WO2000034345A1

WO2000034345A1 - Production of vinylic polymers

Info

Publication number: WO2000034345A1
Application number: PCT/GB1999/004140
Authority: WO
Inventors: Derek John Irvine; Christopher David Borman
Original assignee: Ineos Acrylics Uk Limited
Priority date: 1998-12-10
Filing date: 1999-12-09
Publication date: 2000-06-15
Also published as: AU1669200A; GB9827080D0

Abstract

A method of producing acrylic polymers by atom transfer radical polymerisation (ATRP) methods is described in which the reaction is carried out in the presence of a chain transfer agent. The resulting polymers show improved thermal stability compared with polymers produced by ATRP in the absence of the chain transfer agent, yet maintain an improved polydispersity as compared with comparable polymers produced by conventional free-radical polymerisation methods.

Description

Production of vinylic polymers

The present invention concerns the production of vinylic polymers, especially, but not limited to, acrylic polymers, for example methacrylate and acrylate copolymers and homopolymers, using atom transfer radical polymerisation.

Atom transfer radical polymerisation (ATRP) of unsaturated monomers such as styrene and methyl methacrylate has been reported by Matyjaszewski et al (J. Am. Chem. Soc , ( 1 995), 1 17, 5614; J. Am. Chem. Soc , ( 1997), 1 19, 674; Macromolecules, ( 1 998), 31 , 1 527) and Haddleton et al (Macromolecules, (1997), 30, 21 90; Macromolecules, ( 1 997), 30, 3992). It is a method of living free radical polymerisation which is initiated by the abstraction of a halogen atom from an alkyl halide by a stabilised metal complex (usually copper or ruthenium) to produce an alkyl radical. The alkyl radical then adds to the monomer in a chain reaction which may be terminated by the addition of an abstracted halogen back from the metal complex. Subsequent removal of the halogen may then lead to further addition of monomer. This mode of polymerisation is controlled and normally leads to halogen-terminated polymer of narrow molecular weight distribution in which the molecular weight is dependent upon the concentration of initiator used.

One problem associated with ATRP of methyl methacrylate is that the product polymer chains have halogen end-groups which are thermally labile. When heated these polymers degrade, the result of which is an unzipping of the polymer chains to produce MMA monomer. Therefore PMMA produced directly by ATRP is not thermally stable and this limits its usefulness as an industrial polymer.

It is an object of the present invention to produce an improved vinylic polymer by ATRP methods, especially but not exclusively, an acrylic polymer.

According to the invention, a method of producing an vinylic polymer comprises forming a polymerisation mixture comprising at least: (i) at least one vinylic monomer,

(ii) a transition metal complex or precursor thereof, wherein the transition metal in a first oxidation state is reversibly capable of bonding to a halogen atom X and entering a second oxidation state; (iii) an initiator R-X, where X is a halogen and R is an alkyl, substituted alkyl or halogenated carbon group, such that the acrylic monomer is polymerised by atom transfer radical polymerisation; (iv) a chain transfer agent and enabling polymerisation to occur such that the acrylic monomer is polymerised by atom transfer radical polymerisation.

In a second aspect of the invention, we provide a polymer produced by a method comprising forming a polymerisation mixture comprising:

(i) at least one vinylic monomer, (ϋ) a transition metal complex or precursor thereof, wherein the transition metal in a first oxidation state is reversibly capable of bonding to a halogen atom X and entering a second oxidation state;

(iii) an initiator R-X, where X is a halogen and R is an alkyl, substituted alkyl or halogenated carbon group, such that the acrylic monomer is polymerised by atom transfer radical polymerisation;

(iv) a chain transfer agent and enabling polymerisation to occur such that the acrylic monomer is polymerised by atom transfer radical polymerisation.

The reaction is preferably carried out in the presence of a suitable solvent which can readily be selected by those already familiar with methods of ATRP polymerisation.

Examples of suitable solvents include ethyl acetate, xyiene, toluene, cyclohexane, methyl ethyl ketone and tetrahydrofuran. Although not critical for the polymerisation, the presence of a solvent controls the viscosity of the reaction mixture and assists heat transfer in the reaction medium. A volume of solvent approximately equal to the volume of the monomer, which for many monomer approximates a molar ration of 1 :1 , has been found to be useful. The amount of solvent present does not directly affect the polymerisation reaction although in very dilute mixtures, the reaction rate would be reduced compared with more concentrated reaction mixtures. Bulk polymerisation methods are possible.

Preferred vinylic polymers are acrylic polymers. The at least one vinylic monomer preferably comprises an acrylic monomer such as an alkyl acrylate, alkyl (alkyl)acrylate or acrylic or (alkyl)acrylic acid. Preferred monomers include optionally functionalised alkyl acrylates and alkyl methacrylates, especially, methyl methacrylate (MMA), ethyl methacrylate, n-propyl methacrylate, i-butyl methacrylate, n-butyl methacrylate, t-butyl methacrylate, hydroxyethyl methacrylate, polyalkyleneglycol methacrylates (e.g. polyethylene glycol methacrylates), polyalkyleneglycol acrylates, ethyl acrylate, butyl acrylate, methyl acrylate. Other suitable monomers include styrene (including substituted or functionalised styrenes) acrylonitrile and other vinylic species, e.g. vinyl acetate.

The transition metal M which forms a complex or precursor thereof, wherein the transition metal in a first oxidation state is reversibly capable of bonding to a halogen atom X and entering a second oxidation state, may be selected from e.g. copper, nickel, iron, manganese, chromium or ruthenium or other transition metals but is preferably copper. The transition metal complex may be any of those which have been found suitable for use in ATRP reactions of vinylic monomers, e.g. as described in the literature references supra. The transition metal complex preferably comprises Cu-Ln, where L is a ligand which is preferably a substituted pyridine compound, especially 2,2'- bipyridine or a substituted analogue thereof. Suitable substituted 2,2'- bipyridines include 4,4'-di-(alkyl)-2,2'- bipyridines where the alkyl group comprises a C, .₂₀ alkyl group such as t-butyl, n-heptyl, 5-nonyl or other bulky group. Such complexes may be formed in situ by the reaction of a copper (I) halide with the ligand. The Cu halide is preferably CuCI or CuBr. n is normally 1 - 3, especially 2. Preferably the molar ratio of Cu : ligand is in the range 1 :1 - 5, more preferably about 1 :3. Other suitable transition metal complexes include Ru(PPh₃)₃CI₂ as described by Sawamoto et al (Macromolecules (1995) 28 1721 - 1723). We have found that a suitable concentration of the transition metal is in the range 5 - 500 mM, preferably about 20 - 100 mM, for example about 50 mM.

The initiator is a halogen-containing compound R-X. The halogen X is preferably Cl or Br. Suitable initiator compounds include alkyl and aryl halides and other organic halide compounds such as ethyl-2-bromoisobutyrate (EBIB), 1-phenylethyl bromide (PEB), 1-phenylethyl chloride, p-toluenesulphonyl chloride (PTSC), benzhydryl chloride, 1 ,1 ,1-thchloroacetone, α,α-dichloroacetophenone, carbon tetrachloride. The concentration of the initiator used controls the molecular weight of the polymer because each molecule of initiator may initiate one polymer chain (although it will be appreciated that initiator decomposition is rarely 100% efficient). Therefore the optimum amount of initiator used depends upon the desired molecular weight of the polymer to be made. By varying the concentration of initiator in a given amount of monomer, the polymer molecular weight (Mn) may be varied from < 5,000 to about 100,000. As a guide, using methyl methacrylate monomer, we have found that an initiator concentration of 100 mM produces a polymer having a molecular weight (Mn) of about 5,000, whilst an initiator concentration of 50 mM produces a polymer having a molecular weight of about 10,000 and an initiator concentration of 5 mM produces a polymer having a molecular weight of about 100,000.

The chain transfer agent may be any of those chain transfer compounds which are known to control free-radical polymerisation of vinyl monomers and may include branched or linear species having one or more than one chain transfer site. Suitable chain transfer agents include a mercaptan compound or a catalytic chain transfer agent which is or is derived from a cobalt chelate. Suitable cobalt chelates are described in US 4694054, US 5028677 and US 4680352. Particularly preferred is a cobalt chelate having the following general formula I :

o o

I i

~^{" C θ} ^

^R I I ^κ o .0

in which R = phenyl or alkyl. Suitable chain transfer agents as described in EP-A-199436 include Co(ll)(2,3-dioxyiminobutane-BF₂)₂, Co(ll) (1 ,2-dioxyiminocyclohexane-BF₂)₂ and Co(ll)(1 ,2-diphenyl-1 ,2-dioxyiminoethane-BF₂)₂. Particularly preferred is bis(borondifluorodiphenylglyoximato) cobaltate (II) (CoB-F Ph) in which R = Ph. Alternatively or additionally, the catalytic chain transfer agent is or is derived from cobaloxime (Co(diphenylglyoxime)₂).

Suitable mercaptans include C₃ - C₂₀ alkyl mercaptans which have at least one functional -S-H group which are known in the art as chain transfer agents, in particular for use in acrylic polymers. Examples of suitable mercaptans include but are not limited to propyl mercaptan, butyl mercaptan, nonyl mercaptan, dodecyl mercaptan and others.

Chain transfer agents are well known as additives to control the molecular weight of acrylic polymers made by free radical polymerisation methods. They provide a means for terminating the propagation of the acrylic chains by the abstraction of hydrogen from the mercaptan -S-H group to give a H-terminated alkyl chain and a new -S- radical or by catalysing the formation of a terminal double-bond from the propagating radical site (catalytic chain transfer). Normally the use of such chain transfer agents results in relatively low molecular weight polymer with relatively high polydispersity (Mw/Mn), e.g. about 1.9 - 3. The use of mercaptans in polymethacrylates is also well known to produce more thermally stable polymer than that produced in the absence of mercaptan because the H-terminated end group is more resistant to thermal degradation than C=C terminated polymer such as may arise during normal disproportionation reactions in the absence of a chain transfer agent. When a catalytic chain transfer agent is used in free-radical polymerisation reactions, the resulting polymer is C=C terminated and therefore exhibits no beneficial thermal properties compared to polymer made without chain transfer agent.

The use of ATRP polymerisation techniques enable the molecular weight to be controlled by adjusting the amount of initiator added, because the number of chains initiated is dependent upon the amount of initiator present and propagation may be allowed to continue until substantially all of the monomer has been used up. This produces polymer of low polydispersity, typically < 1.5. Thus the addition of chain transfer agents may be expected to increase the polydispersity, which is often undesirable for a commercial polymer, and their use would not be expected to be necessary because the molecular weight control is inherent with an ATRP system. Surprisingly we have found that the addition of chain transfer agents to ATRP polymerisation reactions of vinylic polymers does not significantly reduce the molecular weight or significantly increase the polydispersity but does significantly increase the thermal stability of the polymers produced. Catalytic chain transfer agents are particularly preferred and their action to increase the thermal stability of ATRP polymers is particularly surprising given that their action in conventional free-radical polymerisation produces unstable double-bond containing end groups.

The amount of chain transfer agent in the reaction mixture varies according to the type of substance used and its mode of action. If a catalytic chain transfer agent (CCT) is used, we have found that the optimum properties of the resulting polymer are achieved when the CCT is present at a level of 3 - 100, more preferably 5 - 15, especially about 8 - 12 e.g. about 10 ppm by weight based on the total weight of the reaction mixture, including solvent. When the level of CCT agent exceeds that of the optimum level for the particular system being polymerised, the polymerisation mechanism is observed to move away from that of ATRP and toward that of CCT control. This results in the level of thermal stability of the polymer decreasing from that exhibited by polymer produced at the optimum level. When a mercaptan is used it is preferably present at a molar concentration of active sites approximately equal to the molar concentration of the initiator used. This is because each initiator molecule may initiate one chain and each mercaptan group may terminate one chain.

The initiator may be mixed with solvent and added to a pre-mixed mixture of the monomer, transition metal complex (or precursors thereto) and solvent (if used) which has been preheated to the desired reaction temperature. This method may be advantageous because the dissociation of the initiator may then be controlled to occur at the reaction temperature. The chain transfer agent may be mixed with the monomer and transition metal complex before addition of the initiator to the reaction mixture or, when the initiator is added separately as described above, it may be mixed with the initiator and solvent and added at the same time as the initiator. We have found that it is beneficial for the chain transfer agent to be present in the reaction mixture when the polymerisation is initiated whether it is added with the initiator or present in the uninitiated mixture of monomer and transition metal complex.

The polymerisation is preferably carried out at an elevated temperature, e.g. between about 50 and 150 °C, more preferably between about 80 - 120 °C e.g. about 90 - 100 °C, but the optimum temperature suitable for each reaction system may vary depending on the effect of temperature on the rate of generation of radicals from the initiator used and reaction rate required.

The invention will be further described in the following examples.

In all examples, methyl methacrylate (MMA, supplied by ICI Acrylics) was further purified using a 4A molecular sieve column under dry nitrogen. Anhydrous ethyl acetate, copper(l)bromide, ethyl-2-bromoisobutyrate (EBIB), 1-phenylethyl bromide (PEB), p-toluenesulphonyl chloride (PEB) and 2,2'-bipyridine (bpy) (all supplied by Aldrich) were used without further purification. The CCT catalyst used was bis(borondifluorodiphenylglyoximato) cobaltate (II) (CoB-F Ph, supplied by DuPont). The mercaptan was dodecyl mercaptan (DDM) (Aldrich). A stock solution of CoB-FPh was prepared in anhydrous ethyl acetate by adding solid catalyst to a dry Schlenck flask and removing oxygen by vacuum-nitrogen purging. Ethyl acetate solvent was then added using a dry glass syringe and the mixture stirred vigorously until ail solid dissolved. Example 1 (comparative)

Method of producing polymethyl methacrylate by ATRP

All reactant concentrations are given in mol dm^"3 (M) of the total reaction mixture. The ATRP polymerisation was conducted as follows: 46.7 mM Copper(l)bromide (Cu(l)Br) and 141 mM 2,2'-bipyridine (bpy) were introduced to a 100 ml round-bottomed flask fitted with a sidearm condenser and oxygen was removed by three successive vacuum-nitrogen purges. 5.1 M of ethyl acetate and 4.67 M MMA were then added using dry glass syringes. The resultant mixture was then heated with stirring, under an atmosphere of dry nitrogen, to 90°C before addition of 23.9 mM initiator via a dry glass Hamilton syringe. The temperature of the reaction mixture was monitored throughout and maintained at 90 °C. The initial ratios of [Cu(l)Br]₀ : [bpy]₀ : [initiator],, = 2:6:1.

The PMMA produced in this way was isolated by resuspension of the reaction mixture in tetrahydrofuran (Fisher) followed by filtration through a small column of alumina (activated, Brockmann I, 58A, Aldrich) to remove the Cu, and finally precipitation into hexane (Fisher). The polymer was analysed by X-ray fluorescence which showed that the Cu content of the PMMA product was <20ppm. Conversion was determined gravimetrically after the polymer had been dried in a vacuum oven at 80°C for 3 h and was >95%.

The thermal stability of the resulting polymer samples was investigated using a TA instruments Hi-Res TGA 2950 Thermogravimetric Analyzer run under an inert atmosphere of dry nitrogen. Samples (~10mg) were placed on a platinum pan and heated from 0 - 600 °C at a rate of 10 °C/min and the weight loss against temperature recorded. In the tables of results, the thermal stability is estimated by the weight loss recorded at 300 °C.

Examples 2 - 3

The method of Example 1 was followed using chain transfer agents. The required amount of CoB-F Ph stock solution was introduced to the reaction flask using a dry glass Hamilton syringe. Final CoB-FPh concentrations were ~ 8μM. When dodecyl mercaptan was used, the required volume was introduced with the ethyl acetate and MMA using a dry glass Hamilton syringe. Mercaptan concentrations were always identical to the initiator concentration. The additives used, molecular weight of resulting polymer and thermal stability at 300 °C are shown in Table 1.

Examples 4 (comparative) - 6

The method of Examples 1 - 3 was followed using p-toluene sulfonyl chloride (PTSC) initiator and the results are shown in Table 1. The results show that the ATRP polymerisation of MMA in the presence of chain transfer agents produces polymer of low polydispersity (<2) and that the thermal stability as evidenced by the weight loss recorded by TGA at 300 °C was considerably improved compared to ATRP polymer made without chain transfer additive and comparable to the mercaptan-ended polymer made by free-radical polymerisation in Example 9.

Table 1

Example 7 - comparative

A free-radical polymerisation of MMA (3.1 1 M) using azobisisobutyronitrile (AIBN) initiator (1.6 mM) in toluene solvent (6.3 M) was carried out in the presence of DDM chain transfer agent (9.4 mM). The reaction temperature was 85 °C and reached 30% conversion after 6 hours. The resulting polymer had Mn = 32,300 Mw = 51 ,700 and % wt loss at 300 °C = 9.1.

Examples 8 - 10

These experiments show ATRP polymerisations in the presence of chain transfer agents according to the invention using different amounts of initiator and additives to produce polymers of different molecular weight.

Table 2

Claims

1. A a method of producing an vinylic polymer comprises forming a polymerisation mixture comprising:

(i) at least one vinylic monomer, (ii) a transition metal complex or precursor thereof, wherein the transition metal in a first oxidation state is reversibly capable of bonding to a halogen atom X and entering a second oxidation state; (iii) an initiator R-X, where X is a halogen and R is an alkyl, substituted alkyl or halogenated carbon group, such that the acrylic monomer is polymerised by atom transfer radical polymerisation;

2. A method as claimed in claim 1 , wherein said at least one vinylic monomer is selected from an alkyl acrylate, alkyl (alkyl)acrylate, acrylic or (alkyl)acrylic acid, styrene, substituted or functionalised styrenes, acrylonitrile and vinyl acetate.

3. A method as claimed in claim 2, wherein said at least one vinylic monomer is selected from methyl methacrylate (MMA), ethyl methacrylate, n-propyl methacrylate, i-butyl methacrylate, n-butyl methacrylate, t-butyl methacrylate, hydroxyethyl methacrylate, poly alkyleneglycol methacrylates, poly alkyieneglycol acrylates, ethyl acrylate, butyl acrylate, methyl acrylate.

4. A method as claimed in any of claims 1 - 3, wherein the transition metal complex comprises Cu-Ln, where L is a ligand which is a substituted pyridine compound and n = 1 to 3.

5. A method as claimed in any of claims 1 - 4, wherein the initiator compound is selected from ethyl-2-bromoisobutyrate (EBIB), 1-phenylethyl bromide (PEB), 1-phenylethyl chloride, p-toluenesulphonyl chloride (PTSC), benzhydryl chloride, 1 ,1 ,1-trichloroacetone, α,α-dichloroacetophenone, carbon tetrachloride.

6. A method as claimed in any of the preceding claims, wherein said polymerisation reaction is carried out in the presence of a solvent.

7. A method as claimed in any of the preceding claims, wherein said chain transfer agent comprises a catalytic chain transfer agent which is or is derived from a cobalt chelate.

8. A method as claimed in claim 7, wherein said cobalt chelate is present at a level of 3 - 100 ppm by weight based on the total weight of the reaction mixture, including solvent, if

5 present.

9. A method as claimed in any of claims 1 - 6, wherein said chain transfer agent comprises an alkyl mercaptan.

10. A method as claimed in claim 9, wherein said mercaptan is present at a level such that the molar concentration of active chain transfer sites provided by the mercaptan is 0 approximately equal to the molar concentration of the initiator.

1 1. An acrylic polymer which has been manufacture by a method as claimed in any one of claims 1 - 10.

12. A vinylic polymer formed by atom transfer radical polymerisation methods, which polymer contains the residue of a chain transfer agent.

15 13. A vinylic polymer as claimed in claim 12 which is an acrylic polymer comprising the residue of at least one monomer selected from the group comprising methyl methacrylate , ethyl methacrylate, n-propyl methacrylate, i-butyl methacrylate, n-butyl methacrylate, t-butyl methacrylate, hydroxyethyl methacrylate, poly alkyleneglycol methacrylates, poly alkyleneglycol acrylates, ethyl acrylate, butyl acrylate and methyl acrylate.