of silicone/vinyl polymer block and graft copolymers. [1] as it ... listed in Table 1, with organic polymers. ... pressure sensitive adhesives, and water repellent ma-.
Silicon Chemistry 1: 107–120, 2002. © 2002 Kluwer Academic Publishers. Printed in the Netherlands.
107
Graft and block copolymers with polysiloxane and vinyl polymer segments D. Graiver 1∗ , G.T. Decker 1 , Y. Kim 2 , F.J. Hamilton 2 & H.J. Harwood 2 1 Dow
Corning Corporation, Midland, MI 48640, U.S.A. of Akron, Maurice Morton Institute of Polymer Science, Akron, OH 44325-3909, U.S.A. ∗ Present address: Michigan State University, Advanced Materials Experimental Station, Midland, MI 48640, U.S.A. 2 University
(Received in revised form 25 January 2002; accepted 26 February 2002)
Key words: graft copolymers, block copolymers, polysiloxane
Abstract A convenient new process to make silicone/organic block and graft copolymers has been recently demonstrated. This dual copolymerization process combines conventional condensation polymerization of the siloxane segments with free radical polymerization of the organic vinyl polymer segments. The copolymerization process is relatively simple and economical compared with other copolymerization techniques as it uses commonly available starting materials and available process equipment. Silicone segments containing alkene side chains or end-groups are prepared in the usual way by polycondensation using an acid or base catalyst. The double bonds of the alkene groups are oxidized to carbonyls which are then used to initiate vinyl monomer polymerization and link the siloxane with the vinyl segments. This initiation step is based on a redox system of copper(II) salts which generates free radicals on the alpha carbons next to the carbonyl groups. This copolymerization process is relatively fast and proceeds at high yields.
Introduction Much effort has been directed toward the preparation of silicone/vinyl polymer block and graft copolymers [1] as it is desirable to combine some of the inherent properties of polydimethylsiloxane (PDMS), as listed in Table 1, with organic polymers. In such block and graft copolymers, the polysiloxane and the vinyl polymer segments are linked by chemical bonds and can only microphase-separate into small domains. The physical properties of the individual polymer segments, their relative concentrations and molecular weights, and to a large extent the preparation process can impact the overall morphology and properties of such copolymers. Thus, it is expected that incorporation of siloxane segments would affect the surface properties of the copolymers due to their low surface energy and that such block copolymers could be useful as surfactants, foam control additives, pressure sensitive adhesives, and water repellent materials. The high gas permeability of the polysiloxane
segments may lead to improved membranes and their low Tg and modulus could be used to develop materials with improved impact resistance. Other properties such as improved flame resistance, heat stability, lubricity and flow properties may also be modified. On the other hand, the relatively poor mechanical properties of PDMS can be greatly enhanced by incorporating crystalline or high Tg vinyl polymer segments that would eliminate the current practice and the need to use reinforcing agents in the production of silicone elastomers. Although several polymerization processes for preparing block and graft copolymers containing polysiloxane and organic polymer segments have been demonstrated, including the use of free radical procedures [2], much of the synthetic work has been based on conventional living ionic polymerization processes. Although many different block copolymers were successfully prepared by this process, only those containing polyether segments are commercially available
108 Table 1. Polydimethylsiloxanes – inherent properties. Surface
Low surface tension High water repellency Good wetting, spreading, flow-out aid
Bulk
High thermal stability High oxidative resistance Low reactivity Unique room temperature cure chemistry Large free volume – High gas permeability Low activation energy for viscous flow Low boiling and freezing points Small variation of physical constants with temperature High dielectric strength Flexible polymer chain
Environment
Essentially non-toxic Low fire hazard Low environmental hazard
Scheme 1.
today. Some of the main reasons are the difficulties and the expense associated with ionic polymerization processes and the availability of the hexamethyltricyclosiloxane (D3 ) as a starting material. In this paper we wish to report a convenient method to link PDMS segments with vinyl polymer segments to obtain graft and block copolymers of well defined structure. This method is based on a redox system that generates free radicals from enolates of aldehydes and ketones using copper(II) salts. This particular redox system has been known [3–5] to yield α-acyl carbon-centered radicals, which couple to form 1,4-dicarbonyl compounds in nearly quantitative yields and under very mild conditions. We found that in the presence of vinyl monomers these radicals can initiate polymerization reactions and that these reactions are very valuable for the synthesis of polymers with useful end-group functionality, block copolymers and graft copolymers [6–10]. It has been found that this process is very flexible and that many aldehydes and ketones can be used as initiators including acetone, phenylacetone, methyl ethyl ketone, butyraldehyde, phenylacetaldehyde, etc. Recently, we extended this chemistry to novel polydimethylsiloxane (PDMS) copolymers [11]; PDMS containing substituents with a double bond (e.g. hexenyl) were prepared in the usual way by acid catalyzed polymerization of of cyclics such as octamethylcyclotetrasiloxane (D4 ) or low molecular weight silanol-terminated PDMS. In a subsequent step, the hexenyl groups were converted to aldehyde-containing groups by exposing the polymers to ozone [12] The relative stability of the siloxane polymers to ozone compared with the high reactivity of the double bonds toward ozone, led to a quantitative conversion of all the olefins to alde-
hydes with no apparent changes in the silicone backbone (e.g. siloxane bond cleavage or cross-linking). Furthermore, the relative stability of the ozonide intermediate allowed us to determine its structure and optimize the ozonolysis reaction conditions. These aldehyde-functional polysiloxanes were then reacted with various vinyl monomers, in conjunction with the copper salt redox system, to obtain a variety of multi-segment block and graft copolymers.
Experimental Polycondensations producing 1-hexene-containing PDMS The polysiloxane segments were prepared in the usual way by polymerizing mixtures of D4 cyclics (octamethyltetracyclosiloxane), and hexenylsilanes using triflic acid (TFA) as a catalyst and an appropriate chain terminator to control the molecular weight of the polymer. The polymerization reaction was run for 16 h at 80 ◦ C and then terminated by neutralization with a weak base (e.g. ε-caprolactam). The neutralization salt and excess ε-caprolactam were filtered out and the volatile fraction (cyclics and low MW linear species) was removed by evaporation. Polysiloxanes useful for preparing copolymers of several different architectures had different placements of the hexenyl
109 groups along their backbones; linear block copolymers were prepared from telechelic hexene-functional polysiloxanes. These polysiloxanes were prepared by polymerizing a mixture of D4 and dihexenyltetramethyldisiloxane. Dihexenyltetramethyldisiloxane in this case was used as the chain terminator and its concentration in the polymerization reaction determined the molecular weight of the polysiloxane. Polysiloxanes suitable for graft copolymer preparations were prepared by copolymerizing D4 , methyldimethoxy-1hexenylsilane and tetramethyldisiloxane. In this case, the concentration of tetramethyldisiloxane was used to control the molecular weight of the polysiloxane while the concentration of methyldimethoxy-1-hexenesilane determined the density of the sites along the polysiloxane backbone that eventually became available for grafting. Synthesis of aldehyde-functional polysiloxane Polydimethylsiloxanes containing 1-hexenyldimethylsiloxane end groups were dissolved in hexane or methylene chloride (1 g/ml). These solutions were cooled to –20 ◦ C and ozone was introduced through a diffuser and bubbled through the solutions (an outlet was connected to a NaI solution to decompose any excess O3 that had not reacted and prevent any ozone release into the atmosphere). Treatment of the reaction mixtures with ozone was stopped when the solutions developed a blue color indicating that all the olefins were consumed. Surprisingly, the ozonide intermediate was relatively stable and could be isolated and characterized by 13 C-NMR. Figure 1 clearly shows 13 C-resonances at 93 and 104 ppm that are attributed to the carbons next to oxygen in the ozonide ring. Also present are resonances of the carbons (17.5, 22.5, 27 and 31 ppm) present in the alkyl group connecting the ozonide to the polysiloxane as well as the intense resonances of Si–CH3 carbons at ∼ 0 ppm. In a typical experiment, the solvent was not removed. Zinc powder/acetic acid (1:1 molar ratio) was added and the reaction mixtures were heated for one hour at 40 ◦ C to complete the conversion of the ozonide groups on the polymers to aldehyde groups. The reaction mixtures were then filtered to remove zinc oxide, washed with water to remove excess acetic acid and then dried. The solvent was removed by distillation to obtain the aldehyde-functional polysiloxanes in high yields (ca. 93%). Figure 2 shows the 13 C-NMR spectrum of one such aldehyde-functional polysiloxane, where resonances related to the ozonide are absent
Table 2. Pendent hexenyl and aldehyde-functional polysiloxanes. Polymer
Hexenyl functional DP Mn Mw/Mn (NMR) (SEC)
Aldehyde functional Mn Mw/Mn (SEC)
A B C
66 152 335
3,800 7,000 10,600
4,660 7,400 8,800
1.92 2.11 2.61
2.12 2.67 3.37
and instead typical aldehyde resonance (202 ppm) and resonances due to the alkyl bridge between 15 and 45 ppm are present. Only a very small amount of carboxylic acid resonance (∼ 7%) is evident, ∼ 177 ppm. The ozonolysis did not affect the siloxane bonds and the molecular weight of the polymers remained essentially the same as is shown in Table 2 for aldehydefunctional polysiloxanes prepared from polysiloxanes containing dimethylsiloxane and hexenylmethylsiloxane units. FTIR spectra of the products exhibited characteristic aldehyde absorption at ∼ 1733 cm−1 . Vinyl copolymerization Typically, copolymerizations were conducted in oneounce bottles under an argon atmosphere using benzene as a solvent to minimize chain transfer reactions. A typical reaction mixture contained 10 ml benzene that had been distilled from CaH2 , a redox system composed of copper octanoate (0.1 g, 2.86 × 10−4 mole), triphenylphosphine (0.3 g, 1.14 × 10−3 mole), and triethylamine (0.1 g, 1 × 10−3 mole), a vinyl monomer (∼ 5 ml) and the aldehyde-functional polydimethylsiloxane. After heating at 70 ◦ C for 21 hr, the reaction mixtures were cooled to room temperature and added to methanol to precipitate the copolymers. The products were washed with fresh methanol and dried to a constant weight under vacuum at room temperature prior to their characterization. NMR measurements 1 H-NMR
spectra of the copolymers (∼ 20 mg) in CDCl3 or D2 O (∼ 0.7 mL) solution containing a small amount of internal standard were recorded at 200 MHz using a Varian Gemini 200 NMR spectrometer, a pulse angle of 90◦ , an acquisition time of 3.74 seconds and a 5 second delay between pulses. 13 C-NMR spectra were recorded at 100 MHz using CDCl3 as the solvent and Me4 Si as the internal reference. A 90◦ pulse, a
110
Figure 1. 13 C-NMR spectrum of ozonide-functional polydimethylsiloxane obtained by ozonolysis of the hexenyl-functional polymer.
pulse delay of 4 seconds and an acquisition time of 1.6 seconds were employed. Mn and Mw/Mn measurements Size exclusion chromatography (SEC) was used for molecular weight characterizations. Measurements were made on approximately 0.16 weight percent solutions of polymers in THF and a Viscotek SEC instrument equipped with refractive index, viscosity, and light scattering detectors. Viscotek TriSEC software was employed for the calculations.
Results and discussion Double bonds are known to be readily attacked by ozone and this ozonolysis reaction is generally very fast and results in the complete destruction of the double bonds. However, the strong oxidation power of ozone leads to a multitude of products [13]. We found out that the reaction of ozone with alkene substituents
on a polysiloxane chain is relatively fast, quantitative, and very specific. Evidently, the high stability of the siloxane (Si-O-Si) and the Si-C bonds and to a large extent, also the C-H bonds is much higher than that of C=C bonds. As a result, the exposure to ozone caused essentially no change in the polysiloxane polymer with no noticeable side reactions. Furthermore, the ozonide intermediate was surprisingly stable. At present, the reason for this relatively high stability is not well understood. Perhaps it relates to the polarity of the siloxane bonds, which could interact with the zwitterionic nature of the ozonide structure [13]. It is apparent, however, that the conversion of the ozonide intermediate to aldehyde is much slower than the attack of ozone on the double bonds and mild heating of the solutions is required to complete this reaction. The position of the aldehyde groups along the polysiloxane chain determined the architecture of the copolymers, as these aldehyde groups are the loci of free radical initiation. Thus, linear block copolymers were obtained using polysiloxane segments hav-
111
Figure 2. 13 C-NMR spectrum of aldehyde-functional polydimethylsiloxane obtained from the ozonide-functional polymer.
ing telechelic carbonyl groups where vinyl polymer segments were initiated from the chain ends of the polysiloxane segments. Under these conditions, diblock – AB, triblock – ABA or multiblock – (AB)n (where A is the vinyl polymer segment and B is the polysiloxane segment) copolymers can be prepared depending on the efficiency of the initiation and the mode of termination. Thus, if the free radical initiation step is inefficient or incomplete, not all the polysiloxane chain ends would react with the vinyl monomer. This would lead to an AB type copolymer, provided that only one chain-end was active. More important is the mode of termination; provided that termination is primarily by disproportionation, ABA architecture would be obtained. However, if the primary mode of free radical termination is by combination, one would obtain an (AB)n type copolymer. In the case of graft copolymer formation, termination of growing vinyl polymer radicals by combination can be expected to cause crosslinking and gelation of the polymerization
mixture. Termination by disproportionation is therefore desirable for graft copolymer synthesis. Methacrylate esters are therefore almost uniquely preferred for such preparations. However, it will be shown later in this paper that t-butyl acrylate is an appropiate monomer for preparing graft copolymers.
A. Block copolymers 1. Linear polysiloxane-b-poly(methyl methacrylate) Telechelic aldehyde-functional PDMS initiators having 30 (I), 100 (II) and 200 (III) siloxane units per chain were used as starting materials for the preparation of PDMS-b-PMMA copolymers. Information about the copolymers and their preparation is provided in Table 3. Listed in this table are Mn values determined for the copolymers by size exclusion chromatography (SEC) and by NMR analysis and values calculated from the moles of polysiloxane used and
112 Table 3. Synthesis and properties of block copolymers containing polysiloxane and PMMA segments. Parent polysiloxane Silicons per molecule (n)
I 30
I 30
I 30
I 30
II 100
III 200
Mn of polysiloxane (SEC) Wt. polysiloxane (g) Wt. MMA (g) Yield of copolymer (g) % MMA polymerized Mn of copolymer (SEC) Mn of copolymer (NMR) Mn of copolymer (Calc.) Mw/Mn
4.8 K 0.4 5.0 4.9 89 81 K,a 43 K 30 K 1.9
4.8 K 0.4 5.0 5.0 92 78 K,b 46 K 31 K 2.1
4.8 K 0.4 5.0 4.2 76 82 K,c 48 K 30 K 1.9
4.8 K 0.4 5.0 4.6 84 74 K,d 40 K 29 K 2.1
10 K 1.2 5.0 4.3 62 128 K,e 124 K 27 K 2.3
20 K 2.3 5.0 6.8 90 99 K,f 79 K 44 K 1.9
Figure 3. 1 H-NMR spectrum of a PDMS-b-PMMA copolymer.
the yield of copolymer obtained. The latter quantity was calculated using the following formula. Mn(Calc.) =
(Copolymer yield) ∗ (Mn polysiloxane) (Wt. polysiloxane)
These values are lower than those determined by SEC and NMR, suggesting that all the siloxane may not have been employed in initiation reactions. A representative 1 H-NMR spectrum of these copolymers is shown in Figure 3, where resonances typical of PMMA [δ = 0.7–1.3 ppm corresponding to
113 Table 4. Synthesis and properties of block copolymers containing polysiloxane and polystyrene segments. Parent polysiloxane Silicons per molecule (n)
I 30
I 30
II 100
III 200
Wt. polysiloxane (g) Mn of polysiloxane (SEC) Wt. polystyrene (g) Yield of copolymer (g) % Styrene polymerized Mn of copolymer (SEC) Mn of copolymer (1 H-NMR) Mn of copolymer (Calc.) Mw/Mn of copolymer (SEC) Mn of recovered PS Mw/Mn of recovered PS
0.4 4.8 K 5.0 3.0 52 98 K 23 K 20 K 5.7 14 K 3.7
0.4 4.8 K 5.0 2.45 41 83 K 31 K 23 K 6.8 – –
1.2 10 K 5.0 2.26 21 32 K 20 K 14 K 2.2 – –
2.3 20 K 5.0 5.75 69 156 K 36 K 37 K 3.8 36 K 3.3
Figure 4. SEC plots for PMDS-b-PMMA copolymers. The curves are identified by letters provided in the SEC data row of Table 3.
CH3 , δ = 1.3–2.2 ppm corresponding to CH2 , and δ = 3.5–3.7 ppm corresponding to OCH3 ] and polydimethylsiloxane (δ ∼ 0 ppm) segments are to be noted. Assuming one polysiloxane segment per molecule, the number average molecular weights of the copolymers can be calculated from the number (n) of siloxane units in the starting polysiloxanes and the ratios of the areas for resonances observed from 0–0.2 ppm (Si-CH3 ), ASi−Me , and from 0.2–4.0 ppm, Arest , which is due to the other hydrogen atoms present. The following formulas were used to calculate Mn values and the weight percentages of MMA units present in the copolymers from their NMR spectra. Mn (NMR) = 75 n ((Arest /ASi−Me ) + 1) − 25 Wt. % MMA = 100((75 n (Arest /ASi−Me ) − 175)/Mn (NMR) Figure 4 shows the size exclusion chromatography (SEC) results obtained for several of the copolymers. Except for one copolymer, the molecular weight distributions appear to be monomodal, with Mw/Mn values close to 2. Since the number average molecular weights obtained from these curves are larger and sometimes twice as large as those determined by NMR (assuming one polysiloxane segment segment per chain), it is possible that the copolymers have multi-block (i.e., other than diblock or triblock) structures.This is particularly true for those prepared from polysiloxanes containing 30 siloxane units (n = 30). Since propagating PMMA radicals can be expected to terminate by disproportionation, it is difficult to
rationalize the apparent multi-block structures of the copolymers. A possible explanation for this could be that some of the radicals generated on the polysiloxane initiator become involved in termination reactions with propagating PMMA radicals. This would be most noticeable in block copolymerizations initiated by low molecular weight polysiloxanes. 2. Polysiloxane-b-polystyrene (PS) Information about styrene polymerizations that were initiated by polysiloxanes with terminal aldehydefunctionality and containing 30 (I), 100 (II), and 200 (III) siloxane units in combination with a copper octanoate, φ3 P, pyridine redox initiation system, is provided in Table 4. Included are number average molecular weights calculated for the copolymers from their 1 H-NMR spectra using the following equation, where (AST /ASi−Me ) is the ratio of resonance areas due to aromatic protons (δ = 6–7.4 ppm) and methyl protons attached to silicon atoms (δ = 0 ppm). Mn (NMR) = n (124.8 (AST /ASi−Me ) + 74) + 150 This equation is based on the assumption that there are as many polystyrene segments in the copolymer as polysiloxane segments which, in turn, is based on the assumptions that each aldehyde functional group in the parent polysiloxane yields a radical and that termination occurs exclusively by radical combination. Mn values calculated this way are similar to values calculated from the copolymer yields and the moles of polysiloxane initiator present in the polymerization mixture.
114
Figure 5. 1 H-NMR spectra of a PDMS-b-PS copolymer (A) and the product obtained after selectively degrading the PDMS segments.
Also included in Table 4 are Mn and Mw/Mn values measured for the copolymers by SEC using a polystyrene calibration. For most of the polymers these values are about four times the Mn values estimated by NMR, assuming one polystyrene block per polysiloxane block. The Mw/Mn values of the copolymers are also very large. This suggests that the copolymers have structures that can be represented by multi-block structures such as that shown below. PDMS − [PS − PDMS]3 Such structures can be expected to form when growing polystyrene radicals terminate by compination after having been generated by reactions of the polysiloxane aldehyde–functional ends. To obtain additional information on this point, the copolymers were selectively degraded by treatment with toluenesulfonic acid in toluene solution at 120 ◦ C
for 4 hours. This caused the polysiloxane segments to be degraded but did not affect the polystyrene segments. Figure 5 compares the 1 H-NMR spectra of a typical PDMS-b-polystyrene sample before and after treatment with toluenesulfonic acid. It can be seen that resonance assignable to methyl groups on silicon (δ = 0 ppm) is almost absent from the spectrum of the degraded polymer. SEC curves for the parent and degraded copolymers are shown in Figure 6 along with a curve for the starting polysiloxane. Mn and Mw/Mn values values for the polystyrenes recovered from the degradation reactions are provided in Table 4. They are close to Mn (Calc.) and Mn (NMR) values and are substantially less than Mn (SEC) values for the undegraded copolymers. This supports our interpretation that the copolymers have multi-block structures as discussed above.
115 2. PDMS-g-PMMA
Figure 6. SEC of PDMS before copolymerization (A); PDMS-b-PS (B); and PS recovered after selectively degrading the PDMS segments in PDMS-b-PS (C).
Such structures are believed to result from termination of growing polystyrene segment radicals by combination. Each such termination reaction yields a polysiloxane-polystyrene block copolymer (or a multiblock copolymer) with terminal polysiloxane segments that can also bear aldehyde ends. These, in turn, can initiate the polymerization of other polystyrene segments and ultimately larger multiple segment block copolymers can be formed. This view of the polymerization process is supported by the fact that the copolymers have multimodal molecular weight distribution curves and Mw/Mn values that increase with styrene conversion, becoming as large as 6.8.
B. Graft copolymers 1. PDMS-g-polystyrene It was not possible to graft polystyrene on the polysiloxane main chain without obtaining insoluble copolymers. Undoubtedly, the strong tendency of propagating polystyrene radicals to terminate by combination was responsible for the crosslinking that occurs during these vinyl polymerizations. It should be noted here that insoluble networks were obtained even when the siloxane segments had only 2 mole % aldehyde groups per chain. This was the case because 2 mole % represents only an average number of sites. That is, the actual number of grafting sites consisted of a distribution with some polysiloxane chains having more than 2 polystyrene grafts and others less. Thus, termination predominantly by combination should lead to an insoluble network.
Since PMMA radicals terminate predominantly by disproportionation, crosslinking was not expected to be a problem when PMMA was grafted onto the aldehyde-functional polysiloxanes and indeed, soluble, moldable copolymers were obtained. Table 5 lists the results of MMA polymerizations initiated by polysiloxanes containing 2 mole percent pendent aldehyde functionality and containing 30 (IV), 100 (V) and 200 (VI) siloxane units in combination with the copper octanoate redox system described above. As can be seen from this table, the MMA conversions were reasonably high and varied from 83–91%. In addition to obtaining relatively high conversions of MMA to grafts in these experiments, the sizes of the PMMA grafts were also high. The high molecular weights of the PMMA grafts are attributed to the low concentration of aldehyde in these systems. The sizes of the MMA grafts generally fell as the concentration of aldehyde units in the system increased. This relation between the molecular weight and the concentration of the initiator is to be expected if all aldehyde units initiate grafting reactions. Somewhat unexpected, however, are the observed molecular weight distributions of the grafted polymers which appeared to be multi-modal, particularly so for those derived from the highest molecular weight polysiloxanes. This is most likely due to the presence of multiple initiating sites on the polysiloxanes. Figure 7 shows SEC curves observed for the PDMSg-PMMA copolymers. Multi-modal distributions are clearly indicated by the curves for polymers prepared from initiator VI. This is due to the presence of an average of 4 graft sites per molecule in this initiator. Initiators IV and V have an average of one and two graft sites per molecule. 3. PDMS-g-poly(t-butyl methacrylate) and PDMS-g-poly(methacrylic acid) An interesting family of silicone containing graft and block copolymers includes copolymers composed of hydrophobic polysiloxane segments and hydrophilic vinyl polymer segments. One such copolymer is PDMS-g-poly(methacrylic acid). However, such a copolymer can not be prepared directly by our procedure because the redox initiation system contains amines, which would be neutralized by the acidic monomer and this would inactivate it. Another way to obtain PDMS-g-poly(methacrylic acid) was to graft copolymerize t-butyl methacrylate onto a polysiloxane using
116 Table 5. PMMA-grafted polysiloxanes. Parent polysiloxane Silicons per molecule (n)
IV 50
IV 50
V 100
V 100
VI 200
VI 200
Mn polysiloxane (SEC) Wt. MMA (g) Wt. polysiloxane (g) Yield copolymer (g) % MMA polymerized Mn (SEC) Mw/Mn (SEC) MMA/Me2 SiO MMA/branch
3.8 K 5 0.3 4.42 83 222 K,a 4.0 14.2 709
3.8 K 5 0.6 5.12 91 169 K,b 3.2 10.3 513
7K 5 0.55 5.08 91 144 K,c 3.7 9.4 470
7K 5 1.10 5.53 91 132 K,d 3.0 4.9 243
10.6 K 5 1.1 5.48 90 215 K,e 10.5 5.5 948
10.6 K 5 2.2 6.42 89 193 K,f 9.5 1.9 273
Figure 7. SEC plots for PMDS-g-PMMA copolymers. The curves are identified by letters provided in the SEC data row of Table 5.
Figure 8. SEC plots for PMDS-g-poly(t-butyl methacrylate) copolymers. The curves are identified by letters provided in the SEC data row of Table 6.
the same procedure employed to prepare the MMA grafts and then thermally degrade the ester groups to acid groups as shown below.
rylate units were converted to methacrylic acid units (see Calc. % Wt. loss A in Table 6) or that they were converted to methacrylic anhydride units (see Calc. % Wt. loss B). Some depolymerization of the polymethacrylate segments must have also occurred at 200–250 ◦ C. In any case, products obtained by pryolysis of the copolymers were soluble in dilute alkali. Figure 7 compares the 1 H-NMR spectra of a PDMSg-poly(t-butyl methacrylate) sample and the PDMSg-poly(methacrylic acid) derived from it. SEC curves for the PDMS-g-poly(t-butyl methacrylate) samples covered by Table 6 are provided in Figure 8. The molecular weight distributions are sometimes broad and multi-modal. Mn values determined from these curves are approximately twice those calculated for the copolymers from their 1 H-NMR spectra.
The thermal degradation process was studied by TGA and it was observed that the t-butyl methacrylatecontaining copolymers rapidly lost about 50 percent of their weight at 200–250 ◦ C and were nearly completely volatilized (90–95%) at 480 ◦ C. These results are summarized in Table 6. The weight losses obtained at 200–250 ◦ C were larger than those expected based on the compositions of the copolymers, assuming that the t-butyl methac-
117 Table 6. Poly(t-butyl MA)-grafted polysiloxanes. Parent polysiloxane Silicons per molecule (n)
IV 50
IV 50
V 100
V 100
VI 200
VI 200
Mn polysiloxane (SEC) Wt. tBuMA (g) Wt. polysiloxane (g) Yield copolymer (g) % tBuMA polymerized Mn (SEC) Mw/Mn (SEC) tBuMA/Me2 SiO tBuMA/branch %Wt. loss @ 250 ◦ C (TGA) Calc’d. % Wt. loss A Calc’d. % Wt. loss B
3.8 K 5 0.3 4.82 87 253 K,a 1.8 12 610 57 37 43
3.8 K 5 0.6 5.61 96 157 K,b 2.1 6.6 330 53 35 41
7K 5 0.55 5.11 89 172 K,c 2.3 5.6 282 55 35 41
7K 5 1.10 4.86 87 135 K,d 2.2 9.1 455 53 30 35
10.6 K 5 1.1 5.50 87 185 K,e 4.7 2.3 114 40 31 37
10.6 K 5 2.2 6.30 85 115 K,f 3.4 1.2 60 38 26 30
Figure 9. 1 H-NMR spectrum of PDMS-g-poly(t-butyl acrylate).
4. PDMS-g-poly(t-butyl acrylate) and PDMA-g-poly(acrylic acid) Even more desirable than PDMS-g-poly(methacrylic acid) was the preparation of PDMS-g-poly(acrylic acid). However, since polyacrylate radicals generally terminate by combination, it was expected that efforts to graft acrylate esters on the polysiloxanes
would result in cross-linked products, just as was the case for efforts to prepare PDMS-g-PS copolymers. Surprisingly, soluble products were obtained when aldehyde-functional polysiloxanes were used to initiate t-butyl acrylate polymerizations. Figure 9 shows the 1 H-NMR spectrum of one such copolymer. Apparently, this suggests that steric interactions between the propagating t-butyl acrylate radicals prevent them
118 Table 7. Poly(t-butyl acrylate)-grafted polysiloxane. Parent polysiloxane Silicons per molecule (n)
IV 50
IV 50
V 100
V 100
VI 200
VI 200
Mn polysiloxane (SEC) Wt. tBu A (g) Wt. polysiloxane (g) Yield copolymer (g) % tBu A polymerized Mn (SEC) Mw/Mn (SEC) tBu A/Me2 SiO tBu A/Branch % Wt. loss @ 250 ◦ C (TGA) % Wt. loss (NMR) % Wt. loss (pyrolysis) Calc’d. % Wt. loss A Calc’d. % Wt. loss B
3.8 K 5 0.3 4.1 83 126 K,a 2.3 43 610 43 42 47 41 47
3.8 K 5 0.6 4.1 91 89 K,b 2.1 39 330 39 40 43 37 43
7K 5 0.55 3.4 92 143 K,c 3.0 41 282 41 39 44 37 42
7K 5 1.10 5.2 91 94 K,d 2.6 37 455 38 41 43 34 40
10.6 K 5 1.1 4.1 90 254 K,e 6.6 35 114 35 35 42 32 37
10.6 K 5 2.2 5.2 89 180 K,f 4.9 28 60 28 29 35 25 29
Figure 10. SEC plots for PMDS-g-poly(t-butyl acrylate) copolymers. The curves are identified by letters provided in the SEC data row of Table 7.
from terminating by combination and that they must terminate by a process other than by radical combination and perhaps even by disproportionation. Table 7 summarizes the results of these studies. Figure 10 shows SEC data obtained for these copolymers. The t-butyl acrylate-grafted copolymers lost nearly quantitative amounts of butene when heated at 250 ◦ C as shown by TGA (Figure 11), NMR and pyrolysis studies and most were nearly completely volatilized at 500 ◦ C. Although the rate of volatilization seems to be less for high molecular weight grafts than for lower molecular weight ones, essentially all the t-butyl
Figure 11. TGA curve of PDMS-g-poly(t-butyl acrylate).
groups were removed leaving PDMS-g-poly(acrylic acid), as shown by the 1 H-NMR spectrum in Figure 12. In Table 7, weight losses calculated for the copolymers, assuming conversion of t-butyl acrylate units to acrylic acid units (Calc’d. % Wt. loss A) and assuming conversion to acrylic anhydride units (Calc’d. % Wt. loss B) are listed along with weight losses measured by TGA, NMR and direct pyrolysis. Conclusions Hexenyl groups in polydimethylsiloxanes react readily with ozone to form rather stable ozonides that can
119
Figure 12. 1 H-NMR spectrum of PDMS-g-poly(acrylic acid).
be converted to aldehydes by treatment with zinc and acetic acid. These aldehyde functional polymers were used in conjunction with copper octanoate and organic bases to initiate polymerizations of various vinyl monomers (e.g. styrene, MMA) to prepare block and graft block copolymers having distinct polysiloxane and organic vinyl polymer segments. The nature of the free radical termination of the propagating vinyl polymer chains determines the type of block copolymer obtained. Polydimethylsiloxanes with pendent aldehyde-functional groups yielded graft copolymers bearing vinyl polymer grafts when the termination was predominantly by disproportionation. However, insoluble products were obtained with vinyl monomers that tend to terminate by combination (e.g. styrene). Surprisingly, soluble graft copolymers were obtained when t-butyl acrylate was the monomer, however. Although not prepared as of yet, polymers with star structures can probably also be prepared starting with cyclosiloxanes bearing unsaturated substituents. Our procedure for preparing block and graft copolymers from appears to be very versatile.
Acknowledgements The authors are grateful to the Dow-Corning company and the Ohio Board of Regents for sponsoring this study, and to Mr. Jon Page for performing SEC measurements.
References 1. Dean, J.W. 1970 Polym. Sci., Polym. Lett. Ed. 8, 677; Saam, J.C., Ward, A.H. & Fearon G. 1972 Polym. Prepr. (Am. Chem. Soc. Div. Polym. Chem.), 13, 524; Chaumont, P., Beinet, G., Herz, J. & Rempp, P. 1979 Eur. Polym. J. 15, 459; Gagenbien, D., Madec, P.J., & Marechal, E. 1985 Eur. Polym. J. 21 (3), 271; Auman, B.C., Perec, V., Schneider, H.D., Jishan, W. & Canton, H.J. 1987 Polymer 28 (1), 119; Yilgor, I., Sha’aban, A.K., Steckle, W.P., Jr., Tyagi, D., Wilkies, G.L., & McGrath, J.E. 1984 Polymer 25 (12), 1800; Simionescu, C.I., Harabagiu, V., Comanita, E., Hamciuc, V., Giurgiu, D., & Simionescu, B.C. 1990 Eur. Polym. J. 26 (5), 565; Zundel, T., Lestel, L., Teyssie, D., Yu, J.M., & Boileau, S. 1993 Polym. Prepr. (Am. Chem. Soc. Div. Polym. Chem.) 34, 225 and many other. 2. Crivello, J.V., Lee, J.L. & Conlon, D.A., 1986 J. Polym. Sci., Polym. Chem. Ed. 24, 1197 and 1251 3. Sayer, L.M. & Jin, S.-J. 1984 J. Org. Chem. 49, 3498 4. Brackman, W. & Volger, H.C. 1966 Rec. Trav. Chim. 85, 446 5. Jin, S.-J., Arora, P.K. & Sayer, L.M. 1990 J. Org. Chem. 55, 3011 6. Goodrich, S.D. 1991 Investigation of free radical polymerizations initiated by anaerobic copper (II) oxidation of enolates
120 derived from aldehydes and ketones. Ph.D. dissertation. The University of Akron, Akron, OH. 7. Holland, T.V. 1996 Cu(II)-enolate redox initiated polymerizations: aspects of end functionalization and blocking. Ph.D. dissertation. The University of Akron, Akron, OH. 8. Goodrich, S.D. & Harwood, H.J. 1995 U.S. Patent 5,405,913, April 11; 1995 U.S. Patent 5,470,928, November 28. 9. Harwood, H.J., Christov, L., Guo, M., Holland, T.V., Huckstep, A.Y., Jones, D.H., Medsker, R.E., Rinaldi, P.L., Saito, T. & Tung, D.S. 1996 Macromol. Symp. 111, 25. 10. Holland, T.V., Goodrich, S.D., Guo, M., Harwood, H.J., Rinaldi, P.L. & Saito, T. 1995 Polym. Prepr. (Am. Chem. Soc. Div. Polym. Chem.) 36 (2), 91. 11. Graiver, D., Nguyen, B., Hamilton, F.J., Kim, Y. & Harwood, H.J., 1998 Polym. Prepr. (Am. Chem. Soc. Div. Polym. Chem.) 39 (1), 469; Graiver, D., Nguyen, B., Hamilton, F.J., Kim, Y. & Harwood, H.J. 2000. In Silicone and Silicone-Modified Materials, ed. S.J. Clarson, et al. ACS Symposium Series 729, Chapter 30, pp. 445–459 12. Graiver, D., Khieu, A.Q. & Nguyen, B.T. 1998 U.S. Patent 5,708,115, January 13; Graiver, D., Khieu, A.Q. & Nguyen, B.T. 1998 U.S. Patent 5,789,503, August 4; Graiver, D., Khieu, A.Q. & Nguyen, B.T. 1998 U.S. Patent 5,789,516, August 4; Graiver, D., Khieu, A.Q. & Nguyen, B.T. 1998 U.S. Patent 5,739,246, April 14; Graiver, D., Khieu, A.Q. & Nguyen, B.T. 1999 U.S. Patent 5,880,304, March 9. 13. Bailey, P.S., ed. 1978 Ozonation in Organic Chemistry. Olefinic Compounds, Volume 1. New York: Academic Press.
