This invention relates to the preparation of graft copolymers and branched homopolymers.

A graft copolymer is a polymeric substance the molecules of which have a main chain (or backbone) comprising units derived from one species of monomer, connected to which are side-chains, comprising units derived from a different species of monomer.

The literature contains hundreds of papers and patents describing methods for the preparation of graft copolymers but in very few cases has it conclusively been proved that the product has the expected structure. Moreover, it is often the case that the product is contaminated with a homopolymer derived from one of the species of monomer units (main-chain or side-chain) or even with both types of homopolymer. It would therefore be valuable to be able to prepare graft copolymers whose structures can be authenticated, and which are free or substantially free of contamination by homopolymers.

A branched homopolymer, like a graft copolymer, consists of a backbone to which are connected side- chains, but in this case both the backbone and side- chains are formed from the same monomer species. To be able to prepare branched homopolymers of a well-defined and controlled structure would be valuable in helping to provide products with required properties.

It is the object of the present invention, therefore, to provide a method for the synthesis of graft copolymers and branched homopolymers, by making use of the technique, of Group Transfer Polymerization (GTP) which was first published by E.I. Du Pont de Nemours, Inc., in 1983. See, for example, U.S. Patent Nos. 4414372, 4417034 and 4508800.

The essential feature of GTP is the initiation of polymerization of certain monomers such as acrylates,

ethacrylates, lactones, acrylonitrile and methacrylonitrile by means of e.g. a silyl ketene acetal, in conjunction with a catalyst such as tris(dimethylamino) -sulfonium bifluoride. The chemistry of the addition of the first monomer unit can be represented by Reaction I as follows, using methyl ethacrylate (2) as the monomer and l-methoxy-l-(tri- methylsilyloxy) -2-methyl-prop-1-ene (1) as initiator:

(1) (2) (3)

Reaction I

The product (3) is itself a silyl ketene acetal, and it can therefore undergo addition with a further molecule of methyl methacrylate, and the whole process can be repeated with the result that poly(methyl methacrylate) is formed.

The present invention is based on the realization that advantage can be taken of the fact that a polymer chain which has been initiated by a GTP site remains bound to that site. Accordingly, if the GTP site is attached to a polymer backbone, then the polymer grown from that site constitutes a side chain to that backbone. This, then, leads to the possibility of preparing a backbone polymer containing groups which are GTP sites, or which can be transformed into GTP sites, and which can then be used to initiate the formation of grafts or side chains.

Thus, in accordance with the present invention there is broadly provided a method of preparing a graft copolymer or a branched homopolymer, which comprises the steps of:

(a) providing a backbone polymer chain containing GTP-initiating sites, and

(b) effecting the growth of polymer side chains from said GTP-initiating sites.

The general features of the present invention can be illustrated by reference to an example in which the polymer backbone is essentially polystyrene, but which is formed by copolymerizing styrene with a comonomer which contains a group (Y) which can be chemically converted into a silyl ketene acetal or other GTP-initiating site. The comonomer may be a vinyl monomer, for example it may be a substituted styrene or a methacrylate, which contains a Y group at a point which does not interfere with the potential of the compound for undergoing vinyl polymerization. Illustratively then:

is copolymerized with a small quantity of substituted styre

CHo or CH, = C

COCH ₂ Y substituted methyl methacrylate

to produce a copolymer in which the Y groups are distributed statistically in the essentially polystryene backbone chain e.g.:

The Y group may suitably be a hydroxyl group, for example.

After transforming the Y groups to silyl ketene acetal (or other GTP initiating site) , we have an essentially polystyrene backbone containing GTP initiating sites (X) , which can be represented:

Contact of this backbone with an acrylate or methyaerylate monomer will then result in the growth of grafts from the GTP initiating sites (X) , so that the resulting product can be represented as:

PMMA PMMA PMMA

The illustrated product is, of course, a graft copolymer, but if, for example, the backbone polymer was itself polymethylmethacrylate, then the product would be a branched homopolymer.

The backbone polymer used as the starting material for this invention may be any polymer which contains GTP initiating sites, such as silyl ketene acetal groups, and which is chemically unreactive in the reaction by which the side chains are grown from the GTP sites. The preparation of the backbone polymer material containing GTP initiating sites is itself effected by preparing a polymer which contains groups which may be readily converted to GTP initiating sites, as by conventional polymerization or copolymerization methods. In the above general illustration of the -Invention, this was accomplished by copolymerizing styrene with a monomer containing groups such as hydroxyl groups, convertible to GTP initiating sites, but if the polymer already contains

groups convertible to GTP initiating sites, for example cellulose and polyvinyl alcohol contain hydroxyl groups, and these are present in a desired proportion, then obviously no special steps to introduce groups convertible to GTP initiating sites are required. However, the introduction of groups convertible into GTP initiating sites by copolymerization is often preferred since it may provide for better control of the position and number of the eventual side chains.

It is, of course, not necessary that the backbone polymer should be composed only of a single type of monomer unit, but instead it can equally be copolymeric in structure, for example styrene-butadiene copolymers.

Having formed a polymer backbone containing groups convertible to GTP-initiating sites, the next step is to form the GTP-initiating sites themselves. A number of methods can be used for this purpose, depending on the nature of the initial groups present and the final GTP-initiating groups desired. Up to the present, silyl ketene acetal groups have been shown to be the most suitable for initiating GT polymerization of acrylates and methacrylates, and such groups can be readily prepared from hydroxyl groups, for instance by using the technique illustrated in Example 3 below.

In one preferred embodiment of the invention a polystyrene backbone containing hydroxyl groups attached to comonomer residues is obtained by radical polymerization of styrene and 2-isobutyryloxyethyl methacrylate (hereafter abbreviated as "ibem") . The comonomer ibem can readily be synthesised by esterification of 2-hydroxyethyl methacrylate with isobutyryl chloride in the presence of triethylamine at a temperature as low as possible so as to avoid thermal polymerization. Ibem is suitably copolymerized radically with styrene using A1BN as initiator, at an

elevated temperature eg 60°C resulting in a statistical copolymer substantially free of homo-sequences of styrene or ibem. The proportion of ibem comonomer to the styrene is not at all critical, but typically the resulting copolymer may contain 5-20% of ibem units.

The resulting poly(styrene-stat-ibem) copolymer is next treated with a suitable base eg lithium diisopropy1amide, which converts the ester group to the enol form, and excess chlorotrimethylsilane, resulting in the formation of silyl ketene acetal groups along the backbone chain. This reaction is conducted in a solvent, preferably THF, at 0°C. After completion of the reaction, the mixture is preferably filtered under nitrogen to remove the triethylamine hydrochloride salt formed as a by-product, and then the remaining reagents as well as the solvent are removed in vacuo. The poly(styrene-stat-ibem) containing tri ethy1silyl acetal groups can now be grafted with, for example, an acrylate or methacrylate monomer.

The grafting reaction can be performed by adding the comononer and catalyst to a solution of the backbone polymer in a solvent such as THF. Suitable catalysts include Lewis acids, azides, cyanides and bifluorides, but tris(dimethylamino)sulfonium bifluoride, is particularly preferred. It is found that the dilution of the reaction solution and the amount of catalyst used both have an effect on the yield of the graft copolymer which is obtained, the use of relatively dilute solutions and relatively high concentrations of catalyst both helping to enhance the yield. It is convenient to effect the grafting reaction at room temperature.

The above-described reactions to prepare a poly(styrene-stat-ibem) -graft-poly(methylmethacrylate) are illustrated by Reaction II be ^' iow:

e ₃

:Bac bo'ne .Grafts

Poly(styrene-slat-IBEM)-graft-poly(met yl rnethacrylat )

For simplicity of description, the graft or side chain polymers have been referred to as being composed of a single type of monomer unit. It is, however, possible for the side chains to be in the form of statistical copolymers, achieved by using a mixture of two or more different types of monomer unit; or in the form of block copolymers, achieved by changing the monomer species during the grafting reaction.

Further, the above illustrative description should not be thought to limit the invention to the use of comonomers in which the Y group is directly attached to the aromatic ring (in the case of a styrene derivative) or attached to the ester function directly through a methylene unit (in the case of an acrylate or methacrylate) .

A methacrylate monomer of particular interest in the present invention is trimethylsilyl methacrylate (TMSM) , of the formula:

CH ₂ =C(CH ₃ )

C0 ₂ Si(CH ₃ )

This monomer behaves like methyl methacrylate (MM) in its general polymerisation characteristics, and it can be polymerised by GTP without difficulty. The trimethylsilyl group is very labile to acid, so that poly(trimethylsilyl methacrylate) is readily hydrolysed to give poly(methacrylie acid) . If TMSM is used in place of MM in polymerisation processes of the type described in this specification, and the graft polymer is subsequently exposed to the action of acid, the product will be a copolymer comprising both hydrophobic and hydrophilic segments. Polymer molecules containing ionisable groups are dependent on pH for their configurations in solution, and their solution properties can be controlled accordingly.

One example of such a graft copolymer is a system with a poly(methyl methacrylate) backbone and pol (methacrylic acid) grafts; equally, the nature of the components could be reversed. Polymers of this general type find technological and commercial application, for example, as homogenising and/or stabilising agents, e.g. for polymer blends, because the different segments of the graft copolymer molecules can be chosen so as to be individually compatible with the components of the blend. Further useful applications arise from the detergent properties of graft copolymers of mixed hydrophilic/hyrophobic character.

The invention is illustrated by the Examples which follow:

Example 1

Preparation of 2-Isobutyryloxyethγl methacrylate (IBEM) (CH ₂ = CMeC0 ₂ (CH ₂ )0C0CHMe ₂ )

To a solution of 2-hydroxy thyl methacrylate (10 g, 0.07 mole) in dry THF (100 cm ³ ) in a 250 cm ³ three-necked flask equipped with condenser, dropping funnel, magnetic stirrer and N ₂ inlet, Et ₃ N (7 g, 0.07 mole) was added and the reaction mixture was cooled to O'C. Isobutyryl chloride (9.5 g, 0.09 mole) was added through the dropping funnel at a slow rate, so that the temperature was kept as low as possible, (below- 10°C) .

A white solid started to precipitate immediately. After the completion of the addition, the mixture was left stirring at 0°C for 3 hours, then broken up in ice-H ₂ 0 and the organic layer was collected. The solvent was removed using a rotary evaporator. The colourless oil which remained was distilled to give 13.2 g of ibem. Yield : 94% b.p. 49-51°C/l m Hg NMR : δ : 6.1, 5.5 ppm (2 d, 2H) , 4.25 ppm (t, 4H) , 2.8-2.3 ppm (h, 1H) , 1.75 ppm (s, 3H) .

Example 2 Preparation of poly(st-stat-ibem)

In a dry ampoule, kept under N ₂ , 1 cm ³ of 0.1M solution of AIBN in dry CHC1 ₃ was placed, using a syringe. After removing the solvent under high vacuum, the amounts of styrene and ibem indicated in Table I, both freshly distilled, were added under a dry N ₂ atmosphere and the mixture was degassed by the freeze- thaw method. Then the ampoule was sealed and placed in a temperature-controlled water bath at 60°C.

After 150 min, the ampoule was broken and the viscous solution was diluted with PhMe and poured into MeOH. The polymeric product was further purified by twice dissolving in CHC1 ₃ and reprecipitating in MeOH. The polymer was dried ±τ vacuo at 40°C/25 mmHg for 5-6 h. The graft copolymers were analysed by NMR spectroscopy.

Exa ple 3 Preparation of Graft Copolymers

A two-necked round-bottom flask, equipped with a magnetic stirrer, a condenser and a Suba-seal, was charged with lithium diisopropylamide LDA followed by a solution of poly(st-stat-ibem) , prepared as described in Example 2, in dry THF, via a syringe. The LDA and isobutyryl groups of the polymer used, were present in equimolar quantities.

The solution was subsequently stirred for 30 minutes at 0°C. Then an excess of Me ₃ SiCl was added and the mixture was stirred for 1 hour at room, temperature. The white solid which precipitated was filtered under N ₂ and unreacted Me ₃ SiCl and THF were removed in vacuo. The remaining yellowish solid was dissolved in freshly distilled THF arid to this solution the estimated amount of a solution of tris(dimethyla ino)sulfonium bifluoride (TaSHF ₂ ) in THF as catalyst was added. After stirring for 15 minutes, methyl methacrylate monomer was added dropwise. The reaction mixture was stirred for ca. 17-36 h, and then MeOH (3-5 cm ³ ) was added. Solvents and residual unpolymerised CH ₂ = CMeC0 ₂ Me were removed in. vacuo. The remaining yellow solid was purified by twice dissolving in THF and reprecipitating in 40-60"C petroleum ether, to give finally a white polymer, characterised as poly(styrene-stat-2-isobutyryloxyethyl methacrylate)-αraft-poly(methyl methacrylate) copolymer.

The amounts of reactant monomers used, and of the methylmethacrylate (MM) grafted onto the polymer backbone are shown in Table II below:

ABLE II

The catalyst molar concentration remained standard in all runs at about one-tenth of the molar concentration of the ibem groups in the backbone polymer, indicated in the table by the symbol [I] . The reaction time was from 17 to 36 hours, although reaction was complete in about 17 hours.

Table II shows that as the concentration of the ibem in the polymer backbone in solution increases, and despite the variations of the concentration of methyl methacrylate, the proportion of methyl methacrylate which is grafted decreases, very significantly in the case of run Nos. 6-10. For concentrations of ibem groups in the backbone polymer in the solution in the range 3 x 10~ ⁴ - 15 x 10~ ⁴ mol.l ^-1 , conversion was 96-53% whereas for concentrations of backbone polymer greater than 15 x 10~ ⁴ mol.l" ¹ , the conversion rate fell markedly, to as low as 2% in run No. 10.

In a further series of runs, using more dilute solutions of the backbone polymer as well as a five-fold

increase in the amount of catalyst, the results shown in Table III were obtained

TABLE III

N.B. In Tables II and III above, the symbol [I] is used to indicate the molar concentration of the ibem groups in the backbone polymer, and the symbol [MM] is used to indicate the molar concentration of the ethylmethacrylate used as grafting comonomer.

Example 4 A 100 cm ³ round-bottomed flask equipped with a magnetic stirrer, a condenser, and a Suba-seal was charged with lithium diisopropyla ide (LDA) solution (1 cm ³ ), cooled to O'C, followed by a solution of ^p ol ^{y (} st-stat-ibem) (1 g; containing 10% of ibem) in dry THF (10 cm ³ ) of dry THF, via a syringe. After the addition, the solution was stirred to 30 in at 0°C. Then Me ₃ SiCl (6 cm ³ , 0.047 mole) was added and the mixture was stirred for 1 h' at room temperature. After treating the solution as described in Example 3, the remaining solid was dissolved in dry THF (20 cm ³ ) and tris(dimethylamino)sulfonium bifluoride (l cm ³ of a solution 1M in MeCN; lo ^"3 mole) was added. After stirring for 15 min, methyl methacrylate (2 cm ³ , 0. ⁰ 2 mole was added dropwise. Stirring continued for

36 h, then MeOH (2 cm ³ ) was added, and the solution was treated as described above, to give the expected graft copolymer. Conversion was quite high.

Example 5

The same procedure as the one described in Example 4 was followed except that anhydrous ZnCl (0.1 g, 7.3 x 10~ ⁴ mole) was used as cocatalyst and dry CH C1 ₂ (10 cm ³ ) as solvent. Conversion was quite high.

In general, however,zinc chloride has been found to be a less effective catalyst for the GTP grafting process than tris(dimethyla ino) sulfonium bifluoride.

Example 6

Step 1: Preparation of Polv(methyl methacrylate-stat-ibenQ

Following the general procedure described in Example 2, poly(methyl methacrylate-stat-ibem) was prepared from the following reactants:

methyl methacrylate: 9 ml (0.08 mole); and 2-isobutyryloxyethyl methacrylate: 2 ml (9 x 10~ ³ mole)

These monomers were reacted in dry THF (30 ml) in the presence of AlBN (0.16 g) under nitrogen at 60°C for 6 hours.

Step 2: Preparation of Poly(methyl methacrylate-stat- ibem^ graft methyl methacrylate

The general procedure described in Example 3 was used.

In the first stage, 1 g of poly(methyl methacrylate-stat-ibem) containing 10% of ibem (5 x 10 ⁴ moles g ibem) was reacted with 10 ml of Me ₃ SiCl in 50 ml of dry THF containing 1.25 ml of LDA. The Me ₃ SiCl was added at 0°C, and the reactants were stirred at room temperature for one hour. Then 1 cm ³ of a 1 M solution of TaSHF ₂ in MeCN(5 x 10~ ⁵ moles) was added, and finally freshly distilled methyl methacrylate (4 cm ³ , 0.04 moles) was introduced and the reaction mixture was stirred overnight.

Poly(methyl methacrylate-stat-ibem) graft methyl methacrylate was recovered in an amount of 3.4 g. The conversion rate was 74%.