental Section Resin Synthesis. Novolac resins were prepared by reaction of 1500 g of commercial grade phenol with 685 g of a 50% formaldehyde solution (PIF ratio = 1:0.71) in addition of catalyst. The reactor was a 5 L threeneck flask equipped with a stirrer, a cooling condenser, a thermometer, and a heating mantle. Four high-ortho novolac resins (AI-&) were synthesized in the pH range 4-6 using varying amounts of bivalent metal acetate (ZnAcz) catalyst. Decreasing ratios of phenollformaldehyde,going from 1:0.39 to 1:0.81, were used to prepare five random novolacs with 7.5 g of oxalic acid as catalyst instead of zinc acetate. The temperature for all reaction mixtures was allowed to rise steadily t o 100-102 "C followed by reflux for several hours until the free formaldehyde content fell below 0.5%. After removing water under atmospheric pressure, the temperature was raised to 140 "C for 90 min and the excess of phenol was distilled off under vacuum. When the viscosity reached a value between 1900 and 3000 CP and the desired melting point was obtained, the reaction was stopped. In the high-ortho novolacs the presence of free phenol amounted to 7-lo%, while in the random novolacs it was less then 1%. lSCNMR Spectra. All NMR samples were prepared by dissolving 200 mg of resin oligomer in 200 p L of THF and 600 p L of CDCl3 (internal shift reference at 77.0 ppm). 13C spectra were recorded at 25 "C on a Varian Unity 400 spectrometer. Spectral parameters used were a spectral width of 20 555 Hz, a 90" pulse width of 7.0 ps, a filter bandwidth of 20 600 Hz, an acquisition time of 0.859 s, and 5000 repetitions. To eliminate NOE
0 1995 American Chemical Society 0888-5885/95/2634-1364$09.00/0
Ind. Eng. Chem. Res., Vol. 34, No. 4,1995 1365
HIGH-ORTHO-NOVOLAC
...
/Q ..
OH
RANDOM-NOVOLAC
Figure 1. Reaction schema of the synthesis of novolac resins.
010,
olp, and plp indicate the position of the methylene bridges.
Table 1. Chemical Shift and TIRelaxation Times of a High-Ortho Novolac (As) in THF-CDC13, with Different Concentrations of Chromium Acetylacetonatea Ti (s) carbonb c 1
c4s c3,5 c2s,6s-c3,5 c4u c2u,6u
6 (ppm) 156.3-156.5 153.1-153.3 149.7-150.0 131.5-131.7 129.6-129.7 129.1- 129.2 127.0-127.1 120.3-119.7 119.4-119.5 115.5-115.6 115.1-115.2
0 mM C$' 15 6.8 3.6 2.8 1.2 3.1 3.3 1.1 2.9 0.99 3.3
10 mM Cr3+
20 mM C$+
30 mM C$+
0.41 0.31 0.39 0.47 0.42 0.77 0.55 0.48 1.0 0.21 0.40
0.27 0.22 0.31 0.39 0.32 0.54 0.33 0.39 0.72 0.14 0.26
0.17 0.14 0.21 0.34 0.23 0.38 0.24 0.27 0.47 0.10 0.16
a Of each region only signals with the largest numeric value for the 21 ' are given. CI is the region of the phenolic carbons; c2u,6u is the region of the ortho-unsubstituted aromatic carbons; (23.5 is the region of the meta aromatic carbons; ClU and ClS are the regions for the para-unsubstituted and -substituted aromatic carbons, respectively; c 2 s , 6 s - c 3 , 5 is the region where the resonances of ortho-substituted and meta (trisubstituted) aromatic carbons overlap. Temperature was 25 "C.
effects, the decoupler was only gated on during acquistion (inverse gated decoupling). To reduce the 2'1 relaxation times in order to improve the signal-to-noise ratio, 5.6 mg of chromium acetylacetonate (20 mM) was added, permitting the acquisition of quantitative spectra 5 the longest by using a delay time of only 3.6 s ( ~ times relaxation time). 2'1 relaxation times were measured by means of the inversion recovery method, showing the longest value of 0.72 s for the unsubstituted para aromatic carbon resonance signals (Table 1). Attached proton test (APT) spectra were acquired to discriminate between protonated and nonprotonated aromatic carbon resonances (Pethrick and Thomson, 198613).
Results and Discussion Chemical Shift Assignments of Phenol Formaldehyde Novolac Resins. Solution 13C NMR spectral assignments were made referring to a series of corresponding model compound studies (Knop and Pilato, 1985; Pethrick and Thomson, 1986a; Sojka et al., 1979; Casiraghi et al., 1982; Werstler, 1986a,b; de Breet et al., 1977; Bogan, 1991). As a representative example, the 13C NMR spectrum of a high-ortho novolac (AI) is shown in Figure 2A. It shows peaks in three regions: high-field resonances between 30 and 40 ppm are assigned to the aliphatic methylene carbons, and the complex region from 113 to 135 ppm is due to the
aromatic non-phenoxy carbons, while the signals a t 146-157 ppm stem from the phenoxy carbons. The ortho-ortho, ortho-para, and para-para methylene bridges being separated by an upfield y-substituent effect of about 5 ppm per y-hydroxyl (Sojka et al., 1979)appear a t 30,35, and 40 ppm, respectively. These chemical shifts are in good agreement with previously reported studies if solvent effects are considered (Werstler, 1986a). The aromatic regions at 113-117 and 118-122 ppm are assigned t o the resonances of unsubstituted ortho and para aromatic carbon atoms, respectively. The aromatic meta carbons as well as those of alkylated ortho and para carbons resonate between 125 and 135 ppm. Due to the complex structure of the novolac resins, this region is subject to extensive signal overlap, which increases as a function of the number of phenolic units. APT spectra were acquired to achieve more clarity in this area (Figure 3). In the APT spectrum a clear differentiation is seen between the substituted para carbons resonating at 131.2-135 ppm and the meta carbons of free phenol and mono- and disubstituted phenol rings at 128.1-131.2 ppm. The region between 125 and 128.1 ppm contains the alkylated ortho carbon resonances as well as the meta carbon resonance signals of branched (trisubstituted) rings. These assignments are completely in agreement with the literature (Knop and Pilato, 1985;Pethrick and Thomson, 1986a; Sojka et al., 1979). In the phenoxy
1366 Ind. Eng. Chem. Res., Vol. 34, No. 4, 1995
A
B
c2s,6s-c3,5
ise
148
138
1213
iie
iee
9e
80
7e
60
50
40
38
PP"
Figure 2. (A, top) 13CNMR spectrum of a high-ortho novolac resin (AI)in THF-CDCl3; 2000 repetitions with a 30 s delay. (B, bottom) Full quantitative 13C NMR spectrum of resin AI, in addition of 5.6 mg (20 mM) of chromium acetylacetonate; 5000 repetitions with a 3.6 s delay. C1 is the region of the phenolic carbons; c2u,6u is the region o$ the ortho-unsubstituted aromatic carbons; C4u and C l Sare the regions for the para-unsubstituted and -substituted aromatic carbons, respectively; c2s,&-C3,5 is the region where the resonances of ortho-substituted and meta aromatic carbons overlap. o/o, oip, and p/p are the regions of the methylene bridges. Table 2. Chemical Shift and the Relative Ratio of the Phenolic, Ortho, Meta, and Para Carbons of Pure Phenol in THF-CDCLs" 6 (ppm)
fb = li2sw flJ = lilsw
c1
c2,6
c3,5
c4
155.4 1.00 1.00
115.5 2.33 2.00
129.7 2.26 2.01
120.8 1.13 1.00
The relative ratio required is 1:2:2:1. C1 is the phenolic carbon; the ortho carbons; C3,5 are the meta carbons; Cq is the para carbon. (I
c2.6 are
region, the isolated resonance signal at 156 ppm yields interesting information about the amount of free phenol present (Table 2). The ortho, meta, and para carbon resonances of free phenol are situated around 115.1, 129.2, and 119.4 ppm, respectively. Results of Quantitative 13C NMR Spectra. 1. Degree of Polymerization. The degree of polymerization ( n )of a novolac resin can be calculated from the aliphatic/aromatic ratio, or other 'group ratios (Kim et al., 1990), using the following equation (Flory, 1953):
n = 141- r )
(1)
where r is the methylene/aromatic ratio (CHd(V&)). In previous studies the values for the number of methylene bridges per aromatic ring ( r )were accurate only within 15-25% and did not give reliable results for the degree of polymerization (Kim et al., 1990,1992, 1993; Kim and Amos, 1991).
The latter can also be calculated indirectly by using the integration values of the ortho and para aromatic carbons since only these positions of phenol rings in PF resins are substituted with methylene groups (Kim et al., 1993). . This degree of polymerization (n') is then related to the substitution fraction (fs),the sum of substituted ortho and para carbons divided by the sum of substituted and unsubstituted ortho and para carbons, in the following way:
[SI = [Os]' + [PSI'
(2)
ru1= [OJ' + [PJ'
(3)
(4) n'= 1/(1- 1.5fs)
(5)
[VI and [SI are the number of unsubstituted and substituted aromatic carbons per phenolic ring, respectively. The number of alkylated para aromatic carbons ([PSI') and unsubsituted ortho ([0,1'> and para ([PJ') aromatic carbons, all per phenolic ring, can be obtained by direct integration of the respective areas in the NMR spectra. The number of alkylated ortho aromatic carbons per phenolic ring, [OJ,can be obtained by subtracting the intensity of meta carbons of the trisubstituted phenol rings from the intensity of the region 125128.1 ppm. The intensity of meta carbons from branched rings can be calculated unambiguously by subtracting
Ind. Eng. Chem. Res., Vol. 34, No. 4, 1995 1367
B
l
136
~
'
"
1
'
~
134
"
1
~
'
~
'
l
132
~
~
'
~
"
"
130
'
1
"
~
'
1
i 28
"
~
'
1
'
~
126
~
'
1
"
~
~
124
1
"
'
~
~
'
122
~
"
"
~
~
1
120
"
~
'
~
"
~
~
1
"
PP"
Figure 3. (A, bottom) Full quantitative 13C NMR spectrum of the overlap region between 125 and 135 ppm of a random novolac resin (B5)in THF-CDCl3, in addition of 5.6 mg of chromium acetylacetonate. (B,top) 13CNMR APT spectrum of the overlap region of resin Bs.
the intensity of meta carbons of free phenol and monoand disubstituted phenol rings ([CH,]) from the double intensity of one-sixth of the aromatic intensity (= total meta intensity). Several reasons were given for deviations from quantitativeness as there are restricted molecular motions leading to reduced signal intensities and severe overlap (broad resonances caused by short TZrelaxation effects), NOE effects, long TIrelaxation times, etc. 2. Optimization of Quantitative NMR Characteristics. In this contribution a systematic 13C NMR study was undertaken to investigate which phenomena were a t the basis of the inaccuracies observed. In order to optimize the resolution (which was almost unaffected by doubling the resin concentration) and the signal-tonoise ratio in the l3C spectra, the effect of temperature was studied by means of TIrelaxation measurements at 25 and 40 "C. Unfortunately, no significant improvement of the resolution was observed. Moreover, the T1 relaxation times increased as a function of temperature meaning that, concerning the molecular correlation time, these PF resins are situated in the extreme narrowing range (Sanders and Hunter, 1987). On the other hand, it also signifies that TZrelaxation effects caused by restricted molecular motions can not be responsible for the observed quantitative inaccuracies (2'2 relaxations times are rather long in the extreme narrowing range). Since the longest TI relaxation times a t 25 "C are of the order of 15 s, requiring delay times of at least 75 s (a total acquisition time of 104 h for 5000 repetitions), the influence of a paramagnetic relaxation agent (chromium acetylacetonate) on the 21 ' relaxation and spectral appearance was examined. An excellent compromise
between spectral characteristics (Figure 2B, minor line broadening and no significant chemical shift changes within the integration regions) and TIrelaxation was obtained by adding 5.6 mg (20 mM) of chromium acetylacetonate (Table 1). Table 1 shows that the longest TIrelaxation times in the presence of 5.6 mg of chromium acetylacetonate is 0.72 s, allowing to obtain quantitative results with a delay time of only 3.6 s (total acquisition time of 6 h for 5000 repetitions). Moreover, NOE effects (which were already avoided by inverse gated decoupling) are suppressed by chromium acetylacetonate. Paramagnetic relaxation agents namely provide an additional relaxation mechanism that suppresses the C-H dipole relaxation responsible for NOE enhancement (Sanders and Hunter, 1987). Despite these precautions (TI relaxation, digital resolution, signal to noise, NOE, 90" pulse corresponding t o frequency range of one-fourth of the spectral width), neither the 13Cspectra of the chromium acetylacetonate containing resin solutions (THF-CDC13:200/600 pL)nor those of pure phenol solutions (THF-CDCl3:200/600 p L ) were fully quantitative as shown by the deviation from 1:2:2:1 for the phenolic to ortho to meta t o para ratio (Table 2). However, it must be kept in mind that standard (commercial)NMR parameter settings include a filter bandwidth setting of around one-half the spectral width (for reasons of signal to noise) causing a signal reduction toward the edges of the spectrum. Increasing the filter bandwidth to a value equal t o the spectral width resulted in quantitative measurements for the PF resins as well as for pure phenol (Tables 2 and 3). Table 3 shows that, for example, the ratio of one-sixth of the total aromatic intensity t o the phenolic intensity no longer significantly differs from unity.
~
'
"
~
~
~
l
1368 Ind. Eng. Chem. Res., Vol. 34, No. 4, 1995 Table 3. Free Phenol Extent and NMR-Based Data of Some Novolac Resins" sample Ai A2 A3 -44
Bi B2 B3 B4 B5
Clf, free phenol (%)
r
[UI
9.17 8.10 9.38 8.88 0.02 0.46 0.05 0.19 0.93
0.742 0.726 0.723 0.719 0.647 0.710 0.763 0.792 0.803
1.46 1.42 1.45 1.45 1.62 1.48 1.35 1.26 1.23
[SI 1.55 1.58 1.56 1.55 1.39 1.52 1.65 1.73 1.77
1.5 fs
d
d'
0.770 0.791 0.777 0.775 0.691 0.759 0.825 0.870 0.885
0.06 0.13 0.11 0.11 0.08 0.10 0.12 0.16 0.16
-0.01 0.00 0.00 0.00 0.01 0.00 0.00 0.01 0.00
(1/6Ar)/Arco~ 1.01 1.00 1.00 1.00 1.01 1.01 1.00
0.99 1.00
a AI, high-ortho novolac 1, etc.; B1, random novolac 1, etc.; Clf is the extent of free phenol obtained from the intensity of the isolated resonance signal of the phenoxy carbon originating from free phenol ( ~ 1 0 0divided ) by li6Ar; r , [SI, [Ul, fs, d , and d' are parameters defined in eqs 1-4, 6, and 7; (l/GAr)/Arco~is the ratio obtained from one-sixth of the total aromatic carbons divided by the phenolic carbon intensity.
Deviation from Ideal Quantitativeness. The accuracy reflecting the quantitative character of the NMR data can be expressed by calculating the deviation of phenol functionality, d (Kim and Amos, 1991) and d'. They describe the deviations from ideal functionality for the aliphatic and aromatic regions, respectively. d = (3.0 - 2r - [Ul)
d' (3.0 - [SI - [UI)
(7)
The functionality of phenol is 3.0. Large d deviations were obtained previously (Kim and Amos, 1991). The values of d and d' (Table 3) improved significantly by applying the proposed acquisition precautions, confirming the quantitative character of the NMR method. Moreover, d' values offer better results over d values, indicating that the aromatic region is used in favor of the aliphatic region due to the relative larger intensity of the aromatic carbons. Another verification of the improved accuracy by using the aromatic region versus the aliphatic region can be given by comparing the PIF ratio after reaction, defined as lh or ll(l.5fs), with the initial PdF ratio. For this purpose the following equation has been used:
PIF-PJP = PJF
(8)
where PdF and PIF are the molar ratios of phenol/ formaldehyde before and after reaction, respectively. PdP is the inverse of the yield with respect to phenol, which can be calculated from the initial number of phenol rings (in moles) and the number of phenol rings in the novolac resin after reaction. The latter can be obtained from the molecular weight of the resin because it consists only of phenol rings and methylene bridges. The mean molecular weight (M,) of one monomer unit as it is built in the resin can be calculated via eq 9:
M , = MnIn (or n') where M , is the number average molecular weight, calculated with the following equation (Casiraghi et al., 1982),
M,, = 106(n - 2)
+ 200 (or n')
(10)
So the number of phenol rings, P, (in moles), in the novolac resin after reaction can be calculated from the weight of the resin divided by M,. The results obtained for PdP are reported in Table 4. The PdF ratios obtained by using the aromatic region (by means of f s ) are much more in agreement with the initial ones than those calculated by means of the aliphatic region ( r ) .
Table 4. Inverse of the Yield with Respect to Phenol and the PhenoVFormaldehvde Ratio before Reactiona Ai A2
A3 A4 Bi Bz B3 B4 B5
1.08 1.13 1.11
1.09 1.14
1.11
1.12 1.78 1.32 1.21 1.16 1.12
1.77 1.31 1.18 1.15 1.11
1.11
1:0.68 1:0.64 L0.65 1:0.65 1:0.36 k0.54 1:0.65 1:0.69 L0.72
1:0.71 1:0.70 1:0.70 1:0.70 1:0.39 1:0.57 1:0.68 1:0.75 1:0.79
L0.71 L0.71 1:0.71 1:0.71 1:0.39 L0.55 L0.70 L0.77 1:0.81
a PdP and PdF, (PdPIp)' and (PdF)' are parameters defined in eq 8 obtained by using r or fs, respectively; (PdF)initialis the initial phenoYformaldehyde ratio before reaction.
Some Typical Resin Structure Characteristics of ComparableNovolac Samples. All further results discussed in this paper resulted from solutions containing 200 mg of resin in THFlCDCls (2001600pL) and 20 mM chromium acetylacetonate that were measured a t 25 "C under optimal quantitative spectral conditions. Tables 5 and 6 present some typical resin characteristics. 1. The Molecular Weight, Mn.The number average molecular weight was calculated by means of the degree of polymerization (n or n') according to eq 10. Due to the more favorable signal intensities to measure the fs ratio compared to r, M , is by preference obtained through n'. Cyclic derivatives were not taken into account because intramolecular reactions are unlikely in bulk and the molecular size of the oligomers is rather small. It should be noted that the confidence limits of n values obtained from eqs 1 and 5 become very significant when r or 1.5fs approaches 'unity causing a decreasing reliability with increasing molecular weight. However, this trend is inherent to all molecular weight determination methods based on end group determinations. 2. The Extent of Branching. Also the extent of branching in these phenol formaldehyde novolacs was ascertained from the I3C NMR spectra. Integration of the phenolic carbon resonances, originating from trisubstituted rings, has been used for estimation of the degree of branching in PF resins (Kim et al., 1993). However, considering the complexity of the phenolic carbon region due to overlap of the resonances of mono-, di-, and trisubstituted phenol rings, calculations using this method seem to be unreliable. Alternatively, the extent of branching (a)can be determined from the intensity ratio of meta carbons of trisubstituted rings to all meta
Ind. Eng. Chem. Res., Vol. 34, No. 4, 1995 1369 Table 5. NMR Data of Some Novolac Samples" sample Ai A2 A3 A4
Bi B2 B3 B4 B5
n
n'
M,
M,'
CH,
3.88 3.65 3.61 3.56 2.83 3.45 4.22 4.80 5.08
4.36 4.78 4.49 4.44 3.24 4.15 5.72 7.67 8.66
400 375 370 365 288 353 436 497 527
449 495 464 459 331 428 594 802 906
1.70 1.72 1.67 1.70 1.61 1.62 1.63 1.66 1.68
U.
0.15 0.14 0.17 0.15 0.19 0.19 0.18 0.17 0.16
a n and n' are defined in eqs 1and 5; M, and M ,' are defined in eq 10, based on the values of n and n'; CH, and a are defined in eq 11.
carbons as shown in the following equation:
a = (2 - [CH,1)/2
(11)
[CHmIcorresponds to the number of meta carbons, originating from free phenol and mono- and disubstituted phenol rings, 2 is the numeric value of all meta carbons, and (2 - [CHm]) is the number of meta aromatic carbons corresponding to branched rings, all expressed per phenolic ring. The degree of branching yields interesting information about the gelation of the polymer. The gelation point is reached when a equals 0.5 for the trifunctional phenol formaldehyde system (Flory, 1953). The a values reported here (Table 5 ) indicate that all novolacs are far from gelation. Addition of catalyst (ZnAc2) does not appear t o affect the degree of branching in a consistent manner. In the random resins, on the other hand, a seems to decrease with increasing molecular weight. This cannot be explained by statistical means. Branching becomes less likely at high molecular weights, probably due to kinetic aspects. This is in agreement with an earlier literature rapport based on viscosity measurements (Kim and Amos, 1991). 3. The Number of Unreacted Ortho and Para Positions and the Isomeric Distribution. Another important parameter which could be obtained from the aliphatic region of the 13C NMR spectrum is the number of unreacted para and ortho positions per phenolic ring. In curing reactions an increased concentration of free para positions will result in a greater reaction rate because para positions are favored over ortho positions (Knop and Pilato, 1985). [PSI,[P,l, [Os], and [0,1 are the number of substituted and unsubstituted para and ortho positions, respectively, per phenolic ring. The results obtained by using the following equations are reported in Table 6:
[PSI= (2pp + op)r
(12)
+ op)r
(14)
[Os] = (200
LO"] = 2 - [Os]
(15)
where pp, 00, and op values present the isomeric distribution in the novolac resin. The number of unreacted para and ortho positions per phenolic ring can also be obtained by dividing the intensity of unsubstituted ortho and para aromatic carbons by the intensity of onesixth of the total aromatic region. The values for [P,]' and [O,]' determined in this way are also reported in Table 6. These results are seemingly in good agreement
Table 6. Number of Unreacted Ortho and Para Positions"
oo(%)
op(%)
pp(%)
[P,]
[O,]
[P,]'
[O,]'
66.8 58.9 56.3 56.7 22.4 24.4 27.1 26.1 26.4
30.7 39.4 42.4 41.5 51.3 50.7 49.1 50.5 50.2
2.47 1.70 1.70 1.80 26.3 24.9 23.8 23.3 23.4
0.74 0.69 0.68 0.68 0.33 0.29 0.26 0.23 0.22
0.78 0.86 0.88 0.89 1.38 1.29 1.22 1.19 1.17
0.73 0.67 0.63 0.65 0.30 0.24 0.21 0.19 0.18
0.73 0.75 0.81 0.81 1.32 1.24 1.14 1.09 1.06
a 00, op, and pp are the methylene group orientations and present the isomeric distribution; [P,] and [O,] are defined in eqs 13 and 15; [PJ and [O,]'are the content of unreacted para and ortho positions per phenolic ring, obtained directly from the 13C NMR spectra.
with [P,l and [O,]. Only small differences appear, again due to the greater intensity of the aromatic region relative to the aliphatic region. The results presented here are useful in describing the structure of novolac resins. The isomeric distribution in high-ortho novolac resins differs from those in random resins. In the latter there are almost equal amounts of para-para bridges and ortho-ortho bridges (f25%),while in high-ortho novolacs the ortho-ortho bridges are predominant due to the addition of catalyst (ZnAcz). Depending on the concentration of bivalent catalyst, the substitution in ortho position is favored. This is reflected in an increased number of unreacted para positions and ortho-ortho bridges, together with a decrease in free ortho positions and ortho-para methylene bridges. Addition of small amounts of ZnAc2 already causes an enormous change in the isomeric distribution, as illustrated by the values for A4 versus B1 in Table 6.
Conclusion The quantitative character of the 13C NMR spectra and the preparation delays have been improved by optimizing the standard acquisition parameters and adding chromium acetylacetonate (higher signal-tonoise ratio). Increased resolution in the aliphatic region is achieved by using a proper THF-CDCl3 solvent mixture. APl' spectra were used to achieve more clarity in the complex part of the aromatic region. Several structural parameters such as the degree of polymerization, the number average molecular weight, the degree of branching, the number of free para and ortho positions, and the isomeric distribution have been calculated with a much better accuracy and precision. All resin parameters can be deduced more accurately by using the aromatic carbon resonances instead of the methylene reonances. Addition of small amounts of bivalent catalyst (ZnAc2) enormously influences the isomeric distribution. Furthermore, lower molecular weight resins are confirmed t o have a higher degree of branching than higher molecular weight resins.
Acknowledgment This work was supported by IWT (Flemish Institute for the Promotion of Scientific Technological Research) and STRIDE (Science and Technology for Regional Innovation and Development in Europe).
1370 Ind. Eng. Chem. Res., Vol. 34,No. 4, 1995
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IE940322+ @
Abstract published in Advance ACS Abstracts, March 1,
1995.
